Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/75—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
  • G01N21/77—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator
  • G01N21/78—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator producing a change of colour
  • G01N21/783—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator producing a change of colour for analysing gases
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/416—Systems
  • G01N27/42—Measuring deposition or liberation of materials from an electrolyte; Coulometry, i.e. measuring coulomb-equivalent of material in an electrolyte
  • G01N27/44—Measuring deposition or liberation of materials from an electrolyte; Coulometry, i.e. measuring coulomb-equivalent of material in an electrolyte using electrolysis to generate a reagent, e.g. for titration

Definitions

• the invention relates to a method for dosing an oxidizing gas making it possible to measure a parameter linked to the quantity of this qaz in a given volume or flow; the invention makes it possible in particular to measure the partial pressure of the oxidizing gas in a gas mixture; it also makes it possible to measure the permeability of a porous material with respect to the oxidizing gas. It extends to a metering device intended for implementing the method.
• the invention is particularly applicable for dosing oxygen.
• Oxygen dosing methods are numerous and can be classified into three essential categories according to the nature of the oxygen properties that are used. Certain processes use specific physical properties of oxygen, and more particularly its paramagnetic properties (electromagnetic excitation of the qaz to be assayed and analysis of the response). These methods have the defect of requiring purification of the sample which must be rid of gases with high magnetic susceptibility (such as nitrogen oxides), and their implementation requires heavy and expensive materials.
• a third type of process uses a chemical property of oxyqene, namely its spontaneous combustion with hydrogen: the sample to be assayed is put in the presence of hydrogen and either the quantity of heat produced or the variation in thermal conductivity before and after combustion.
• this process was abandoned due to its lack of selectivity: many gases react with hydrogen and interfere in the measurements.
• the reactive compound is consumed during the reaction and the properties of the sensor vary during the measurements, so that the response obtained must take into account the state of the sensor: this constitutes a major difficulty to guarantee simple and reliable measurements. ; in addition, the sensor reagent must be changed after a certain number of measurements.
• the direct introduction of the sample into the electrolyser for measuring the quantity of electricity can cause parasitic redox reactions with other compounds contained in the sample, so that the coulometric measurements carried out do not necessarily provide the image of the parameter to be assayed.
• electrolysers platinum, palladium, nickel Certainly have the property of catalyzing combustion reactions which can lead to parasitic consumption of the oxidizing gas in the presence of a combustible gas in the sample.
• use a coulometric measurement of the quantity of electricity give results which are functions of the composition of the sample and are not applicable in a faithful manner in many cases.
• the present invention relates to a process of the last category (use of the oxidizing property of the oxidizing gas to be dosed) and proposes to provide a new process free from the defects of the aforementioned processes.
• An objective of the invention is in particular to provide an oxygen metering method giving a faithful and constant response over time regardless of the age of the device, guaranteeing good selectivity and benefiting from a very short response time. .
• Another object of the invention is to allow both continuous operation for online measurements and, if necessary, discontinuous operation for measurements on samples or samples.
• the method of the invention is applicable for the determination of other oxidizing gases such as nitrogen oxide, sulfur oxide, halogen, etc., by adapting, as will be seen below, the mediating compound used with metering gas.
• the invention therefore proposes to indicate a method for dosing an oxidizing gas which uses the oxidation properties of this gas to allow it to be measured accurately, that is to say to say to precisely measure a parameter linked to the quantity of this gas contained in a given volume or flow.
• the dosing method according to the invention consists:
