DE102016111469A1 - Method for producing a shaped body, reference electrode, method for producing a reference electrode and electrochemical sensor - Google Patents

Abstract

The invention relates to a method for producing a shaped body (8, 18) comprising: introducing, in particular, a molten salt (32, 42) formed from one or more halide salts into a mold (33, 43, 53); and - solidifying the salt melt (32, 42) to form the one-piece, in particular transparent, shaped body (8, 18) in the mold (33, 43, 53). A reference electrode comprising a shaped body comprises: - a housing (2, 17) in which a reference electrolyte (3, 20) is accommodated, which comprises an electrolyte salt, in particular an alkali metal halide salt; - a reference electrolyte (3, 20) contacting, electrically conductive reference element (4, 19); - A in a wall of the housing (2, 17) arranged transfer (6, 21), via which the reference electrolyte (3, 20) with a housing (2, 17) surrounding the medium is in electrolytic contact; and - an electrolyte salt reservoir contained in the housing (2, 17); wherein the electrolyte salt stock comprises at least one shaped body (8, 18) consisting of at least one salt, in particular the electrolyte salt, prepared according to the method indicated above.

Description

The invention relates to a method for producing a shaped body, a reference electrode, a method for producing a reference electrode and an electrochemical sensor with such a reference electrode. Reference electrodes can be used in various electrochemical sensors.

Electrochemical sensors are used in laboratory and process measurement technology in many areas of chemistry, biochemistry, pharmacy, biotechnology, food technology, water management and environmental metrology for the analysis of measuring media, in particular of measuring liquids. By means of electrochemical measurement techniques, it is possible to record, for example, activities of chemical substances, for example ions, and thus correlated measured variables in liquids. The substance whose concentration or activity is to be measured is also called analyte. Generic electrochemical measuring arrangements can be, for example, potentiometric or amperometric sensors.

Potentiometric sensors usually comprise a measuring electrode and a reference electrode and a measuring circuit. The measuring electrode forms in contact with the measuring medium, e.g. a measuring liquid, a dependent on the concentration or activity of the analyte in the measuring medium potential, while the reference electrode provides a stable, independent of the concentration of the analyte reference potential. The measuring circuit generates a measuring signal representing the potential difference between the measuring electrode and the reference electrode. If necessary, the measuring signal is output by the measuring circuit to a superordinated unit, for example a measuring transducer, connected to the sensor, which further processes the measuring signal.

The measuring electrode of potentiometric sensors comprises a potential-forming element which, depending on the type of potentiometric sensor, may comprise, for example, a redox electrode, an analyte-sensitive coating or an ion-selective membrane. Examples of ion-selective membrane-forming elements are ion-selective electrodes (ISE). An ion-selective electrode has a housing closed by the ion-selective membrane serving as a potential-generating element, in which an inner electrolyte is accommodated, and a lead-out electrode, which is in contact with the inner electrolyte. If the measurement medium is in contact with the potential-forming element, a change in the activity or concentration of the analyte in the measurement medium causes a relative change in the equilibrium galvanic voltage between the measurement medium and the deflection electrode in contact with the potential-forming element via the inner electrolyte. The discharge electrode is electrically conductively connected to the measuring circuit. A special case of such an ion-selective electrode is the known pH glass electrode, which has a chamber sealed by a pH-sensitive glass membrane with an internal electrolyte comprising a buffer system for establishing a stable pH. Ion-selective electrodes are for example in "Working with Ion-Selective Electrodes", K. Cammann, H. Galster, Springer, 1996 , described.

The reference electrode of potentiometric sensors is often designed as an electrode of the second type whose potential depends only indirectly on the composition of the medium to be measured, in particular on the concentration of the analyte in the medium to be measured. Electrodes of the second type generally comprise a reference element which is in contact with a reference electrolyte. The reference element is formed of a metal, wherein at least a part of the surface of the reference element has a coating of a poorly soluble salt of the metal. The reference electrolyte is usually a saturated solution of this sparingly soluble salt. In addition, the reference electrolyte contains a high concentration of the anion of the sparingly soluble salt, usually in the form of an alkali metal salt. This salt is also referred to below as the electrolyte salt. The potential of a second type electrode depends on the concentration of the cation of the sparingly soluble salt in the reference electrolyte. Due to the very high anion concentration in the reference electrolyte, the concentration of the cation of the sparingly soluble salt and thus the potential of the electrode remains substantially constant.

An example of such a reference electrode configured as a second type electrode is the silver / silver chloride electrode or the calomel electrode. A silver / silver chloride electrode comprises as reference electrolyte a solution with high chloride concentration, usually a potassium chloride solution, and as a reference element a silver chloride coated silver wire. The reference electrolyte is accommodated in a chamber formed in a housing of the reference electrode. To carry out a potentiometric measurement, the reference electrolyte must be in electrolytic contact with the measurement medium. This contact is made by a transfer, which may for example consist of a through the housing wall passing hole, a porous diaphragm or a gap. The reference element is electrically connected to the already mentioned measuring circuit. The reference electrolyte can by adding a Thickening agent, in particular a polymer thickened.

