EP2710361A1 - Method for assaying nitrates and/or nitrites in a neutral medium - Google Patents

Method for assaying nitrates and/or nitrites in a neutral medium

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Publication number
EP2710361A1
EP2710361A1 EP12728682.1A EP12728682A EP2710361A1 EP 2710361 A1 EP2710361 A1 EP 2710361A1 EP 12728682 A EP12728682 A EP 12728682A EP 2710361 A1 EP2710361 A1 EP 2710361A1
Authority
EP
European Patent Office
Prior art keywords
potential
copper
electrode
ions
nitrate
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP12728682.1A
Other languages
German (de)
French (fr)
Inventor
Catherine Debiemme-Chouvy
Thi Tuyet Maï TRONG LONG
Hubert Cachet
Jean-Pierre Heitzmann
Cédric CHAUMONT
Julien TOURNEBIZE
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Centre National de la Recherche Scientifique CNRS
Original Assignee
Centre National de la Recherche Scientifique CNRS
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Filing date
Publication date
Application filed by Centre National de la Recherche Scientifique CNRS filed Critical Centre National de la Recherche Scientifique CNRS
Publication of EP2710361A1 publication Critical patent/EP2710361A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/18—Water
    • G01N33/188—Determining the state of nitrification

Definitions

  • the invention relates to an electrochemical sensor comprising a working electrode, a reference electrode and a counter electrode.
  • electrochemical sensors are particularly suitable for the determination of nitrate ions and / or nitrite ions, especially in situ and at a high frequency. It is typically, but not exclusively, applied to potentiostatic nitrate and / or nitrite ion dosing in aqueous media of substantially neutral pH.
  • the principle of these already existing sensors is to arrange these three electrodes in an arrangement for which they are found parallel to each other, and bathing all three in an electrolytic solution.
  • This sensor geometry is bulky, and is not suitable for use in the field. In fact, for outdoor applications, the sensors must be able to be handled without any particular precaution, and must be able to withstand various and varied shocks inherent to this type of activity.
  • such an arrangement of the electrodes creates protuberances, likely to catch or bump into external natural elements, such as branches or stones. This will result in damage to the sensor, which may render it inoperable or may lead to erroneous measurements.
  • the electrochemical sensors according to the invention have a compact geometry, limiting their size and giving them a character of great robustness, for use in the field, efficient and reliable.
  • the subject of the invention is an electrochemical sensor comprising a working electrode, a reference electrode, and a counter electrode.
  • the main characteristic of a sensor according to the invention is that the three electrodes are arranged coaxially in said sensor. By thus having the same axis of rotation, the three electrodes are found in a optimized functional configuration, favoring a small footprint of the sensor.
  • the electrochemical sensor is an amperometric sensor.
  • the senor is delimited by an elongated insulating casing.
  • the insulative housing is designed not only to maintain the three electrodes in a compact arrangement, but also to provide effective protection of said electrodes, vis-à-vis external elements against which they could come hang on or shock.
  • This resistant housing combined with a compact geometry of the sensor, allows an easy handling of this sensor in external environment, without risk of damage due to untimely shocks.
  • An elongated shape of the sensor allows it to be easily inserted into an artificial external structure, such as a specific apparatus allowing for example to apply different potentials to the working electrode, or in a natural external structure, such as for example loose soil.
  • the housing is essentially insulating from the electric current, but it can also be heat, limiting the thermal conduction.
  • the housing is cylindrical and is made of an electrically insulating material such as polyvinyl chloride (PVC).
  • PVC polyvinyl chloride
  • the cylindrical shape is preferred for ease of manufacture.
  • PVC is a light and resistant material, particularly suitable for a small object, brought to be handled in an external environment and without any particular precaution.
  • PVC is a common material, whose manufacture is well controlled.
  • the reference electrode is placed in the center of the working electrode.
  • This particular arrangement allows the two electrodes to be very close to each other, this proximity being particularly sought to minimize the ohmic drop, in the case of measurements made in waters with low salt content, and which are therefore little conductive.
  • an ohmic drop can be a source of error on the value of the electrical potential actually applied to the working electrode.
  • the casing has an external shoulder, making it possible to distinguish a first part of smaller diameter and in which are placed the working electrode, the counter-electrode and a part of the reference electrode, and a second part enclosing the remainder. of the reference electrode.
  • the electrodes are grouped at one end of the housing.
  • the portion of the smaller diameter housing and containing most of the electrodes is intended to be inserted into an outer structure, the shoulder then serving as a stopper during the final fixing of the sensor in said structure.
  • the portion of the smaller diameter housing may have an external thread allowing the sensor to be screwed onto an outer structure.
  • a cover made of an electrically insulating material such as PVC, is placed at the free end of the first part, between the working electrode and the counter electrode.
  • This insulating cover can be represented either by an insert and fixed to the housing, or be an integral part of the housing by forming therewith a single piece.
  • the working electrode has a central channel in which is inserted an end of the reference electrode, said end being closed by a porous body.
  • This body may for example be constituted by a porous ceramic.
  • the porous body is of cylindrical shape and is locked in translation by means of two mechanical retaining stops.
  • This arrangement avoids having to stick the porous body in the reference electrode, as is commonly practiced in existing configurations.
  • the adhesive may overflow the porous body and come to close, at least partially the channel of the reference electrode.
  • the working electrode is made of copper (ie copper electrode).
  • the counter-electrode may be made of stainless steel, and the reference electrode may be an Ag / AgCl type electrode or a calomel type electrode (ECS) detailed in FIG. following the description.
  • ECS calomel type electrode
  • An Ag / AgCl electrode is an electrode comprising an Ag wire covered with a layer of AgCl and immersed in a saturated solution of KCl. It will be preferred to use a reference electrode that does not include mercury.
  • the electrochemical sensors according to the invention have a simple geometry, allowing them to be manufactured quickly and easily without major risk of defects. They also have the advantage of being efficient for use in the field, while remaining of a small footprint and having a resistant structure. Finally, they have the advantage of being able to be configured to cooperate with an external structure, such as for example an electronic device making it possible to apply different potentials to the working electrode and to measure the different currents flowing through the working electrode corresponding to these potentials.
  • the present invention also relates to a method for determining nitrate and / or nitrite ions in a substantially neutral pH solution.
  • a copper electrode for the potentiostatic dosing of nitrate and / or nitrite ions in an aqueous medium of substantially neutral pH.
  • Numerous methods for quantitative determination of nitrate and / or nitrite ions are known, in particular at high acid pH.
  • This document presents a method for assaying nitrate ions using a copper electrode. This assay method comprises a first step of applying a potential of between 0 and 0.2 V.
  • the plot of the curve of the intensity as a function of the potential applied to the copper electrode then makes it possible to identify the current peak due to the reduction of the nitrate ions to NH 4 + ions, and to deduce from this the concentration in nitrate ions by calibration in the acidified aqueous medium.
  • the object of the present invention is to overcome the drawbacks of the techniques of the prior art by proposing in particular a simple and direct method for the determination of nitrate and / or nitrite ions which makes it possible to guarantee reliable measurements, without it being necessary to add any additive in the solution to be analyzed, in particular to modify the pH.
  • the subject of the present invention is a method for the determination of nitrate and / or nitrite ions in solution, using a copper electrode, said method being characterized in that it is carried out in potentiostatic mode, and that it comprises the dosage steps. consisting of: a. applying a PA potential to a submerged copper electrode in a substantially neutral pH analyte solution, for measuring the oxygen reducing current likely to be present in said solution to be analyzed, then b. applying a PB potential to the copper electrode so as to reduce the nitrate ions to obtain the measurement of the reduction current II corresponding to the reduction of the nitrate ions to nitrite ions and the reduction of the oxygen, then c.
  • the dosing steps a and b make it possible to determine the nitrate reduction current, corrected for the stream 10, and the a, b and c determination steps make it possible to determine the nitrite ion reduction current corrected for the currents II. and 10.
  • the oxygen likely to be present in the solution to be analyzed tends to distort the determination of the nitrate and / or nitrite ions, by overestimating the results of the assay.
  • the application of this PA potential advantageously makes it possible to propose a nitrate and / or nitrite ion assay method which optimally guarantees reproducible and reliable measurements.
  • the potential PA applied to the step of assay advantageously makes it possible to avoid deaerating the solution to be analyzed.
  • copper electrode means a copper electrode called “unalloyed”, consisting solely of copper, the copper may be solid or electro-deposited. Of course, the copper electrode may contain unavoidable impurities such as other metal elements. In this case, the copper electrode is composed of at least 99.9% by weight of copper, the remainder being said inevitable impurities.
  • substantially neutral pH means a solution (or an electrolyte) whose pH is close to neutrality, that is to say ranging from 5 to 9, and preferably about 7.
  • the solution to be analyzed may be an aqueous medium such as natural water or wastewater.
  • Steps a, b and optionally c assay are performed "in potentiostatic mode".
  • This type of assay is well known and consists in measuring the intensity of the current at a given potential (ie fixed potential).
  • Current intensity is, in the present invention, a function of the concentration of dioxygen and nitrate and / or nitrite ions.
  • steps a, b and optionally c are reduction steps in order to determine the nitrite and / or nitrate ions (i.e., quantitative detection), the potentials applied to each of these steps are of the cathode type.
  • the potentiostatic mode used in steps a to c advantageously makes it possible to apply a constant potential for a given time, which is technically much simpler than to vary this potential with a given speed such as in potentiodynamic mode.
  • the potentiostatic dosage according to the invention is therefore very different from the potentiodynamic dosage (cyclic voltammetry) in which the potential applied to the copper electrode varies as a function of time (potential sweep).
  • the potential PA applied to the copper electrode makes it possible to measure the current of reduction of the oxygen into OH " ions according to the following equation 1 (Eq 1):
  • the potential PA may be equal to about -0.6 V / SCE.
  • the potentials described in the present invention are typically expressed with respect to a saturated calomel electrode type reference electrode (i.e. KCI saturated calomel electrode), noted in the present description "ECS".
  • the duration of the application (or imposition) of the potential PA may be at least a few seconds, for example between 20 and 30 seconds.
  • the potential PB applied to the copper electrode makes it possible to reduce the nitrate ions to nitrite ions according to the following equation 2 (Eq 2): N0 3 " + 2e " + H 2 0 ⁇ N0 2 " + 2 OH " (Eq2)
  • the application of the potential PB makes it possible to obtain the measurement of the reduction current II corresponding to the reduction of nitrate (Eq 2) and dioxygen (Eq 1) ions.
  • the potential PB is equal to approximately -0.9 V / SCE.
  • the duration of the application (or imposition) of the potential PB may be at least a few seconds, and for example between 20 and 30 seconds.
  • the potential PC applied to the copper electrode makes it possible to reduce the nitrate ions and the nitrite ions according to equations 3 and 4 (Eq 3 and Eq 4) as follows:
  • the application of the potential PC makes it possible to obtain the measurement of the reduction current 12 corresponding to the reduction of nitrate ions (Eq 3), nitrite ions (Eq 4) and oxygen (Eq 1).
  • the potential PC is equal to approximately -1.1 V / SCE.
  • the duration of the application (or imposition) of the potential PC may be at least a few seconds, and for example between 20 and 30 seconds.
  • nitrate ions and / or nitrite ions can thus be easily and reliably obtained by taking into account the current (10) of reduction of oxygen.
  • the assay method according to the invention is preferably carried out with a flow rate of the analyte solution which is substantially constant.
  • a flow rate of the analyte solution which is substantially constant.
  • the flow velocity of the solution to be analyzed may be zero or may be different from zero if it does not vary with time.
  • the solution to be analyzed is renewed after each step b when the assay consists of measuring only the currents 10 and II, or after each step c when the assay consists of measuring the currents. 10, 11 and 12, in order to guarantee in time the reproducibility of the measurements made with the copper electrode.
  • the various reduction reactions included in steps a, b and c tend to increase the pH of the solution to be analyzed.
  • chemical species of calcium and / or magnesium type likely to be in the solution to be analyzed may form calcium-magnesium deposits on the surface of the copper electrode and thus limit the use of the copper electrode and the reproducibility of measurements made with said electrode.
  • the assay method of the invention may further comprise a step d following the dosage step c, said step of maintaining the copper electrode at a potential PD, said PD potential being lower than the quiescent potential of the copper electrode, so as to avoid the copper electrode to oxidize when the latter is no longer used for said assay for a given period.