• a redox mediator compound having, on the one hand, an oxidized state, on the other hand, a reduced state in leguel it is reducing vis-à-vis the oxidizing gas to be dosed, said compound being able to change state by rocky eiect and having different absorption spectra in its reduced state and in its oxidized state,
• oxidizing gas is meant the gaseous medium to be analyzed in which the oxidizing gas is found).
• the method of the invention leads to carrying out the measurement, not on the oxidizing gas itself, but on a mediating compound chosen as a function of the oxidizing gas to be determined in order to give with it a redox reaction modifying the chemical state of the mediator and then making it possible to know the quantity of mediator which has reacted by absorbance measurement.
• This absorbance measurement independent of the reqeneration electrolyser, provides completely free measurements of the parasitic reactions which can be induced in the electrolyser by the electrodes. It should be noted that, if necessary, an additional coulometric measurement can be carried out in this electro-lyzer, but this measurement is only a means of verification.
• the selectivity depending on the choice of the mediating compound, can be adapted to the application envisaged.
• the mediating compound is only subject to electronic modification during the reaction with the oxidizing gas and does not change chemical species; the regeneration of said mediator compound from the oxidized state to the reduced state thus takes place by a simple electronic transfer which can in particular be easily carried out by an electrolysis of this compound in dissolved form.
• the mediating compound is used in a form dissolved in a conductive solution, this compound is circulated after cathodic regeneration to a cell for bringing said compound and the oxidizing gas into contact and returned after reaction said mediating compound towards cathodic regeneration.
• the invention is particularly applicable to the determination of oxygen.
• a compound from the following group is advantageously chosen as mediating compound: flavin, viologene, quinone.
• flavin, viologene, quinone are advantageously chosen as mediating compound: flavin, viologene, quinone.
• the method of the invention will be implemented by causing a fraction of said oxidizing gas to diffuse through a porous wall and by bringing the dissolved mediating compound into contact with the oxidizing gas having diffused through said porous wall .
• This porous wall which has no role of selectivity simply plays the role of interface between the liquid medium and the gaseous medium and also makes it possible to reduce the quantities of oxidizing qaz brought into contact with the mediating compound so that the latter always remains in excess.
• the method can be used to measure the permeability of the porous wall by admitting into the contacting cell a mixture of known composition.
• the process of the invention can be carried out batchwise: then predetermined quantities of mediating compound and oxidizing gas are admitted into the contacting cell, these compound and gases are left in the presence for a predetermined period, the mediator compound is then removed and the amount of mediator compound that has reacted is determined by absorbance measurement. This type of discontinuous measurement will in particular be used for the dosing of samples.
• the process of the invention can also be implemented continuously to measure a flow of oxidizing gas: the mediating compound and the oxidizing gas are then caused to circulate in the contacting cell with predetermined flow rates, the the mediating compound is removed continuously and the amount of absorbance is determined by the absorbance measurement mediating compound which reacted per unit of time.
• this mode of implementation will be chosen because it allows continuous monitoring of a system and its regulation.
• the mediating compound is admitted batchwise into the contacting cell, while the oxidizing gas is caused to circulate in it for a specified time.
• the method of the invention makes it possible to measure any parameter linked to the quantity of oxidizing gas contained in a gaseous mixture: partial pressure, permeability of a porous material with respect to the oxidizing gas ...
• the invention extends to a device for metering an oxidizing qaz such as oxygen with a view to implementing the method defined above; the device according to the invention comprises:
• circulation means adapted to allow operating and stopping sequences to be carried out
• an absorbance meter downstream of the contacting cell, an absorbance meter to determine the quantity or the flow rate of oxidized mediator compound