Amperometric sensors may comprise, for example, a three-electrode circuit having a working electrode, a counterelectrode and a non-current-carrying reference electrode. Such a sensor may, for example, have a, in particular potentiostatic, control circuit which is designed to predetermine a setpoint voltage between the working electrode and the reference electrode and to detect the current flowing between the working electrode and the counterelectrode. The reference electrode, which is not current-carrying, can be configured in the same way as a reference electrode of a potentiometric measuring arrangement.

Since the reference electrode is in electrolytic contact during operation via its transfer with the measuring medium, the reference electrolyte can be obtained by outdiffusion of the salts dissolved in the reference electrolyte, in particular the anion of the sparingly soluble salt, such as, for example, Chloride in the case of a silver / silver chloride electrode, deplete. This effect is the stronger the lower the concentration of the corresponding salts in the medium to be measured, for example in drinking water applications. This salt loss can lead to a drift of the reference potential. By regularly calibrating the sensor, it is possible to counteract the drift for some time, as long as the reference potential has not yet moved too far from its nominal value or its original value, which means the end of the lifetime of the reference electrode. However, it is desirable to have as long as possible a safe period of operation of the reference electrode, during which the least possible time-consuming calibration has to be carried out.

It is therefore, especially for reference electrodes in drinking water applications, attempts to compensate for the salt loss of the reference electrolyte by a salt reservoir present in the electrode, which is configured in the form of undissolved, present in the reference electrolyte salt rings. If the reference electrode loses dissolved salt during operation through the transfer, the salt supply is available for continuous refreshment of the salt concentration in the reference electrolyte. A drift, which requires a more frequent calibration or leads to the early end of the life of the reference electrode, only occurs when the additional salt supply is used up. If the reference electrode is designed as a single electrode for potentiometric or amperometric measurements, the salt rings can be so formed in a front region of the housing of the reference electrode, i. be arranged in the region of the housing, which also includes the transfer, that they enclose the reference element.

The salt rings used in known sensors are as compacts by pressing salt particles, usually with the addition of auxiliaries, e.g. of waxes, which mediate adhesion between the salt particles, designed. In general, the compacts have a density that is significantly lower than the density of a crystal of the corresponding salt, which is related to the fact that in the volume of the compact there are a plurality of microscopic cavities and / or grain boundaries. This is visually already recognizable by the fact that the compacts appear opaque white.

As a result, the compacts present in the reference electrolyte dissolve relatively quickly during operation. If adhesion-promoting auxiliaries are present in the compacts, clouding of the reference electrolyte can occur when the rings are dissolved. This can lead to the condition of the reference element and possibly also of the diverting element, if the sensor is no longer visually detectable, being a combination electrode with a reference electrode running annularly around the measuring electrode.

It is therefore the object of the invention to provide a reference electrode with a salt supply which overcomes the disadvantages of the prior art. In particular, the reference electrode should allow a longer service life. Furthermore, the object of the invention is to provide a method for producing a shaped body, which can be used as salt reservoir in such a reference electrode.

This object is achieved by the method according to claim 1, the reference electrode according to claim 7, and the method for producing a reference electrode according to claim 11. Advantageous embodiments are specified in the subclaims.

• – Einbringen einer, insbesondere einer aus einem oder mehreren Salzen, vorzugsweise Halogenidsalzen, gebildeten, Salzschmelze in eine Form; und
• – Erstarrenlassen der Salzschmelze unter Bildung des, insbesondere einstückigen, Formkörpers in der Form.

The method according to the invention for producing a shaped body comprises the following steps:

• - introducing, in particular, a molten salt formed from one or more salts, preferably halide salts, into a mold; and
• - Solidification of the molten salt to form the, in particular one-piece, shaped body in the mold.

When solidification of the molten salt is formed as a molded body, a one-piece cast salt body whose shape and geometry are determined by the shape. The molded body produced in this way is transparent has a density which is approximately as large as the density of a salt single crystal and thus significantly larger than the density achievable by pressing salt crystals into a compact. The density of the shaped body produced by the process according to the invention is only slightly smaller than the density of a corresponding salt crystal, in particular in the range of 95 to 100% of the density of the salt crystal or even between 99 and 100% of the salt crystal. Thus, moldings produced by this process from pure (ie pure up to negligible, unavoidable impurities pure) potassium chloride to a density of 1.9 g / cm ³ and higher. It has been found that moldings produced by the process according to the invention, when used as electrolyte salt reservoir in a reference electrode of one of the electrochemical sensors described above, have a significantly longer service life than electrolytic salt compacts produced by known pressing processes. A possible explanation for this is that a molded article produced by the method according to the invention has considerably fewer cavities and / or grain boundaries than a compact. This has the consequence that moisture spreads more slowly in the volume of the molding than in a conventional compact, so that the molding remains intact longer. A further advantage of the shaped body produced by the process according to the invention is that it is formed from a melt comprising only a single or several salts, ie without adhesion-promoting auxiliaries. Upon dissolution of compacts comprising such auxiliaries known from the prior art, turbidity of the reference electrolyte results from the auxiliary substances which are not completely soluble in the reference electrolyte. If the turbidity is too strong, the reference element is no longer visible from the outside, so that the condition of the reference element (eg a discoloration or a tear) is no longer visually recognizable. This disadvantage is avoided when using a molded body produced by the above-mentioned method as a salt reservoir of the reference electrode, since the shaped body itself is transparent and goes completely dissolving during gradual dissolution.