  • the PD potential advantageously prevents the copper from oxidizing (ie corroding), which could lead to the formation of a large film of copper oxide on the surface of the copper electrode, thus preventing to obtain reproducible measurements.
  • the resting potential of the copper electrode (or open circuit potential) can be for example approximately equal to -0.10 V / SCE.
  • the method according to the present invention may further comprise copper electrode treatment steps, prior to assay steps a and b, or assay steps a, b and c, consisting of: i. applying a first potential to the copper electrode so as to reduce the copper oxides present on the surface of the copper metal electrode, ii. applying a second potential to the copper electrode so as to oxidize the metallic copper formed in step i to cupric ions, copper oxides possibly forming in step ii, and iii. applying a third potential to the copper electrode so as to reduce the copper oxides possibly formed in step ii, steps i to iii being performed by immersing the copper electrode in a supporting electrolyte having a substantially neutral pH .
  • the electrolyte supporting steps i to iii is preferably an aqueous carrier electrolyte.
  • the carrier electrolyte is the solution to be analyzed during assaying steps a and b, or assaying steps a, b and c.
  • Steps i to iii are of course performed "in potentiostatic mode" (or “in amperometric mode”). Steps i to iii are referred to as “conditioning steps” of the electrode, prior to dosing steps a and b, or to dosing steps a, b and c, per se. These conditioning steps, consisting in applying to the copper electrode a sequence of three successive potentials in potentiostatic mode, advantageously make it possible to obtain a surface of the electrode consisting essentially of copper in the oxidation state 0 and therefore free of of copper oxide, in order to have a constant and reproducible active surface.
  • the concentration of nitrate ions and / or nitrite present in the solution to be analyzed is proportional to the intensities measured at the potentials PB and PC applied to the conditioned copper electrode.
  • This potentiostatic dosage thus makes it possible to easily obtain the concentration of nitrate and / or nitrite ions with optimum reliability.
  • the first potential makes it possible to reduce copper oxides of the CuO and / or Cu 2 0 type to metallic copper, said copper oxides notably originating from the oxidation of metallic copper. by the oxygen of the air.
  • the first potential is cathodic type, it must allow the reduction of all copper oxides. Typically, it may be less than or equal to -0.85 V / SCE, and preferably equal to -1.15 V / SCE. Ideally, the duration of application of the first potential may be at least a few seconds, and for example between 20 and 30 seconds.
  • the second potential makes it possible to oxidize the metallic copper formed in step i into cupric ions (Cu 2+ ).
  • the second potential is therefore anodic type.
  • the second potential may be greater than the quiescent potential of the copper electrode, in particular greater than 5 to 200 mV with respect to said quiescent potential, and preferably greater than 5 to 100 mV with respect to said quiescent potential.
  • a second higher potential of more than 200 mV with respect to the resting potential would not necessarily be acceptable because it could induce an excessive dissolution of the metallic copper and thus the active surface of the electrode would become inhomogeneous as explained herein. -after.
  • the quiescent potential corresponds to the open circuit potential or, in other words, to the "natural" potential measured when the copper electrode is not subjected to the application of any potential.
  • This quiescent potential is therefore conventionally measured between the copper electrode and a reference electrode, for example of the ECS type, without any potential being applied between these two electrodes.
  • the resting potential of the copper electrode may be approximately equal to -0.10 V / SCE.
  • the second potential when the open circuit potential is -0.100 V / SCE, the second potential may be greater than or equal to -0.095 V / SCE and less than or equal to 0.100 V / SCE, and preferably less than or equal to 0.000 V / SCE.
  • the duration of application of the second potential may be at least a few seconds, for example between 20 and 30 seconds.
  • copper oxides such as, for example, CuO and / or Cu 2 O, may optionally be formed.
  • step ii makes it possible to guarantee reproducible measurements one after the other, guaranteeing an active surface that remains constant and homogeneous (smooth surface) as and when dosages are carried out.
  • the third potential makes it possible, in turn, to reduce the copper oxides that may have formed during step ii.
  • the third potential is therefore cathodic type. It may be less than or equal to -0.85 V / SCE, and preferably equal to -1.15 V / SCE. Ideally, the duration of application of the third potential may be at least a few seconds, and for example between 20 and 30 seconds.
  • steps i, ii and iii makes it possible to obtain a copper electrode whose surface has been conditioned and which then makes it possible, during the determination of the nitrate and / or nitrite ions, of steps a, b and c, to measure a reduction current that is reproducible and proportional to the concentration of nitrate and / or nitrite ions in the solution to be analyzed.
  • steps i to iii, prior to steps a and b, or steps a, b and c, are repeated before each step a, thus making it possible to perform continuous dosages while ensuring reproducibility. measurements and keeping the performance of the copper electrode constant.
  • a reference electrode for example of the calomel electrode type in saturated solution of KCI (ECS) or of type
  • the electrochemical sensor can advantageously be the electrochemical sensor with compact geometry such as as defined in the present invention.
  • the electrochemical sensor may further comprise an electronic device for applying the different potentials to the copper electrode and for measuring the different currents (i.e. current intensities) passing through the copper electrode corresponding to these potentials. It may further include a temperature probe.
  • Another object of the invention relates to a copper electrode conditioned by steps i to iii of the assay method as defined in the present invention.
  • Said copper electrode is characterized in that its surface consists essentially of copper in the oxidation state 0 (Cu (0)), that is to say copper in metallic form.
  • the term "substantially” is understood to mean an atomic percentage (% at.) Content of copper in oxidation state 0 which is greater than 80 at%, and preferably greater than 90 at%, with respect to the sum of the contents in% at. of all the constituents comprising at least one metal element, present on the surface of the copper electrode.
  • All of said constituents of course, comprise copper in metallic form (Cu (O)), and may furthermore comprise copper in oxidation state +1 (Cu (I)) and / or copper in the form of oxidation state +11 (Cu (II)). More particularly, the surface of the copper electrode comprises less than 10 at%. copper oxide (s), preferably less than 5 at%. of copper oxide (s).
  • the surface of the copper electrode does not comprise copper oxide, and particularly preferably the surface of the copper electrode consists only of copper in the state of copper. oxidation 0.
  • Another object of the invention relates to the use of such a conditioned copper electrode, for the determination of nitrate and / or nitrite ions in substantially neutral pH solution, said assay being carried out in potentiostatic mode.
  • Another object of the invention relates to the electrochemical sensor according to the present invention, namely the compact geometry electrochemical sensor, comprising a copper electrode as a working electrode, said copper electrode being conditioned by steps i to iii of the assay method as defined in the present invention.
  • Another object of the invention relates to the use of such an electrochemical sensor comprising said conditioned copper electrode, for the determination of nitrate and / or nitrite ions in a substantially neutral pH solution, said assay being carried out in potentiostatic mode.
  • Another subject of the invention concerns the method for determining the nitrate and / or nitrite ions in solution of the invention, using the electrochemical sensor (compact geometry) of the invention, when the working electrode of said electrochemical sensor is a copper electrode, the copper electrode possibly having undergone the conditioning steps i to iii.
  • the electrochemical sensor with compact geometry advantageously makes it possible to be able to significantly limit the ohmic drop in order to guarantee a potential at the copper electrode closest to the one applied, in particular in the solutions to be analyzed which are not heavily loaded with salts or not very conductive.
  • the ohmic drop (RI) is the product of the current flowing through the copper electrode by the ohmic resistance of the solution to be analyzed (ie electrolyte).
  • FIG. 1 shows a longitudinal axial sectional view of an electrochemical sensor according to the invention.
  • an electrochemical sensor 1 designed for the in situ and high frequency assay of nitrate ions and / or nitrite ions, especially in natural environments, such as for example rivers, comprises a cylindrical outer housing 2 in which a working electrode 3 made of copper, a counter-electrode 4 made of stainless steel, and a reference electrode 5 of the Ag / AgCl type, constituted by an Ag wire covered by a layer of aluminum, are disposed. AgCl and immersed in a saturated solution of KO.
  • the housing 2 is a hollow part of cylindrical shape, and comprises an outer shoulder 6 for distinguishing a first portion 7 and a second portion 8, the diameter of said first portion 7 being smaller than that of the second portion 8.
  • the internal channels of these two parts 7,8 are in continuity with one another, the internal channel of the first portion 7 having a smaller diameter than the internal channel of the second part.
  • the reference electrode 5 is delimited by a glass tube 13, placed in a cylindrical body 9, made of plastic material, and having an external shoulder 10 making it possible to distinguish a first part 11 and a second portion 12, the diameter of the first portion 11 being smaller than that of the second portion 12.
  • the tube 13 which contains the silver wire and the KCl solution, is fixed inside this hollow body 9, by means of a fastener involving a Versilic TM silicone plug 14, and a ring 25 silicone also Versilic TM type.
  • work 3 is in the form of a hollow cylindrical part, placed at the free end of the first part 7 of the housing 2, so that its axis of revolution coincides with the axis of revolution of said housing 2.
  • the positioning of the working electrode 3 at this end is such that a flat annular surface 18 of said working electrode 3 is flush with the free end of the first portion 7 of the housing 2, said electrode being housed inside said first part 7.
  • the counter-electrode 4 made of stainless steel is a hollow cylindrical piece inserted in the wall of the first part 7 of the casing 2, the maximum thickness of said counter-electrode 4 being substantially equal to the thickness of the wall of said first part. hollow 7.
  • the counter-electrode 4 is set back from the free end of the first part 7 of the housing 2.
  • the housing 2 is filled with black araldite 16, and the hollow cylindrical body 9 surrounding the reference electrode 5 containing gel 15 and the tube 13 is immersed in said araldite 16.
  • Said hollow body 9 is connected to the working electrode 3 via a hollow interface piece 17 having an internal channel of constant diameter, said interface piece 17 acting as an adapter between the reference electrode 5 and the working electrode 3.
  • This interface piece 17 is divided into three segments 19,20,21 which are distinguished from each other at their outer diameter.
  • the first segment 19 is housed in the internal channel of the first portion 11 of the hollow cylindrical body 9 delimiting the reference electrode 5, so that its outer surface is in contact with the inner surface of said first portion 11.
  • the second segment 20, which extends the first segment 19 has an outer diameter greater than that of the first segment 19.
  • This second segment 20 forms an annular bead inserted between the end of the first portion 11 of the hollow cylindrical body 9 delimiting the reference electrode 5 and the flat annular surface 26 of the working electrode 3, which is opposite. at the flat annular surface 18 leveling the end of the first part 7 of the housing 2.
  • This bead 20 is in contact with both said working electrode 3 and the hollow cylindrical body 9 delimiting the reference electrode 5.
  • the second segment 20 is extended by a third segment 21, whose outer diameter is smaller than that of the first segment 19.
  • This third segment 21 is of small thickness and is housed in the internal channel of the working electrode 3, so that the external surface of said third segment 21 comes into contact with the inner surface of the working electrode 3.
  • This third segment 21 is flush with the free end of the first part 7 of the casing 2, and has an edge 23 bent inwards so as to locally reduce the internal diameter of said third segment 21.
  • This interface piece 17 is made of PVC.
  • a PVC hollow tube 22 is glued inside the internal channel of the interface piece 17, one end of said hollow tube 22 being placed at the level of the middle of the second segment 20, and the other emerging end of the first segment 19, inside the hollow body 9 delimiting the reference electrode.
  • the outer wall of the cylindrical tube 22 is glued to the inner wall of the interface piece 17.
  • a solid cylindrical body 24, made of porous ceramic, is placed in the internal channel of the interface piece 17 , between one end of the tube 22 adhered inside said channel, and the folded rim 23 of the third segment 21, said end of the tube 22 and said flange 23 constituting retaining stops of said solid body 24.
  • the oxygen likely to be present in the solution to be analyzed is first reduced according to the Eql equation, by applying a PA potential (step a) of -0.6 V / SCE for 20 to 30 seconds, in order to measure the corresponding reduction current.
  • nitrate ions (and also oxygen) are reduced according to equation Eq2, by applying a PB potential (step b) of -0.9 V / SCE for 20 to 30 seconds, in order to measure the reduction current II corresponding.
  • the corrected reduction current (I N itrate) of the nitrate ions, at -0.9 V / ECS, is then defined as:
  • IlMitrate H - 10.