• an electrolyser having two compartments, a cathode compartment provided with a cathode and arranged to receive the mediating compound downstream of the contacting cell and to return it after reduction to said cell, and an anode compartment provided with a anode and filled with a conductive solution to ensure electrical continuity,
• the contacting cell comprises a porous wall permeable to oxidizing gas and impermeable to the solution of mediating compound, said porous wall dividing the cell into two compartments: an oxidizing gas inlet compartment and a circulation compartment for the mediating compound.
• the contacting cell comprises a single compartment equipped with aqitation means.
• FIG. 1 is an overall schematic view of this metering device
• FIG. 2 is a vertical section of the contacting cell of the device
• FIG. 3 is a vertical section of the regeneration electrolyzer
• FIG. 4 is a diagram providing the absorbance spectrum of a flavin, on the one hand, in the oxidized state (curve A), on the other hand, in the reduced state (curve B),
• Figures 5 to 9 are diagrams obtained in the examples of implementation described below, respectively providing curves of evolution of absorbance and intensity (Figure 5), calibration curves ( Figures 6, 7 and 9) and an absorption spectrum (Figure 8).
• the device shown by way of example in FIGS. 1, 2 and 3 is intended for the determination of oxygen.
• This device comprises a sealed circulation loop composed of a contacting cell 1 having a conduit the inlet of a solution of mediating compound in the form reduced and an outlet duct 1b, an absorbance meter constituted by a spectrophotometer 2 receiving the solution by the outlet outlet duct 1b, a degasser 3, a circulation pump 4, a flow meter 5, an electrolyser 6 'whose inlet conduit 6a receives the oxidized solution and the outlet conduit 6b is looped back to the inlet conduit -la- of the contacting cell 1.
• the various conduits of this loop and the fittings are made of stainless steel to avoid any parasitic entry of oxygen or oxidizing gas.
• the spectrophotometer 2 can be of the "Hewlett Packard" type and has a sealed cell provided for the circulation of the solution to be analyzed.
• Degasser 3 of the conventional type, is an argon bubbler degasser; it allows the loop to be filled beforehand with the solution of mediating compound and contains a reserve of solution acting as a buffer in the loop.
• the pump 4 is of the peristaltic pump type, with a portion of flexible hose (necessary for its operation) as short as possible and made of material such as
• the contacting cell 1 is shown in detail in FIG. 2.
• This cell is formed by two parallelepipedal half-shells 1c, 1d, in the example in "ait ⁇ glass", which are held one against the other by means of four threaded clamping rods such as the, with interposition of seals lf.
• a porous wall 7, permeable to oxygen, is pinched between the two shells; in the example, this wall is constituted by a "cellophane” membrane applied to a grid made of inert synthetic material.
• This porous wall is impermeable to the solution of mediator compound and divides the cell into two compartments: a compartment 8 for circulation of the solution of mediator compound and a compartment 9 for admitting the gas to be assayed.
• the inlet and outlet ducts 1b open into the compartment 8 at the ends thereof so as to avoid the formation of dead volumes.
• gas inlet 1g and outlet li conduits open into the gas compartment 9.
• a flow meter 10 is fitted to the inlet conduit 1g to measure the flow admitted into cell 1 at all times.
• electrolyser 6 shown in detail in FIG. 3, is formed by an enclosure made up of two parallelepipedal half-shells 6c, 6d, in the example in "altuqlass", which are kept tight. one against the other as before for cell 1.
• a membrane 11 not permeable to the mediating compound but permeable to small ions divides the electrolyser into an anode compartment 12 and into a cathode compartment 13.
• This membrane is in particular of the reverse osmosis membrane type.
• the inlet 6a and outlet 6b conduits open into the cathode compartment 13 which contains several metal grids, in the example of platinum, such as 14, separated by grids of inert synthetic material 15 playing the role of turbulence promoters; the grids 14 are connected by a platinum wire which extends outside through a sealed channel for the power supply.
• a Luggin capillary 16 penetrates into the cathode compartment and opens into a sealed reservoir 17 into which a reference electrode 18 of the conventional calomel type, saturated, is immersed.
• the anode compartment contains a platinum plate 19 connected to a wire which extends outside for the power supply.
• This anode compartment is connected by inlet conduits 6g and 6i to an auxiliary loop containing a conductive solution.