  • the amount of nitrate present in the solution to be analyzed can then easily and reliably be determined. If it is furthermore desired to determine the quantity of nitrite in said solution to be analyzed, the method can be continued by reducing the nitrate and nitrite ions (and also the oxygen) according to equations Eq3 and Eq4 by applying a potential PC (step c ) of -1.1 V / DHW for 20 to 30 seconds, in order to measure the corresponding reduction current 12.
  • the amount of nitrite present in the solution to be analyzed can then easily and reliably be determined.

Abstract

The invention relates to a method for assaying nitrate and/or nitrite ions in a solution, using a copper electrode, said method being characterised in that it is carried out in a constant-potential mode, and in that it includes the assaying steps involving: a. applying a potential PA to a copper electrode submerged in a solution to be analysed having a substantially neutral pH, in order to measure the reduction current 10 of the dioxygen likely to be contained in said solution to be analysed; then b. applying a potential PB to the copper electrode, such as to reduce the nitrate ions in order to obtain the measurement of the reduction current 11 corresponding to the reduction of nitrate ions into nitrite ions and to the reduction of the dioxygen; and c. optionally, applying a potential PC to the copper electrode, such as to reduce the nitrate ions and the nitrite ions in order to obtain the measurement of the reduction current 12 corresponding to the reduction of nitrate and nitrite ions into ammonia and to the reduction of the dioxygen.

Description

Méthode de dosage des nitrates et/ou des nitrites en milieu neutre  Method for the determination of nitrates and / or nitrites in a neutral medium
L'invention se rapporte à un capteur électrochimique, comprenant une électrode de travail, une électrode de référence et une contre électrode. Ces capteurs électrochimiques sont particulièrement adaptés au dosage des ions nitrate et/ou des ions nitrite, notamment in situ et à fréquence élevée. Elle s'applique typiquement, mais non exclusivement au dosage potentiostatique des ions nitrate et/ou des ions nitrite en milieux aqueux de pH sensiblement neutre. The invention relates to an electrochemical sensor comprising a working electrode, a reference electrode and a counter electrode. These electrochemical sensors are particularly suitable for the determination of nitrate ions and / or nitrite ions, especially in situ and at a high frequency. It is typically, but not exclusively, applied to potentiostatic nitrate and / or nitrite ion dosing in aqueous media of substantially neutral pH.
Les capteurs électrochimiques faisant intervenir ces trois types d'électrodes existent et ont déjà été mis en œuvre, pour diverses applications. Le principe de ces capteurs déjà existants consiste à disposer ces trois électrodes selon un arrangement pour lequel elles se retrouvent parallèles entre elles, et baignant toutes les trois dans une solution électrolytique. Cette géométrie de capteur est encombrante, et n'est pas adaptée à une utilisation sur le terrain. En effet, pour des applications en extérieur, les capteurs doivent pouvoir être manipulés sans précaution particulière, et doivent pouvoir résister à des chocs divers et variés inhérents à ce type d'activité. Or, une telle disposition des électrodes crée des protubérances, susceptibles de s'accrocher ou de se cogner à des éléments naturels extérieurs, comme par exemple des branchages ou des pierres. Il en résulterait un endommagement du capteur, pouvant le rendre inopérant ou pouvant conduire à des mesures erronées. Electrochemical sensors involving these three types of electrodes exist and have already been implemented, for various applications. The principle of these already existing sensors is to arrange these three electrodes in an arrangement for which they are found parallel to each other, and bathing all three in an electrolytic solution. This sensor geometry is bulky, and is not suitable for use in the field. In fact, for outdoor applications, the sensors must be able to be handled without any particular precaution, and must be able to withstand various and varied shocks inherent to this type of activity. However, such an arrangement of the electrodes creates protuberances, likely to catch or bump into external natural elements, such as branches or stones. This will result in damage to the sensor, which may render it inoperable or may lead to erroneous measurements.
Les capteurs électrochimiques selon l'invention, possèdent une géométrie compacte, limitant leur encombrement et leur conférant un caractère de grande robustesse, pour une utilisation sur le terrain, performante et fiable. The electrochemical sensors according to the invention have a compact geometry, limiting their size and giving them a character of great robustness, for use in the field, efficient and reliable.
L'invention a pour objet un capteur électrochimique comprenant une électrode de travail, une électrode de référence, et une contre électrode. La principale caractéristique d'un capteur selon l'invention, est que les trois électrodes sont disposées de façon coaxiale dans ledit capteur. En possédant ainsi le même axe de rotation, les trois électrodes se retrouvent dans une configuration fonctionnelle optimisée, favorisant un faible encombrement du capteur. Préférentiellement, le capteur électrochimique est un capteur ampérométrique. The subject of the invention is an electrochemical sensor comprising a working electrode, a reference electrode, and a counter electrode. The main characteristic of a sensor according to the invention is that the three electrodes are arranged coaxially in said sensor. By thus having the same axis of rotation, the three electrodes are found in a optimized functional configuration, favoring a small footprint of the sensor. Preferably, the electrochemical sensor is an amperometric sensor.
Avantageusement, le capteur est délimité par un boîtier isolant de forme allongée. Le boîtier isolant est conçu, non seulement pour maintenir les trois électrodes selon un arrangement compact, mais également pour assurer une protection efficace desdites électrodes, vis-à-vis des éléments extérieurs contre lesquels elles pourraient venir s'accrocher ou se choquer. Ce boîtier résistant associé à une géométrie compacte du capteur, permet une manipulation aisée de ce capteur en milieu extérieur, sans risque d'endommagement dû à des chocs intempestifs. Une forme allongée du capteur lui permet d'être facilement inséré dans une structure extérieure artificielle, telle qu'un appareillage spécifique permettant par exemple d'appliquer différents potentiels à l'électrode de travail, ou dans une structure extérieure naturelle, telle que par exemple un sol meuble. Le boîtier est essentiellement isolant du courant électrique, mais il peut également l'être de la chaleur, en limitant la conduction thermique. Advantageously, the sensor is delimited by an elongated insulating casing. The insulative housing is designed not only to maintain the three electrodes in a compact arrangement, but also to provide effective protection of said electrodes, vis-à-vis external elements against which they could come hang on or shock. This resistant housing combined with a compact geometry of the sensor, allows an easy handling of this sensor in external environment, without risk of damage due to untimely shocks. An elongated shape of the sensor allows it to be easily inserted into an artificial external structure, such as a specific apparatus allowing for example to apply different potentials to the working electrode, or in a natural external structure, such as for example loose soil. The housing is essentially insulating from the electric current, but it can also be heat, limiting the thermal conduction.
De façon préférentielle, le boîtier est cylindrique et est réalisé en un matériau électriquement isolant tel que du polychlorure de vinyle (PVC). La forme cylindrique est privilégiée pour des facilités de fabrication . Le PVC est un matériau léger et résistant, particulièrement adapté à un objet de petite taille, amené à être manipulé en milieu extérieur et sans précaution particulière. De plus, le PVC est un matériau usuel, dont la fabrication est bien maîtrisée. Preferably, the housing is cylindrical and is made of an electrically insulating material such as polyvinyl chloride (PVC). The cylindrical shape is preferred for ease of manufacture. PVC is a light and resistant material, particularly suitable for a small object, brought to be handled in an external environment and without any particular precaution. In addition, PVC is a common material, whose manufacture is well controlled.
De façon avantageuse, au moins une partie de l'électrode de référence est placée au centre de l'électrode de travail. Cette disposition particulière permet aux deux électrodes d'être très proches l'une de l'autre, cette proximité étant particulièrement recherchée pour minimiser la chute ohmique, dans le cas de mesures effectuées dans des eaux peu chargées en sels, et qui sont donc peu conductrices. En effet, une chute ohmique peut constituer une source d'erreur sur la valeur du potentiel électrique réellement appliqué à l'électrode de travail. Préférentiellement, le boîtier présente un épaulement externe, permettant de distinguer une première partie de plus faible diamètre et dans laquelle sont placées l'électrode de travail, la contre électrode et une partie de l'électrode de référence, et une deuxième partie renfermant le reste de l'électrode de référence. Avec ce type de configuration, les électrodes sont regroupées à l'une des extrémités du boîtier. La partie du boîtier de plus faible diamètre et regroupant l'essentiel des électrodes, est destinée à être insérée dans une structure extérieure, l'épaulement servant alors de butée lors de la fixation finale du capteur dans ladite structure. De façon préférentielle, la partie du boîtier de plus faible diamètre peut présenter un filetage extérieur permettant au capteur d'être vissé sur une structure extérieure. Advantageously, at least a portion of the reference electrode is placed in the center of the working electrode. This particular arrangement allows the two electrodes to be very close to each other, this proximity being particularly sought to minimize the ohmic drop, in the case of measurements made in waters with low salt content, and which are therefore little conductive. Indeed, an ohmic drop can be a source of error on the value of the electrical potential actually applied to the working electrode. Preferably, the casing has an external shoulder, making it possible to distinguish a first part of smaller diameter and in which are placed the working electrode, the counter-electrode and a part of the reference electrode, and a second part enclosing the remainder. of the reference electrode. With this type of configuration, the electrodes are grouped at one end of the housing. The portion of the smaller diameter housing and containing most of the electrodes, is intended to be inserted into an outer structure, the shoulder then serving as a stopper during the final fixing of the sensor in said structure. Preferably, the portion of the smaller diameter housing may have an external thread allowing the sensor to be screwed onto an outer structure.
Avantageusement, un capot, en un matériau électriquement isolant tel que du PVC, est placé au niveau de l'extrémité libre de la première partie, entre l'électrode de travail et la contre électrode. Ce capot isolant peut être représenté, soit par une pièce rapportée et fixée au boîtier, soit faire partie intégrante du boîtier en constituant avec celui-ci une seule et même pièce. Advantageously, a cover, made of an electrically insulating material such as PVC, is placed at the free end of the first part, between the working electrode and the counter electrode. This insulating cover can be represented either by an insert and fixed to the housing, or be an integral part of the housing by forming therewith a single piece.
De façon préférentielle, l'électrode de travail possède un canal central dans lequel vient s'insérer une extrémité de l'électrode de référence, ladite extrémité étant obturée par un corps poreux. Ce corps peut par exemple être constitué par une céramique poreuse. Il s'agit d'une configuration particulière, pour laquelle l'électrode de travail et l'électrode de référence sont disposées de façon concentrique, l'une par rapport à l'autre, permettant un gain de place considérable au niveau du capteur. Preferably, the working electrode has a central channel in which is inserted an end of the reference electrode, said end being closed by a porous body. This body may for example be constituted by a porous ceramic. This is a particular configuration, for which the working electrode and the reference electrode are arranged concentrically, relative to each other, allowing a considerable space saving at the sensor.
De façon avantageuse, le corps poreux est de forme cylindrique et est bloqué en translation au moyen de deux butées mécaniques de retenue. Cette disposition permet d'éviter d'avoir à coller le corps poreux dans l'électrode de référence, comme cela est pratiqué couramment dans les configurations déjà existantes. En effet, la colle peut déborder du corps poreux et venir obturer, au moins partiellement le canal de l'électrode de référence. En introduisant deux butées solides, pour bloquer le corps, on s'affranchit ainsi d'un collage aléatoire pouvant dégrader les conditions de fonctionnement du capteur. Selon un mode de réalisation préféré d'un capteur selon l'invention, l'électrode de travail est réalisée en cuivre (i.e. électrode de cuivre). Advantageously, the porous body is of cylindrical shape and is locked in translation by means of two mechanical retaining stops. This arrangement avoids having to stick the porous body in the reference electrode, as is commonly practiced in existing configurations. Indeed, the adhesive may overflow the porous body and come to close, at least partially the channel of the reference electrode. By introducing two solid stops, to block the body, it is thus freed from a random bond that can degrade the operating conditions of the sensor. According to a preferred embodiment of a sensor according to the invention, the working electrode is made of copper (ie copper electrode).
Concernant la contre-électrode et l'électrode de référence, la contre- électrode peut être réalisée en inox, et l'électrode de référence peut être une électrode de type Ag/AgCI, ou une électrode de type calomel (ECS) détaillée dans la suite de la description. With respect to the counter electrode and the reference electrode, the counter-electrode may be made of stainless steel, and the reference electrode may be an Ag / AgCl type electrode or a calomel type electrode (ECS) detailed in FIG. following the description.
Une électrode de type Ag/AgCI est une électrode comportant un fil d'Ag recouvert d'une couche d'AgCI et immergé dans une solution saturée de KCI. On préférera utiliser une électrode de référence ne comprenant pas de mercure. An Ag / AgCl electrode is an electrode comprising an Ag wire covered with a layer of AgCl and immersed in a saturated solution of KCl. It will be preferred to use a reference electrode that does not include mercury.
Les capteurs électrochimiques selon l'invention, possèdent une géométrie simple, leur permettant d'être fabriqués rapidement et facilement sans risque majeur de défauts. Ils ont de plus l'avantage d'être performants pour une utilisation sur le terrain, tout en demeurant d'un encombrement réduit et en possédant une structure résistante. Enfin, ils ont l'avantage de pouvoir être configurés pour coopérer avec une structure extérieure, comme par exemple un dispositif électronique permettant d'appliquer différents potentiels à l'électrode de travail et de mesurer les différents courants traversant l'électrode de travail correspondant à ces potentiels. La présente invention se rapporte également à une méthode de dosage des ions nitrate et/ou nitrite en solution de pH sensiblement neutre. The electrochemical sensors according to the invention have a simple geometry, allowing them to be manufactured quickly and easily without major risk of defects. They also have the advantage of being efficient for use in the field, while remaining of a small footprint and having a resistant structure. Finally, they have the advantage of being able to be configured to cooperate with an external structure, such as for example an electronic device making it possible to apply different potentials to the working electrode and to measure the different currents flowing through the working electrode corresponding to these potentials. The present invention also relates to a method for determining nitrate and / or nitrite ions in a substantially neutral pH solution.
Elle s'applique typiquement, mais non exclusivement, à l'utilisation d'une électrode de cuivre pour le dosage potentiostatique des ions nitrate et/ou nitrite en milieu aqueux de pH sensiblement neutre. De nombreuses méthodes pour doser quantitativement les ions nitrate et/ou nitrite sont connues, notamment à pH fortement acide. On peut citer par exemple le document intitulé « The effect of surface préparation of a copper électrode on the réduction of nitrate ions » de Aljaz Coh et Boris Pihlar (Acta Chimica Slovenia 43/1/1996, pp. 5-12). Ce document présente une méthode de dosage des ions nitrate à l'aide d'une électrode de cuivre. Cette méthode de dosage comprend une première étape consistant à appliquer un potentiel compris entre 0 et 0,2 V vs. Ag/AgCI à l'électrode de cuivre pendant un temps déterminé de sorte à oxyder le cuivre métallique de ladite électrode et à former ainsi une couche d'oxyde de cuivre, et une seconde étape consistant à doser, par réduction, les ions nitrate en effectuant un balayage du potentiel appliqué (dosage dit « en mode potentiodynamique ») à l'électrode de cuivre, les première et seconde étapes étant effectuées en milieu fortement acide dans une solution du type 0,1 M HCIO4/0,9 M NaCI04. D'ailleurs, même si ce document mentionne qu'une couche d'oxyde de cuivre se forme, il est difficilement envisageable qu'en milieu fortement acide, la couche d'oxyde de cuivre puisse exister. It typically, but not exclusively, applies to the use of a copper electrode for the potentiostatic dosing of nitrate and / or nitrite ions in an aqueous medium of substantially neutral pH. Numerous methods for quantitative determination of nitrate and / or nitrite ions are known, in particular at high acid pH. For example, the document entitled "The effect of surface preparation of a copper electrode on the reduction of nitrate ions" by Aljaz Coh and Boris Pihlar (Acta Chimica Slovenia 43/1/1996, pp. 5-12). This document presents a method for assaying nitrate ions using a copper electrode. This assay method comprises a first step of applying a potential of between 0 and 0.2 V. Ag / AgCl at the copper electrode for a period of time so as to oxidize the metallic copper of said electrode and thereby form a layer of copper oxide, and a second step of dosing, by reduction, the nitrate ions into performing a sweep of the applied potential (so-called "potentiodynamic mode" assay) at the copper electrode, the first and second steps being carried out in a strongly acidic medium in a solution of the type 0.1 M HCIO 4 / 0.9 M NaClO 4 . Moreover, even if this document mentions that a layer of copper oxide is formed, it is difficult to envisage that in strongly acidic medium, the copper oxide layer may exist.
Le tracé de la courbe de l'intensité en fonction du potentiel appliqué à l'électrode de cuivre permet alors d'identifier le pic de courant dû à la réduction des ions nitrate en ions NH4 +, et d'en déduire ainsi la concentration en ions nitrate par étalonnage dans le milieu aqueux acidifié. The plot of the curve of the intensity as a function of the potential applied to the copper electrode then makes it possible to identify the current peak due to the reduction of the nitrate ions to NH 4 + ions, and to deduce from this the concentration in nitrate ions by calibration in the acidified aqueous medium.
En outre, selon cette méthode, il est préférable d'ajouter dans le milieu acide à analyser des ions Cu2+ afin de catalyser la réduction des ions nitrate lors de l'étape de dosage. In addition, according to this method, it is preferable to add in the acid medium to be analyzed Cu 2+ ions in order to catalyze the reduction of nitrate ions during the assay step.
Toutefois, lors de la mise en œuvre de cette méthode de dosage en mode potentiodynamique, c'est-à-dire avec un balayage progressif du potentiel appliqué à l'électrode de cuivre, on observe une diminution progressive du courant mesuré due à un empoisonnement de l'électrode, et il est donc impossible de faire des mesures reproductibles les unes à la suite des autres. De plus, la mesure de la concentration en ions nitrate ne peut être effectuée de façon techniquement simple puisque la variation du potentiel avec une vitesse donnée, en mode potentiodynamique, implique l'utilisation d'appareillages électroniques relativement complexes. En outre, par cette méthode, il n'est pas possible de doser les ions nitrate dans un milieu naturel non modifié car il est nécessaire d'ajouter au milieu à analyser un acide fort, voire des ions Cu2+. However, during the implementation of this method of dosage in potentiodynamic mode, that is to say with a progressive scan of the potential applied to the copper electrode, there is a gradual decrease in the measured current due to poisoning. of the electrode, and it is therefore impossible to make reproducible measurements one after the other. In addition, the measurement of the nitrate ion concentration can not be carried out in a technically simple manner since the variation of the potential with a given speed, in potentiodynamic mode, involves the use of relatively complex electronic equipment. In addition, by this method, it is not possible to assay the nitrate ions in an unmodified natural medium because it is necessary to add to the medium to be analyzed a strong acid, or even Cu 2+ ions.
Enfin, les mesures en milieu acide sont limitées puisqu'elles ne permettent pas d'observer la réduction des ions nitrite. En effet, lorsque le milieu est acidifié, les ions nitrite initialement présents dans le milieu sont sous forme d'acide nitreux (HN02, pKa = 3,3). L'acide nitreux est instable et se dismute alors en nitrates et NO. Finally, measurements in acidic medium are limited since they do not make it possible to observe the reduction of nitrite ions. Indeed, when the medium is acidified, the nitrite ions initially present in the medium are in the form of nitrous acid (HN0 2 , pKa = 3.3). Nitrous acid is unstable and is then disproportionated to nitrates and NO.
Le but de la présente invention est de pallier les inconvénients des techniques de l'art antérieur en proposant notamment une méthode de dosage simple et directe des ions nitrate et/ou nitrite permettant de garantir des mesures fiables, sans qu'il soit nécessaire d'ajouter un quelconque additif dans la solution à analyser, notamment pour en modifier le pH. The object of the present invention is to overcome the drawbacks of the techniques of the prior art by proposing in particular a simple and direct method for the determination of nitrate and / or nitrite ions which makes it possible to guarantee reliable measurements, without it being necessary to add any additive in the solution to be analyzed, in particular to modify the pH.
La présente invention a pour objet une méthode de dosage des ions nitrate et/ou nitrite en solution, utilisant une électrode de cuivre, ladite méthode étant caractérisée en ce qu'elle est réalisée en mode potentiostatique, et qu'elle comprend les étapes de dosage consistant à : a. appliquer un potentiel PA à une électrode de cuivre immergée dans une solution à analyser de pH sensiblement neutre, pour mesurer le courant 10 de réduction du dioxygène susceptible d'être présent dans ladite solution à analyser, puis b. appliquer un potentiel PB à l'électrode de cuivre de sorte à réduire les ions nitrate pour obtenir la mesure du courant II de réduction correspondant à la réduction des ions nitrate en ions nitrite et à la réduction du dioxygène, puis c. optionnellement, appliquer un potentiel PC à l'électrode de cuivre de sorte à réduire les ions nitrate et les ions nitrite pour obtenir la mesure du courant 12 de réduction correspondant à la réduction des ions nitrate et nitrite en ammoniaque et à la réduction du dioxygène. Ainsi, les étapes de dosage a et b permettent de déterminer le courant de réduction des ions nitrate, corrigé du courant 10, et les étapes de dosage a, b et c permettent de déterminer le courant de réduction des ions nitrite, corrigé des courants II et 10. De ce fait, les mesures des courants (i.e. ou intensités de courant), permettant de doser quantitativement les ions nitrate et/ou les ions nitrite, obtenues grâce à cette méthode sont fiables : il n'y a pas de surévaluation de la teneur en ions nitrate et en ions nitrite. The subject of the present invention is a method for the determination of nitrate and / or nitrite ions in solution, using a copper electrode, said method being characterized in that it is carried out in potentiostatic mode, and that it comprises the dosage steps. consisting of: a. applying a PA potential to a submerged copper electrode in a substantially neutral pH analyte solution, for measuring the oxygen reducing current likely to be present in said solution to be analyzed, then b. applying a PB potential to the copper electrode so as to reduce the nitrate ions to obtain the measurement of the reduction current II corresponding to the reduction of the nitrate ions to nitrite ions and the reduction of the oxygen, then c. optionally, applying a PC potential to the copper electrode so as to reduce nitrate ions and nitrite ions to obtain the measurement of the reduction current 12 corresponding to the reduction of nitrate and nitrite ions to ammonia and the reduction of oxygen. Thus, the dosing steps a and b make it possible to determine the nitrate reduction current, corrected for the stream 10, and the a, b and c determination steps make it possible to determine the nitrite ion reduction current corrected for the currents II. and 10. As a result, the measurements of the currents (ie, or current intensities), making it possible to quantitatively determine the nitrate ions and / or the nitrite ions, obtained by this method are reliable: there is no overvaluation of the content of nitrate ions and nitrite ions.
En effet, le dioxygène susceptible d'être présent dans la solution à analyser tend à fausser le dosage des ions nitrate et/ou nitrite, en surévaluant les résultats du dosage. L'application de ce potentiel PA permet avantageusement de proposer une méthode de dosage des ions nitrate et/ou nitrite garantissant de façon optimale des mesures reproductibles et fiables. Indeed, the oxygen likely to be present in the solution to be analyzed tends to distort the determination of the nitrate and / or nitrite ions, by overestimating the results of the assay. The application of this PA potential advantageously makes it possible to propose a nitrate and / or nitrite ion assay method which optimally guarantees reproducible and reliable measurements.
En outre, le potentiel PA appliqué à l'étape a de dosage permet avantageusement d'éviter de désaérer la solution à analyser. In addition, the potential PA applied to the step of assay advantageously makes it possible to avoid deaerating the solution to be analyzed.
On entend par l'expression « électrode de cuivre » une électrode de cuivre dite « non alliée », constituée uniquement de cuivre, le cuivre pouvant être massif ou électro-déposé. Bien entendu, l'électrode de cuivre peut contenir des impuretés inévitables comme par exemple d'autres éléments métalliques. Dans ce cas, l'électrode de cuivre est composée d'au moins 99,9% en poids de cuivre, le reste étant lesdites impuretés inévitables. The term "copper electrode" means a copper electrode called "unalloyed", consisting solely of copper, the copper may be solid or electro-deposited. Of course, the copper electrode may contain unavoidable impurities such as other metal elements. In this case, the copper electrode is composed of at least 99.9% by weight of copper, the remainder being said inevitable impurities.
On entend par les termes « pH sensiblement neutre », une solution (ou un électrolyte) dont le pH est proche de la neutralité, c'est-à-dire pouvant aller de 5 à 9, et de préférence environ égal à 7. A titre d'exemple, la solution à analyser peut être un milieu aqueux tel qu'une eau naturelle ou une eau usée. The term "substantially neutral pH" means a solution (or an electrolyte) whose pH is close to neutrality, that is to say ranging from 5 to 9, and preferably about 7. For example, the solution to be analyzed may be an aqueous medium such as natural water or wastewater.