• This solution is identical to that of the main circulation loop with the exception of the mediating compound which is absent from it (phosphate buffer solution of pH 7).
• This auxiliary loop is equipped with circulation means such as a pump 20 and an argon deoxygenation bubbler 21.
• the electrodes 14, 18 and 19 are electrically connected to a conventional type potential regulation system such as potentiostat 22, allowing applying a predetermined constant potential difference between the cathode 14 and the reference electrode 18, this potential difference being in particular independent of the intensity of the electrolysis current.
• a conventional type potential regulation system such as potentiostat 22
• a coulometer 23 is arranged in the electrical circuit in order to measure the quantity of electricity which passes through the electrolyser. In the examples referred to below, this measurement is carried out by a recorder placed in the electrical circuit on the anode side, which gives access to the electrical intensity which crosses the circuit and to the amount of electricity by integration over the intervals. of time considered.
• the cathode loop is filled with a solution of mediating compound. Filling takes place at the degasser.
• the flavon mononucleotide was used at a concentration of 0.23.10 -3 mole / l.
• the spectra of the oxidized and reduced forms of the mononucleotide flavin are given in FIG. 4: this compound has a very different behavior in its oxidized state (curve A) and in its reduced state (curve B).
• the anode loop is filled with a conductive solution identical to the solution used to dissolve the mediating compound (0.2 mole / l phosphate buffer solution pH 7.0).
• the solutions are circulated at a sufficiently high rate (160 cm 3 / min) and the mediating compound is reduced by bringing the cathode to the adequate potential (-0.6 V relative to the saturated calomel electrode) and by maintaining an argon current in the contacting cell (2,000 cm 3 / min).
• the wavelength chosen must correspond to a maximum difference between the spectra of the oxidized and reduced forms of the mediating compound (450 nm for the case described).
• a known partial pressure of oxygen is admitted into the stream of nitrogen entering the contacting cell for a given time (3 minutes), after which the flow of argon is restored. The instant when oxygen is admitted into the gas flow is taken as the origin of time.
• the intensity and absorbance measurements give lines from 0.1 atm to 1 atm.
• the experience is repeated with methylviologen as the mediating compound.
• the spectra of the oxidized and reduced forms of the compound are given in FIG. 8 (curve E: oxidized form; curve F: reduced form).
• the absorbance is monitored at 606 nm.
• the reduced form has an absorbance greater than the oxidized form and reoxidation by oxygen causes a decrease in absorbance, unlike the previous case.
• the potential of the electrolyser is of the order of -0.75 V relative to the saturated calomel electrode.
• FIG. 9 gives the calibration curve obtained with the variation in absorbance as a function of the partial pressure of oxygen.
• the circulation of nitrogen in the contacting cell is cut off and a stream of compressed air is admitted at an approximately equal rate (2000 cm 3 / min) for a given time (3 minutes), after which the flow of argon is restored.
• the instant when air is admitted into the cell is taken as the origin of time.
• the absorbance measurements successively give: 0.048 - 0.054 - 0.052 - 0.051, that is, using the calibration curve in FIG. 7, partial oxygen pressures of 0.19 - 0.215 - 0.21 - 0.21 ATM.
• the average value of 0.205 ⁇ 0.01 atm corresponds well to the average oxygen content in the air.
• the measurement reported in FIG. 5 was carried out with a sheet of 0.02 cm thick of cellulosic material of the "cellophane" type used as a separator in the contacting cell; the exchange surface between the gas and the solution of mediating compound, fixed by the geometry of the cell, is 15 cm 2 : the partial pressure of oxygen in the gas stream flowing in the cell is 0.5 atm ; the flow rate of the mediator compound solution of 40 cm 3 - / min.
• the permeability of a material is defined as:
• the value of Q can be calculated from the monitoring of the change in absorbance during the measurement ( Figure 5):

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• Measuring Or Testing Involving Enzymes Or Micro-Organisms (AREA)

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Citations (4)

• 1988
  • 1988-12-26 FR FR8817292A patent/FR2641080A1/fr not_active Withdrawn
• 1989
  • 1989-12-08 WO PCT/FR1989/000640 patent/WO1990007709A1/fr unknown

Patent Citations (4)

Non-Patent Citations (2)

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