Les étapes a, b et optionnellement c de dosage sont réalisées « en mode potentiostatique ». Ce type de dosage est bien connu et consiste à mesurer l'intensité du courant à un potentiel donné (i.e. potentiel fixe). L'intensité de courant est, dans la présente invention, fonction de la concentration en dioxygène et en ions nitrate et/ou nitrite. Steps a, b and optionally c assay are performed "in potentiostatic mode". This type of assay is well known and consists in measuring the intensity of the current at a given potential (ie fixed potential). Current intensity is, in the present invention, a function of the concentration of dioxygen and nitrate and / or nitrite ions.
Les étapes a, b et optionnellement c étant des étapes de réduction afin de doser les ions nitrite et/ou nitrate (i.e. détection quantitative), les potentiels appliquées à chacune de ces étapes sont de type cathodique. Since steps a, b and optionally c are reduction steps in order to determine the nitrite and / or nitrate ions (i.e., quantitative detection), the potentials applied to each of these steps are of the cathode type.
Le mode potentiostatique utilisé dans les étapes a à c permet avantageusement d'appliquer un potentiel constant pendant un temps déterminé, ce qui est techniquement beaucoup plus simple que de faire varier ce potentiel avec une vitesse donnée tel qu'en mode potentiodynamique. Le dosage potentiostatique selon l'invention est donc bien différent du dosage potentiodynamique (voltamétrie cyclique) dans lequel le potentiel appliqué à l'électrode de cuivre varie en fonction du temps (balayage en potentiel). The potentiostatic mode used in steps a to c advantageously makes it possible to apply a constant potential for a given time, which is technically much simpler than to vary this potential with a given speed such as in potentiodynamic mode. The potentiostatic dosage according to the invention is therefore very different from the potentiodynamic dosage (cyclic voltammetry) in which the potential applied to the copper electrode varies as a function of time (potential sweep).
Dans un mode de réalisation particulier selon l'étape a, le potentiel PA appliqué à l'électrode de cuivre permet de mesurer le courant 10 de réduction du dioxygène en ions OH" selon l'équation 1 (Eq l) suivante : In a particular embodiment according to step a, the potential PA applied to the copper electrode makes it possible to measure the current of reduction of the oxygen into OH " ions according to the following equation 1 (Eq 1):
02 + 4e" + H20→ 4 OH" (Eq l) 0 2 + 4th " + H 2 0 → 4 OH " (Eq l)
A titre d'exemple, le potentiel PA peut être égal à environ -0,6 V/ECS. Les potentiels décrits dans la présente invention sont typiquement exprimés par rapport à une électrode de référence du type électrode au calomel saturée (i.e. électrode au calomel en solution saturée de KCI), notée dans la présente description « ECS ». By way of example, the potential PA may be equal to about -0.6 V / SCE. The potentials described in the present invention are typically expressed with respect to a saturated calomel electrode type reference electrode (i.e. KCI saturated calomel electrode), noted in the present description "ECS".
Idéalement, la durée de l'application (ou de l'imposition) du potentiel PA peut être d'au moins de quelques secondes, et par exemple comprise entre 20 et 30 secondes. Ideally, the duration of the application (or imposition) of the potential PA may be at least a few seconds, for example between 20 and 30 seconds.
Dans un mode de réalisation particulier selon l'étape b, le potentiel PB appliqué à l'électrode de cuivre permet de réduire les ions nitrate en ions nitrite selon l'équation 2 (Eq2) suivante : N03 " + 2e" + H20→ N02 " + 2 OH" (Eq2) In a particular embodiment according to step b, the potential PB applied to the copper electrode makes it possible to reduce the nitrate ions to nitrite ions according to the following equation 2 (Eq 2): N0 3 " + 2e " + H 2 0 → N0 2 " + 2 OH " (Eq2)
L'application du potentiel PB permet d'obtenir la mesure du courant II de réduction correspondant à la réduction des ions nitrate (Eq2) et du dioxygène (Eq l). A titre d'exemple, le potentiel PB est égal à environ -0,9 V/ECS. The application of the potential PB makes it possible to obtain the measurement of the reduction current II corresponding to the reduction of nitrate (Eq 2) and dioxygen (Eq 1) ions. By way of example, the potential PB is equal to approximately -0.9 V / SCE.
Idéalement, la durée de l'application (ou de l'imposition) du potentiel PB peut être d'au moins de quelques secondes, et par exemple comprise entre 20 et 30 secondes. Ideally, the duration of the application (or imposition) of the potential PB may be at least a few seconds, and for example between 20 and 30 seconds.
Dans un mode de réalisation particulier selon l'étape c, le potentiel PC appliqué à l'électrode de cuivre permet de réduire les ions nitrate et les ions nitrite selon les équations 3 et 4 (Eq3 et Eq4) suivantes : In a particular embodiment according to step c, the potential PC applied to the copper electrode makes it possible to reduce the nitrate ions and the nitrite ions according to equations 3 and 4 (Eq 3 and Eq 4) as follows:
N03 " + 8 e" + 6 H20→ NH3 + 9 OH" (Eq3) N0 3 " + 8 e " + 6 H 2 0 → NH 3 + 9 OH " (Eq 3)
N02 " + 6 e" + 5 H20→ NH3 + 7 OH" (Eq4) N0 2 " + 6 e " + 5 H 2 0 → NH 3 + 7 OH " (Eq 4)
L'application du potentiel PC permet d'obtenir la mesure du courant 12 de réduction correspondant à la réduction des ions nitrate (Eq3), des ions nitrite (Eq4) et du dioxygène (Eq l). The application of the potential PC makes it possible to obtain the measurement of the reduction current 12 corresponding to the reduction of nitrate ions (Eq 3), nitrite ions (Eq 4) and oxygen (Eq 1).
A titre d'exemple, le potentiel PC est égal à environ -1,1 V/ECS. By way of example, the potential PC is equal to approximately -1.1 V / SCE.
Idéalement, la durée de l'application (ou de l'imposition) du potentiel PC peut être d'au moins de quelques secondes, et par exemple comprise entre 20 et 30 secondes. Ideally, the duration of the application (or imposition) of the potential PC may be at least a few seconds, and for example between 20 and 30 seconds.
La teneur en ions nitrate et/ou en ions nitrite peut être ainsi obtenue facilement et de façon fiable en tenant compte du courant (10) de réduction du dioxygène. The content of nitrate ions and / or nitrite ions can thus be easily and reliably obtained by taking into account the current (10) of reduction of oxygen.
La méthode de dosage selon l'invention est de préférence réalisée avec une vitesse d'écoulement de la solution à analyser qui est sensiblement constante. En effet, la réduction des ions nitrate et/ou nitrite étant un phénomène limité notamment par la diffusion des ions nitrate et/ou nitrite de la solution à analyser vers l'électrode de cuivre, une solution à analyser dont la vitesse d'écoulement ne serait pas constante pourrait engendrer des erreurs sur les mesures de la teneur en ions nitrate et/ou nitrite. The assay method according to the invention is preferably carried out with a flow rate of the analyte solution which is substantially constant. Indeed, the reduction of nitrate and / or nitrite ions being a phenomenon limited in particular by the diffusion of the nitrate and / or nitrite ions of the solution to be analyzed to the copper electrode, an analyte solution whose flow rate does not would not be constant could lead to errors in measurements of nitrate and / or nitrite ion content.
A titre d'exemple, la vitesse d'écoulement de la solution à analyser peut être nulle ou peut être différente de zéro si elle ne varie pas dans le temps. By way of example, the flow velocity of the solution to be analyzed may be zero or may be different from zero if it does not vary with time.
Par ailleurs, il est préférable que la solution à analyser, lors des étapes de dosage, soit renouvelée après chaque étape b lorsque le dosage consiste à mesurer uniquement les courants 10 et II, ou après chaque étape c lorsque le dosage consiste à mesurer les courants 10, Il et 12, afin de pouvoir garantir dans le temps la reproductibilité des mesures réalisées avec l'électrode de cuivre. En effet, les différentes réactions de réduction comprises dans les étapes a, b et c, tendent à augmenter le pH de la solution à analyser. Par exemple, des espèces chimiques du type calcium et/ou magnésium susceptibles de se trouver dans la solution à analyser peuvent former des dépôts calco-magnésiens à la surface de l'électrode de cuivre et ainsi limiter l'utilisation de l'électrode de cuivre et la reproductibilité des mesures effectuées avec ladite électrode. On peut également citer la présence d'ammoniaque après l'étape de réduction des ions nitrate et nitrite (étape b et c), qui classiquement peut réagir avec la surface de l'électrode de cuivre et ainsi fausser la reproductibilité desdites mesures. En outre, dans un mode de réalisation bien particulier, la méthode de dosage de l'invention peut comprendre en outre une étape d faisant suite à l'étape c de dosage, ladite étape d consistant à maintenir l'électrode de cuivre à un potentiel PD, ledit potentiel PD étant inférieur au potentiel de repos de l'électrode de cuivre, de sorte à éviter à l'électrode de cuivre de s'oxyder lorsque cette dernière n'est plus utilisée pour ledit dosage pendant une période donnée. Le potentiel PD permet avantageusement d'éviter au cuivre de s'oxyder (i.e. se corroder), ce qui pourrait conduire à la formation d'un film important d'oxyde de cuivre à la surface de l'électrode de cuivre, empêchant ainsi d'obtenir des mesures reproductibles. Le potentiel de repos de l'électrode de cuivre (ou potentiel de circuit ouvert) peut être par exemple environ égal à -0,10 V/ECS. Furthermore, it is preferable that the solution to be analyzed, during the determination steps, is renewed after each step b when the assay consists of measuring only the currents 10 and II, or after each step c when the assay consists of measuring the currents. 10, 11 and 12, in order to guarantee in time the reproducibility of the measurements made with the copper electrode. Indeed, the various reduction reactions included in steps a, b and c tend to increase the pH of the solution to be analyzed. For example, chemical species of calcium and / or magnesium type likely to be in the solution to be analyzed may form calcium-magnesium deposits on the surface of the copper electrode and thus limit the use of the copper electrode and the reproducibility of measurements made with said electrode. It is also possible to mention the presence of ammonia after the step of reducing nitrate and nitrite ions (step b and c), which classically can react with the surface of the copper electrode and thus distort the reproducibility of said measurements. In addition, in a very particular embodiment, the assay method of the invention may further comprise a step d following the dosage step c, said step of maintaining the copper electrode at a potential PD, said PD potential being lower than the quiescent potential of the copper electrode, so as to avoid the copper electrode to oxidize when the latter is no longer used for said assay for a given period. The PD potential advantageously prevents the copper from oxidizing (ie corroding), which could lead to the formation of a large film of copper oxide on the surface of the copper electrode, thus preventing to obtain reproducible measurements. The resting potential of the copper electrode (or open circuit potential) can be for example approximately equal to -0.10 V / SCE.
La méthode selon la présente invention peut comprendre en outre des étapes de traitement de l'électrode de cuivre, préalables aux étapes de dosage a et b, ou aux étapes de dosage a, b et c, consistant à : i. appliquer un premier potentiel à l'électrode de cuivre de sorte à réduire les oxydes de cuivre présents à la surface de l'électrode en cuivre métallique, ii. appliquer un second potentiel à l'électrode de cuivre de sorte à oxyder le cuivre métallique formé à l'étape i en ions cuivriques, des oxydes de cuivre pouvant éventuellement se former lors de l'étape ii, et iii. appliquer un troisième potentiel à l'électrode de cuivre de sorte à réduire les oxydes de cuivre éventuellement formés à l'étape ii, les étapes i à iii étant réalisées par immersion de l'électrode de cuivre dans un électrolyte support ayant un pH sensiblement neutre. L'électrolyte support des étapes i à iii est de préférence un électrolyte support aqueux. The method according to the present invention may further comprise copper electrode treatment steps, prior to assay steps a and b, or assay steps a, b and c, consisting of: i. applying a first potential to the copper electrode so as to reduce the copper oxides present on the surface of the copper metal electrode, ii. applying a second potential to the copper electrode so as to oxidize the metallic copper formed in step i to cupric ions, copper oxides possibly forming in step ii, and iii. applying a third potential to the copper electrode so as to reduce the copper oxides possibly formed in step ii, steps i to iii being performed by immersing the copper electrode in a supporting electrolyte having a substantially neutral pH . The electrolyte supporting steps i to iii is preferably an aqueous carrier electrolyte.
De façon particulièrement préférée, l'électrolyte support est la solution à analyser lors des étapes de dosage a et b, ou des étapes de dosage a, b et c. Dans ce cas, on parle de conditionnement et de dosage réalisés in situ, au sein de la même solution de pH sensiblement neutre. In a particularly preferred manner, the carrier electrolyte is the solution to be analyzed during assaying steps a and b, or assaying steps a, b and c. In this case, we speak of conditioning and dosing carried out in situ, within the same substantially neutral pH solution.
Les étapes i à iii sont bien entendu réalisées « en mode potentiostatique » (ou « en mode ampérométrique »). Les étapes i à iii sont dites « étapes de conditionnement » de l'électrode, préalables aux étapes de dosage a et b, ou aux étapes de dosage a, b et c, proprement dites. Ces étapes de conditionnement, consistant à appliquer à l'électrode de cuivre une séquence de trois potentiels successifs en mode potentiostatique, permettent avantageusement d'obtenir une surface de l'électrode constituée essentiellement de cuivre à l'état d'oxydation 0 et donc exempte d'oxyde de cuivre, afin d'avoir une surface active constante et reproductible. Steps i to iii are of course performed "in potentiostatic mode" (or "in amperometric mode"). Steps i to iii are referred to as "conditioning steps" of the electrode, prior to dosing steps a and b, or to dosing steps a, b and c, per se. These conditioning steps, consisting in applying to the copper electrode a sequence of three successive potentials in potentiostatic mode, advantageously make it possible to obtain a surface of the electrode consisting essentially of copper in the oxidation state 0 and therefore free of of copper oxide, in order to have a constant and reproducible active surface.
Grâce à l'invention, la concentration en ions nitrate et/ou nitrite présente dans la solution à analyser est proportionnelle aux intensités mesurées aux potentiels PB et PC appliqués à l'électrode de cuivre conditionnée. Ce dosage potentiostatique permet ainsi d'obtenir facilement la concentration en ions nitrate et/ou nitrite avec une fiabilité optimale. Thanks to the invention, the concentration of nitrate ions and / or nitrite present in the solution to be analyzed is proportional to the intensities measured at the potentials PB and PC applied to the conditioned copper electrode. This potentiostatic dosage thus makes it possible to easily obtain the concentration of nitrate and / or nitrite ions with optimum reliability.
Dans la méthode de dosage de l'invention, le premier potentiel (étape i) permet de réduire les oxydes de cuivre du type CuO et/ou Cu20 en cuivre métallique, lesdits oxydes de cuivre provenant notamment de l'oxydation du cuivre métallique par l'oxygène de l'air. Le premier potentiel est donc de type cathodique, il doit permettre la réduction de tous les oxydes de cuivre. Typiquement, il peut être inférieur ou égal à -0,85 V/ECS, et de préférence égal à -1,15 V/ECS. Idéalement, la durée d'application du premier potentiel peut être d'au moins de quelques secondes, et par exemple comprise entre 20 et 30 secondes. In the assay method of the invention, the first potential (step i) makes it possible to reduce copper oxides of the CuO and / or Cu 2 0 type to metallic copper, said copper oxides notably originating from the oxidation of metallic copper. by the oxygen of the air. The first potential is cathodic type, it must allow the reduction of all copper oxides. Typically, it may be less than or equal to -0.85 V / SCE, and preferably equal to -1.15 V / SCE. Ideally, the duration of application of the first potential may be at least a few seconds, and for example between 20 and 30 seconds.
Les demi-équations électrochimiques de la réduction des oxydes de cuivre sont les suivantes : The electrochemical half-equations for the reduction of copper oxides are as follows:
CuO + 2 e" + H20→ Cu° + 2 OH" CuO + 2 e " + H 2 0 → Cu ° + 2 OH "
Cu20 + 2 e" + H20→ 2 Cu° + 2 OH" Le second potentiel (étape ii) permet d'oxyder le cuivre métallique formé à l'étape i en ions cuivriques (Cu2+) . Le second potentiel est donc de type anodique. Cu 2 0 + 2 e " + H 2 0 → 2 Cu + 2 OH " The second potential (step ii) makes it possible to oxidize the metallic copper formed in step i into cupric ions (Cu 2+ ). The second potential is therefore anodic type.
Typiquement, le second potentiel peut être supérieur au potentiel de repos de l'électrode de cuivre, notamment supérieur de 5 à 200 mV par rapport audit potentiel de repos, et de préférence supérieur de 5 à 100 mV par rapport audit potentiel de repos. Typically, the second potential may be greater than the quiescent potential of the copper electrode, in particular greater than 5 to 200 mV with respect to said quiescent potential, and preferably greater than 5 to 100 mV with respect to said quiescent potential.
Un second potentiel supérieur de plus de 200 mV par rapport au potentiel de repos ne serait pas nécessairement acceptable du fait qu'il pourrait induire une dissolution trop importante du cuivre métallique et de ce fait la surface active de l'électrode deviendrait inhomogène comme expliqué ci-après. A second higher potential of more than 200 mV with respect to the resting potential would not necessarily be acceptable because it could induce an excessive dissolution of the metallic copper and thus the active surface of the electrode would become inhomogeneous as explained herein. -after.
Le potentiel de repos, mentionné dans la présente invention, correspond au potentiel de circuit ouvert ou, en d'autres termes, au potentiel « naturel » mesuré quand l'électrode de cuivre n'est soumise à l'application d'aucun potentiel. Ce potentiel de repos est donc classiquement mesuré entre l'électrode de cuivre et une électrode de référence, par exemple de type ECS, sans qu'aucun potentiel ne soit appliqué entre ces deux électrodes. A titre d'exemple, le potentiel de repos de l'électrode de cuivre peut être environ égal à -0,10 V/ECS. The quiescent potential, mentioned in the present invention, corresponds to the open circuit potential or, in other words, to the "natural" potential measured when the copper electrode is not subjected to the application of any potential. This quiescent potential is therefore conventionally measured between the copper electrode and a reference electrode, for example of the ECS type, without any potential being applied between these two electrodes. By way of example, the resting potential of the copper electrode may be approximately equal to -0.10 V / SCE.
Plus particulièrement, lorsque le potentiel de circuit ouvert est de -0,100 V/ECS, le second potentiel peut être supérieur ou égal à -0,095 V/ECS et inférieur ou égal à 0,100 V/ECS, et de préférence inférieur ou égal à 0,000 V/ECS. Idéalement, la durée d'application du second potentiel peut être d'au moins de quelques secondes, et par exemple comprise entre 20 et 30 secondes. More particularly, when the open circuit potential is -0.100 V / SCE, the second potential may be greater than or equal to -0.095 V / SCE and less than or equal to 0.100 V / SCE, and preferably less than or equal to 0.000 V / SCE. Ideally, the duration of application of the second potential may be at least a few seconds, for example between 20 and 30 seconds.
La demi-équation électrochimique de l'oxydation du cuivre métallique est la suivante : Cu°→ Cu2+ + 2 e" The electrochemical half-equation of the oxidation of metallic copper is as follows: Cu ° → Cu 2+ + 2 e "
Lors de l'application du second potentiel, des oxydes de cuivre, tels que par exemple du CuO et/ou du Cu20, peuvent éventuellement se former. When applying the second potential, copper oxides, such as, for example, CuO and / or Cu 2 O, may optionally be formed.
La réduction des oxydes de cuivre à l'étape i peut conduire à la formation de cuivre divisé (ou cuivre non compact) sur la surface de l'électrode de cuivre, ce qui entraîne une inhomogénéité de surface (rugosité de surface) et une augmentation de la surface active de ladite électrode. De ce fait, l'étape ii permet de garantir des mesures reproductibles les unes après les autres, garantissant une surface active qui reste constante et homogène (surface lisse) au fur et à mesure des dosages effectués. The reduction of copper oxides in step i can lead to the formation of split copper (or noncompact copper) on the surface of the copper electrode, resulting in surface inhomogeneity (surface roughness) and increasing the active surface of said electrode. Because of this, step ii makes it possible to guarantee reproducible measurements one after the other, guaranteeing an active surface that remains constant and homogeneous (smooth surface) as and when dosages are carried out.
Le troisième potentiel (étape iii) permet, quant à lui, de réduire les oxydes de cuivre susceptibles de s'être formés lors de l'étape ii. Le troisième potentiel est donc de type cathodique. Il peut être inférieur ou égal à -0,85 V/ECS, et de préférence égal à -1,15 V/ECS. Idéalement, la durée d'application du troisième potentiel peut être d'au moins de quelques secondes, et par exemple comprise entre 20 et 30 secondes. The third potential (step iii) makes it possible, in turn, to reduce the copper oxides that may have formed during step ii. The third potential is therefore cathodic type. It may be less than or equal to -0.85 V / SCE, and preferably equal to -1.15 V / SCE. Ideally, the duration of application of the third potential may be at least a few seconds, and for example between 20 and 30 seconds.
Ainsi, la combinaison des étapes i, ii et iii permet d'obtenir une électrode de cuivre dont la surface a été conditionnée et qui permet alors, lors du dosage des ions nitrate et/ou nitrite des étapes a, b et c, de mesurer un courant de réduction reproductible et proportionnel à la concentration en ions nitrate et/ou nitrite dans la solution à analyser. Thus, the combination of steps i, ii and iii makes it possible to obtain a copper electrode whose surface has been conditioned and which then makes it possible, during the determination of the nitrate and / or nitrite ions, of steps a, b and c, to measure a reduction current that is reproducible and proportional to the concentration of nitrate and / or nitrite ions in the solution to be analyzed.
Dans un mode de réalisation particulièrement préféré, les étapes i à iii, préalables aux étapes a et b, ou aux étapes a, b et c, sont réitérées avant chaque étape a, permettant ainsi de réaliser des dosages en continu tout en assurant la reproductibilité des mesures et en maintenant constantes les performances de l'électrode de cuivre. Bien entendu, l'homme du métier comprendra aisément que toutes les étapes de la méthode de dosage selon l'invention ne peuvent être effectuées qu'en présence, en plus de l'électrode de cuivre (i.e. électrode de travail), d'une électrode de référence, par exemple de type électrode au calomel en solution saturée de KCI (ECS) ou de typeIn a particularly preferred embodiment, steps i to iii, prior to steps a and b, or steps a, b and c, are repeated before each step a, thus making it possible to perform continuous dosages while ensuring reproducibility. measurements and keeping the performance of the copper electrode constant. Of course, those skilled in the art will readily understand that all the steps of the assay method according to the invention can be carried out only in the presence, in addition to the copper electrode (ie working electrode), of a reference electrode, for example of the calomel electrode type in saturated solution of KCI (ECS) or of type
Ag/AgCI en solution saturée de KCI, et d'une contre-électrode, par exemple en platine ou en acier inoxydable (e.g . inox), formant ensemble un capteur électrochimique, le capteur électrochimique pouvant avantageusement être le capteur électrochimique à géométrie compacte tel que défini dans la présente invention. Ag / AgCl in saturated solution of KCl, and a counter-electrode, for example in platinum or stainless steel (eg stainless steel), together forming an electrochemical sensor, the electrochemical sensor can advantageously be the electrochemical sensor with compact geometry such as as defined in the present invention.
Le capteur électrochimique peut comprendre en outre un dispositif électronique permettant d'appliquer les différents potentiels à l'électrode de cuivre et de mesurer les différents courants (i.e. intensités de courant) traversant l'électrode de cuivre correspondants à ces potentiels. Il peut en outre comprendre une sonde de température. The electrochemical sensor may further comprise an electronic device for applying the different potentials to the copper electrode and for measuring the different currents (i.e. current intensities) passing through the copper electrode corresponding to these potentials. It may further include a temperature probe.
Un autre objet de l'invention concerne une électrode de cuivre conditionnée par les étapes i à iii de la méthode de dosage telle que définie dans la présente invention. Ladite électrode de cuivre est caractérisée en ce que sa surface est essentiellement constituée de cuivre à l'état d'oxydation 0 (Cu(0)), c'est-à-dire de cuivre sous forme métallique. Another object of the invention relates to a copper electrode conditioned by steps i to iii of the assay method as defined in the present invention. Said copper electrode is characterized in that its surface consists essentially of copper in the oxidation state 0 (Cu (0)), that is to say copper in metallic form.
On entend par le terme « essentiellement » une teneur en pourcentage atomique (%at.) de cuivre à l'état d'oxydation 0 qui est supérieure à 80 %at., et de préférence supérieure à 90% at., par rapport à la somme des teneurs en %at. de l'ensemble des constituants comprenant au moins un élément métallique, présent à la surface de l'électrode de cuivre. L'ensemble desdits constituants comprend bien entendu du cuivre sous forme métallique (Cu(0)), et peut en outre comprendre du cuivre à l'état d'oxydation +1 (Cu(I)) et/ou du cuivre à l'état d'oxydation +11 (Cu(II)). Plus particulièrement, la surface de l'électrode de cuivre comprend moins de 10 %at. d'oxyde(s) de cuivre, de préférence moins de 5 % at. d'oxyde(s) de cuivre. The term "substantially" is understood to mean an atomic percentage (% at.) Content of copper in oxidation state 0 which is greater than 80 at%, and preferably greater than 90 at%, with respect to the sum of the contents in% at. of all the constituents comprising at least one metal element, present on the surface of the copper electrode. All of said constituents, of course, comprise copper in metallic form (Cu (O)), and may furthermore comprise copper in oxidation state +1 (Cu (I)) and / or copper in the form of oxidation state +11 (Cu (II)). More particularly, the surface of the copper electrode comprises less than 10 at%. copper oxide (s), preferably less than 5 at%. of copper oxide (s).
Dans un mode de réalisation préféré, la surface de l'électrode de cuivre ne comprend pas d'oxyde de cuivre, et de façon particulièrement préférée la surface de l'électrode de cuivre n'est constituée que de cuivre à l'état d'oxydation 0. In a preferred embodiment, the surface of the copper electrode does not comprise copper oxide, and particularly preferably the surface of the copper electrode consists only of copper in the state of copper. oxidation 0.
Un autre objet de l'invention concerne l'utilisation d'une telle électrode de cuivre conditionnée, pour le dosage des ions nitrate et/ou nitrite en solution de pH sensiblement neutre, ledit dosage étant effectué en mode potentiostatique. Another object of the invention relates to the use of such a conditioned copper electrode, for the determination of nitrate and / or nitrite ions in substantially neutral pH solution, said assay being carried out in potentiostatic mode.
Un autre objet de l'invention concerne le capteur électrochimique selon la présente invention, à savoir le capteur électrochimique à géométrie compacte, comprenant une électrode de cuivre en tant qu'électrode de travail, ladite électrode de cuivre étant conditionnée par les étapes i à iii de la méthode de dosage telle que définie dans la présente invention. Another object of the invention relates to the electrochemical sensor according to the present invention, namely the compact geometry electrochemical sensor, comprising a copper electrode as a working electrode, said copper electrode being conditioned by steps i to iii of the assay method as defined in the present invention.
Un autre objet de l'invention concerne l'utilisation d'un tel capteur électrochimique comprenant ladite électrode de cuivre conditionnée, pour le dosage des ions nitrate et/ou nitrite en solution de pH sensiblement neutre, ledit dosage étant effectué en mode potentiostatique. Another object of the invention relates to the use of such an electrochemical sensor comprising said conditioned copper electrode, for the determination of nitrate and / or nitrite ions in a substantially neutral pH solution, said assay being carried out in potentiostatic mode.
Un autre objet de l'invention concerne la méthode de dosage des ions nitrate et/ou nitrite en solution de l'invention, utilisant le capteur électrochimique (à géométrie compacte) de l'invention, lorsque l'électrode de travail dudit capteur électrochimique est une électrode de cuivre, l'électrode de cuivre pouvant avoir subi ou non les étapes de conditionnement i à iii. Another subject of the invention concerns the method for determining the nitrate and / or nitrite ions in solution of the invention, using the electrochemical sensor (compact geometry) of the invention, when the working electrode of said electrochemical sensor is a copper electrode, the copper electrode possibly having undergone the conditioning steps i to iii.
Grâce à la distance minimisée entre l'électrode de travail et l'électrode de référence, ledit capteur électrochimique à géométrie compacte permet avantageusement de pouvoir limiter de façon significative la chute ohmique afin de garantir un potentiel à l'électrode de cuivre le plus proche de celui appliqué, notamment dans les solutions à analyser peu chargées en sels ou peu conductrices. La chute ohmique (RI) est le produit du courant traversant l'électrode de cuivre par la résistance ohmique de la solution à analyser (i.e. électrolyte). Thanks to the minimized distance between the working electrode and the reference electrode, the electrochemical sensor with compact geometry advantageously makes it possible to be able to significantly limit the ohmic drop in order to guarantee a potential at the copper electrode closest to the one applied, in particular in the solutions to be analyzed which are not heavily loaded with salts or not very conductive. The ohmic drop (RI) is the product of the current flowing through the copper electrode by the ohmic resistance of the solution to be analyzed (ie electrolyte).
D'autres caractéristiques et avantages de la présente invention apparaîtront à la lumière de la description détaillée ci-après d'un mode de réalisation préféré d'un capteur électrochimique en se référant à la figure 1, et d'un dosage des ions nitrate et nitrite selon la méthode de l'invention. Other features and advantages of the present invention will emerge in the light of the following detailed description of a preferred embodiment of an electrochemical sensor with reference to FIG. 1, and a determination of nitrate ions and nitrite according to the method of the invention.
- La figure 1 représente une vue en coupe axiale longitudinale d'un capteur électrochimique selon l'invention. - Figure 1 shows a longitudinal axial sectional view of an electrochemical sensor according to the invention.
Le capteur électrochimique à géométrie compacte Electrochemical sensor with compact geometry
En se référant à la figure 1, un capteur électrochimique 1 selon l'invention, conçu pour le dosage in situ et à fréquence élevée des ions nitrate et/ou des ions nitrite notamment dans les milieux naturels, tels que par exemple des rivières, comprend un boîtier 2 extérieur cylindrique dans lequel sont disposées une électrode de travail 3 en cuivre, une contre-électrode 4 en inox, et une électrode de référence 5 de type Ag/AgCI, constituée par un fil d'Ag recouvert par une couche d'AgCI et immergé dans une solution saturée de KO. Le boîtier 2 est une pièce creuse de forme cylindrique, et comprend un épaulement externe 6 permettant de distinguer une première partie 7 et une deuxième partie 8, le diamètre de ladite première partie 7 étant inférieur à celui de la deuxième partie 8. Referring to FIG. 1, an electrochemical sensor 1 according to the invention, designed for the in situ and high frequency assay of nitrate ions and / or nitrite ions, especially in natural environments, such as for example rivers, comprises a cylindrical outer housing 2 in which a working electrode 3 made of copper, a counter-electrode 4 made of stainless steel, and a reference electrode 5 of the Ag / AgCl type, constituted by an Ag wire covered by a layer of aluminum, are disposed. AgCl and immersed in a saturated solution of KO. The housing 2 is a hollow part of cylindrical shape, and comprises an outer shoulder 6 for distinguishing a first portion 7 and a second portion 8, the diameter of said first portion 7 being smaller than that of the second portion 8.
Les canaux internes de ces deux parties 7,8 se retrouvent en continuité l'un de l'autre, le canal interne de la première partie 7 ayant un diamètre inférieur à celui du canal interne de la deuxième partie. The internal channels of these two parts 7,8 are in continuity with one another, the internal channel of the first portion 7 having a smaller diameter than the internal channel of the second part.
L'électrode de référence 5 est délimitée par un tube 13 en verre, placé dans un corps cylindrique 9, réalisé en matière plastique, et présentant un épaulement externe 10 permettant de distinguer une première partie 11 et une deuxième partie 12, le diamètre de la première partie 11 étant inférieur à celui de la deuxième partie 12. The reference electrode 5 is delimited by a glass tube 13, placed in a cylindrical body 9, made of plastic material, and having an external shoulder 10 making it possible to distinguish a first part 11 and a second portion 12, the diameter of the first portion 11 being smaller than that of the second portion 12.
Le tube 13, qui contient le fil en argent et la solution de KCI, est fixé à l'intérieur de ce corps creux 9, au moyen d'une fixation impliquant un bouchon 14 en silicone de type Versilic™, et une bague 25 en silicone également de type Versilic™. The tube 13, which contains the silver wire and the KCl solution, is fixed inside this hollow body 9, by means of a fastener involving a Versilic ™ silicone plug 14, and a ring 25 silicone also Versilic ™ type.
L'intérieur du corps creux 9 est rempli de gel 15, et le tube 13 se retrouve immergé dans ledit gel 15, de manière à ce que son axe longitudinal soit parallèle à l'axe de révolution du corps creux 13. L'électrode de travail 3 se présente sous la forme d'une pièce cylindrique creuse, placée au niveau de l'extrémité libre de la première partie 7 du boitier 2, de sorte que son axe de révolution soit confondu avec l'axe de révolution dudit boitier 2. The interior of the hollow body 9 is filled with gel 15, and the tube 13 is immersed in said gel 15, so that its longitudinal axis is parallel to the axis of revolution of the hollow body 13. work 3 is in the form of a hollow cylindrical part, placed at the free end of the first part 7 of the housing 2, so that its axis of revolution coincides with the axis of revolution of said housing 2.
De façon plus précise, le positionnement de l'électrode de travail 3 au niveau de cette extrémité est tel, qu'une surface annulaire plane 18 de ladite électrode de travail 3 affleure l'extrémité libre de la première partie 7 du boitier 2, ladite électrode étant logée à l'intérieur de ladite première partie 7. More specifically, the positioning of the working electrode 3 at this end is such that a flat annular surface 18 of said working electrode 3 is flush with the free end of the first portion 7 of the housing 2, said electrode being housed inside said first part 7.
La contre-électrode 4 en inox est une pièce cylindrique creuse, insérée dans la paroi de la première partie 7 du boitier 2, l'épaisseur maximale de ladite contre-électrode 4 étant sensiblement égale à l'épaisseur de la paroi de ladite première partie creuse 7. The counter-electrode 4 made of stainless steel is a hollow cylindrical piece inserted in the wall of the first part 7 of the casing 2, the maximum thickness of said counter-electrode 4 being substantially equal to the thickness of the wall of said first part. hollow 7.
La contre-électrode 4 est placée en retrait de l'extrémité libre de la première partie 7 du boitier 2. Le boitier 2 est rempli d'araldite noire 16, et le corps cylindrique creux 9 entourant l'électrode de référence 5 contenant du gel 15 ainsi que le tube 13, est immergé dans ladite araldite 16. The counter-electrode 4 is set back from the free end of the first part 7 of the housing 2. The housing 2 is filled with black araldite 16, and the hollow cylindrical body 9 surrounding the reference electrode 5 containing gel 15 and the tube 13 is immersed in said araldite 16.
Ledit corps creux 9 est relié à l'électrode de travail 3 par l'intermédiaire d'une pièce d'interface 17 creuse ayant un canal interne de diamètre constant, ladite pièce d'interface 17 servant d'adaptateur entre l'électrode de référence 5 et l'électrode de travail 3. Cette pièce d'interface 17 se décompose en trois segments 19,20,21 se distinguant les uns des autres au niveau de leur diamètre externe. Said hollow body 9 is connected to the working electrode 3 via a hollow interface piece 17 having an internal channel of constant diameter, said interface piece 17 acting as an adapter between the reference electrode 5 and the working electrode 3. This interface piece 17 is divided into three segments 19,20,21 which are distinguished from each other at their outer diameter.
Le premier segment 19 est logé dans le canal interne de la première partie 11 du corps cylindrique creux 9 délimitant l'électrode de référence 5, de manière à ce que sa surface externe soit au contact de la surface interne de ladite première partie 11. The first segment 19 is housed in the internal channel of the first portion 11 of the hollow cylindrical body 9 delimiting the reference electrode 5, so that its outer surface is in contact with the inner surface of said first portion 11.
Le deuxième segment 20, qui prolonge le premier segment 19, possède un diamètre externe supérieur à celui du premier segment 19. The second segment 20, which extends the first segment 19, has an outer diameter greater than that of the first segment 19.
Ce deuxième segment 20 forme un bourrelet annulaire venant s'insérer entre l'extrémité de la première partie 11 du corps cylindrique creux 9 délimitant l'électrode de référence 5 et la surface annulaire plane 26 de l'électrode de travail 3, qui est opposée à la surface annulaire plane 18 arasant l'extrémité de la première partie 7 du boîtier 2. This second segment 20 forms an annular bead inserted between the end of the first portion 11 of the hollow cylindrical body 9 delimiting the reference electrode 5 and the flat annular surface 26 of the working electrode 3, which is opposite. at the flat annular surface 18 leveling the end of the first part 7 of the housing 2.
Ce bourrelet 20 est au contact à la fois avec ladite électrode de travail 3 et le corps cylindrique creux 9 délimitant l'électrode de référence 5. This bead 20 is in contact with both said working electrode 3 and the hollow cylindrical body 9 delimiting the reference electrode 5.
Le deuxième segment 20 est prolongé par un troisième segment 21, dont le diamètre externe est inférieur à celui du premier segment 19. The second segment 20 is extended by a third segment 21, whose outer diameter is smaller than that of the first segment 19.
Ce troisième segment 21 est de faible épaisseur et est logé dans le canal interne de l'électrode de travail 3, de sorte que la surface externe dudit troisième segment 21 vienne au contact de la surface interne de l'électrode de travail 3. This third segment 21 is of small thickness and is housed in the internal channel of the working electrode 3, so that the external surface of said third segment 21 comes into contact with the inner surface of the working electrode 3.
L'extrémité libre de ce troisième segment 21 affleure l'extrémité libre de la première partie 7 du boîtier 2, et présente un rebord 23 plié vers l'intérieur de manière à diminuer localement le diamètre interne dudit troisième segment 21. The free end of this third segment 21 is flush with the free end of the first part 7 of the casing 2, and has an edge 23 bent inwards so as to locally reduce the internal diameter of said third segment 21.
Cette pièce d'interface 17 est réalisée en PVC. Un tube creux 22 en PVC est collé à l'intérieur du canal interne de la pièce d'interface 17, une extrémité dudit tube creux 22 étant placée au niveau du milieu du deuxième segment 20, et l'autre extrémité émergeant du premier segment 19, à l'intérieur du corps creux 9 délimitant l'électrode de référence. This interface piece 17 is made of PVC. A PVC hollow tube 22 is glued inside the internal channel of the interface piece 17, one end of said hollow tube 22 being placed at the level of the middle of the second segment 20, and the other emerging end of the first segment 19, inside the hollow body 9 delimiting the reference electrode.
De façon plus précise, la paroi externe du tube cylindrique 22 est collée à la paroi interne de la pièce d'interface 17. Un corps cylindrique plein 24, réalisé en céramique poreuse, est placé dans le canal interne de la pièce d'interface 17, entre une extrémité du tube 22 collé à l'intérieur dudit canal, et le rebord 23 replié du troisième segment 21, ladite extrémité du tube 22 et ledit rebord 23 constituant des butées de retenue dudit corps plein 24. More specifically, the outer wall of the cylindrical tube 22 is glued to the inner wall of the interface piece 17. A solid cylindrical body 24, made of porous ceramic, is placed in the internal channel of the interface piece 17 , between one end of the tube 22 adhered inside said channel, and the folded rim 23 of the third segment 21, said end of the tube 22 and said flange 23 constituting retaining stops of said solid body 24.
La méthode de dosage selon les étapes a, b et optionnellement c The assay method according to steps a, b and optionally
La méthode de dosage des étapes a, b et optionnellement c est détaillée comme suit. The method of assaying steps a, b and optionally is detailed as follows.
Le dioxygène susceptible d'être présent dans la solution à analyser est tout d'abord réduit selon l'équation Eql, en appliquant un potentiel PA (étape a) de -0,6 V/ECS pendant 20 à 30 secondes, afin de mesurer le courant 10 de réduction correspondant. The oxygen likely to be present in the solution to be analyzed is first reduced according to the Eql equation, by applying a PA potential (step a) of -0.6 V / SCE for 20 to 30 seconds, in order to measure the corresponding reduction current.
Ensuite, on réduit les ions nitrate (et également le dioxygène) selon l'équation Eq2, en appliquant un potentiel PB (étape b) de -0,9 V/ECS pendant 20 à 30 secondes, afin de mesurer le courant II de réduction correspondant. Then, nitrate ions (and also oxygen) are reduced according to equation Eq2, by applying a PB potential (step b) of -0.9 V / SCE for 20 to 30 seconds, in order to measure the reduction current II corresponding.
Ce courant II est ainsi surévalué au regard du courant 10. Le courant de réduction corrigé (INitrate) des ions nitrate, à -0,9 V/ECS, est alors défini tel que : This current II is thus overvalued with respect to the current 10. The corrected reduction current (I N itrate) of the nitrate ions, at -0.9 V / ECS, is then defined as:
IlMitrate = H - 10. On peut alors déterminer facilement et de façon fiable la quantité de nitrate présent dans la solution à analyser. Si l'on souhaite en outre déterminer la quantité de nitrite dans ladite solution à analyser, on peut poursuivre la méthode en réduisant les ions nitrate et nitrite (et également le dioxygène) selon les équations Eq3 et Eq4 en appliquant un potentiel PC (étape c) de -1,1 V/ECS pendant 20 à 30 secondes, afin de mesurer le courant 12 de réduction correspondant. IlMitrate = H - 10. The amount of nitrate present in the solution to be analyzed can then easily and reliably be determined. If it is furthermore desired to determine the quantity of nitrite in said solution to be analyzed, the method can be continued by reducing the nitrate and nitrite ions (and also the oxygen) according to equations Eq3 and Eq4 by applying a potential PC (step c ) of -1.1 V / DHW for 20 to 30 seconds, in order to measure the corresponding reduction current 12.
Ce courant 12 est ainsi surévalué au regard du courant 10. Le courant de réduction corrigé (INitnte) des ions nitrite, à -1,1 V/ECS, est alors défini tel que : This current 12 is thus overvalued with respect to current 10. The corrected reduction current (I N itnte) of nitrite ions, at -1.1 V / ECS, is then defined such that:
12 = 4 X + 10 + IiMitrite- Ainsi, We = 12 - 4 x IN|trate - 10. 12 = 4 X + 10 + IiMitrite- Thus, W e = 12 - 4 x I N | trate - 10.
On peut alors déterminer facilement et de façon fiable la quantité de nitrite présent dans la solution à analyser. The amount of nitrite present in the solution to be analyzed can then easily and reliably be determined.

Claims

REVENDICATIONS
1. Méthode de dosage des ions nitrate et/ou nitrite en solution, utilisant une électrode de cuivre, ladite méthode étant caractérisée en ce qu'elle est réalisée en mode potentiostatique, et qu'elle comprend les étapes de dosage consistant à : a. appliquer un potentiel PA à une électrode de cuivre immergée dans une solution à analyser de pH sensiblement neutre, pour mesurer le courant 10 de réduction du dioxygène susceptible d'être présent dans ladite solution à analyser, puis b. appliquer un potentiel PB à l'électrode de cuivre, de sorte à réduire les ions nitrate pour obtenir la mesure du courant II de réduction correspondant à la réduction des ions nitrate en ions nitrite et à la réduction du dioxygène, puis c. optionnellement, appliquer un potentiel PC à l'électrode de cuivre de sorte à réduire les ions nitrate et les ions nitrite pour obtenir la mesure du courant 12 de réduction correspondant à la réduction des ions nitrate et nitrite en ammoniaque et à la réduction du dioxygène.  A method for the determination of nitrate and / or nitrite ions in solution, using a copper electrode, said method being characterized in that it is performed in potentiostatic mode, and that it comprises the dosing steps consisting of: a. applying a PA potential to a submerged copper electrode in a substantially neutral pH analyte solution, for measuring the oxygen reducing current likely to be present in said solution to be analyzed, then b. applying a potential PB to the copper electrode, so as to reduce the nitrate ions to obtain the measurement of the reduction current II corresponding to the reduction of the nitrate ions to nitrite ions and to the reduction of the oxygen, then c. optionally, applying a PC potential to the copper electrode so as to reduce nitrate ions and nitrite ions to obtain the measurement of the reduction current 12 corresponding to the reduction of nitrate and nitrite ions to ammonia and the reduction of oxygen.
2. Méthode selon la revendication 1, caractérisée en ce que le potentiel PA est égal à environ -0,6 V/ECS. 2. Method according to claim 1, characterized in that the potential PA is equal to about -0.6 V / ECS.
3. Méthode selon la revendication 1 ou 2, caractérisée en ce que le potentiel PB est égal à environ -0,9 V/ECS. 3. Method according to claim 1 or 2, characterized in that the potential PB is equal to about -0.9 V / ECS.
4. Méthode selon l'une quelconque des revendications 1 à 3, caractérisée en ce que le potentiel PC est égal à environ -1,1 V/ECS. 4. Method according to any one of claims 1 to 3, characterized in that the potential PC is equal to about -1.1 V / ECS.
5. Méthode selon l'une quelconque des revendications 1 à 4, caractérisée en ce que la vitesse d'écoulement de la solution à analyser est sensiblement constante. 5. Method according to any one of claims 1 to 4, characterized in that the flow velocity of the solution to be analyzed is substantially constant.
6. Méthode selon l'une quelconque des revendications 1 à 5, caractérisée en ce que la solution à analyser est renouvelée après chaque étape b lorsque le dosage consiste à mesurer uniquement les courants 10 et II, ou après chaque étape c lorsque le dosage consiste à mesurer les courants 10, Il et 12. 6. Method according to any one of claims 1 to 5, characterized in that the solution to be analyzed is renewed after each step b when the assay consists of measuring only the currents 10 and II, or after each step c when the assay consists of to measure the currents 10, 11 and 12.
7. Méthode selon l'une quelconque des revendications 1 à 6, caractérisée en ce qu'elle comprend en outre des étapes de traitement de l'électrode de cuivre, préalables aux étapes de dosage a et b, ou aux étapes de dosage a, b et c, consistant à : i. appliquer un premier potentiel à l'électrode de cuivre de sorte à réduire les oxydes de cuivre présents à la surface de l'électrode en cuivre métallique, ii. appliquer un second potentiel à l'électrode de cuivre de sorte à oxyder le cuivre métallique formé à l'étape i en ions cuivriques, et iii. appliquer un troisième potentiel à l'électrode de cuivre de sorte à réduire les oxydes de cuivre éventuellement formés à l'étape ii, les étapes i à iii étant réalisées par immersion de l'électrode de cuivre dans un électrolyte support ayant un pH sensiblement neutre. 7. Method according to any one of claims 1 to 6, characterized in that it further comprises steps of treating the copper electrode, prior to the a and b dosing steps, or the dosing steps a, b and c, consisting of: i. applying a first potential to the copper electrode so as to reduce the copper oxides present on the surface of the copper metal electrode, ii. applying a second potential to the copper electrode so as to oxidize the metallic copper formed in step i to cupric ions, and iii. applying a third potential to the copper electrode so as to reduce the copper oxides possibly formed in step ii, steps i to iii being performed by immersing the copper electrode in a supporting electrolyte having a substantially neutral pH .
8. Méthode selon la revendication 7, caractérisée en ce que l'électrolyte support des étapes i à iii est la solution à analyser lors des étapes de dosage a et b, ou des étapes de dosage a, b et c. 8. Method according to claim 7, characterized in that the electrolyte supporting steps i to iii is the solution to be analyzed during the assay steps a and b, or the assay steps a, b and c.
EP12728682.1A 2011-05-18 2012-05-14 Method for assaying nitrates and/or nitrites in a neutral medium Withdrawn EP2710361A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR1154318A FR2975494B1 (en) 2011-05-18 2011-05-18 ELECTROCHEMICAL SENSOR WITH COMPACT GEOMETRY FOR THE ASSAY OF NITRATES AND / OR NITRITES AND METHOD FOR DETERMINING NITRATES AND / OR NITRITES IN NEUTRAL MEDIA
PCT/FR2012/051073 WO2012156636A1 (en) 2011-05-18 2012-05-14 Method for assaying nitrates and/or nitrites in a neutral medium

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AT403528B (en) * 1989-04-04 1998-03-25 Urban Gerald MICRO-MULTI-ELECTRODE STRUCTURE FOR ELECTROCHEMICAL APPLICATIONS AND METHOD FOR THEIR PRODUCTION
JP2740587B2 (en) * 1991-07-18 1998-04-15 工業技術院長 Micro composite electrode and method of manufacturing the same
FR2952721B1 (en) * 2009-11-19 2011-12-30 Centre Nat Rech Scient METHOD FOR DETERMINING NITRATES AND / OR NITRITES IN A NEUTRAL ENVIRONMENT

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FR2975493A1 (en) 2012-11-23

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