• – Erzeugen der Salzschmelze durch Schmelzen eines in Form eines oder mehrerer fester Partikel vorliegenden Salzes, insbesondere eines Halogenidsalzes, oder mehrerer in Form eines oder mehrerer fester Partikel vorliegenden Salze, insbesondere mehrerer Halogenidsalze; und
• – Gießen der Salzschmelze in die Form.

In one embodiment, the introduction of the molten salt into the mold may include:

• Producing the molten salt by melting a salt present in the form of one or more solid particles, in particular a halide salt, or several salts present in the form of one or more solid particles, in particular a plurality of halide salts; and
• - pour the molten salt into the mold.

• – Einbringen eines in Form eines oder mehrerer fester Partikel vorliegenden Salzes, insbesondere eines Halogenidsalzes, oder mehrerer in Form eines oder mehrerer fester Partikel vorliegenden Salze, insbesondere mehrerer Halogenidsalze, in die Form; und
• – Schmelzen des Salzes oder der mehreren Salze in der Form.

In an alternative embodiment, the introduction of the molten salt into the mold may include:

• - introducing a present in the form of one or more solid particles salt, in particular a halide salt, or more present in the form of one or more solid particles salts, in particular a plurality of halide salts, in the mold; and
• Melting of the salt or salts in the mold.

After solidification of the molten salt and the formation of the molding, the mold can be removed. The shape may be, for example, a once used, a so-called lost shape or a reusable shape, a so-called permanent shape. As a mold, for example, one made of a deformable, e.g. granular or powdery material such as sand, ceramic powder or ceramic particles or a mold formed of one or a few components from a refractory material such as metal or a metal alloy, in particular steel, ceramic or a refractory glass, e.g. Quartz glass, in question. The molding material is ideally adapted to the properties of the molten salt. In particular, the material of the mold should be chemically inert to the molten salt. For halide salts, a refractory glass, e.g. Quartz glass, or an inert ceramic as a material for the mold.

For, in particular automated, production of a plurality of similar shaped bodies, the mold may be formed in a base body, in which a plurality of other similar shapes are formed, wherein the molten salt is poured from an automatically pivoting crucible, in particular in a single step in the molds. Alternatively, solid salt particles may also be added to the plurality of molds formed in the base body and then melted by heating the base body and then cooled again in the next step until the melt solidifies.

The temperature of the molten salt and / or the mold can be controlled or regulated, in particular when solidifying, according to a predetermined temperature profile. For this purpose, a tempering device in contact with the molten salt or the mold can be used with at least one heating element and / or at least one cooling element. Advantageously, the tempering device has at least one temperature sensor which detects the temperature of the mold and / or the molten salt. Based on the measured values supplied by the temperature sensor heating or cooling element of the temperature control can be controlled or regulated.

For generating and / or keeping warm the molten salt, a heating device can be used. For example, the initially solid salt, for example in the form of salt particles, can be introduced into a furnace in a crucible and melted there. In particular, the comes Heating of the solid salt by convection, radiation or induction in question. From the crucible, the molten salt thus produced can then be poured into the mold. In the alternative method embodiment described above, in which the solid salt is placed in the mold and melted directly in the mold, the mold may be placed in an oven to produce the molten salt in the mold. Here is again a heating by convection or radiation in question. If the mold is electrically conductive, it can also be heated by induction. Heating means, eg thermoelectric elements (eg Peltier elements) or heating conductors can also be attached directly to the mold.

The solidification of the molten salt can be carried out in a process embodiment by leaving the mold at room temperature. Alternatively, the mold may be actively tempered, e.g. For example, active cooling may be achieved by contacting the mold with a heat sink or by supplying a cooling medium to the mold, e.g. be achieved by a water or gas cooling. The cooling may additionally or alternatively be effected by the form contacting thermoelectric elements. By means of a temperature control, which is suitable both for cooling and for heating, more complex temperature profiles can be set or active regulation of the temperature of the mold can be achieved.

• – ein Gehäuse, in dem ein Bezugselektrolyt aufgenommen ist, welcher ein Elektrolytsalz, insbesondere ein Alkalimetallhalogenidsalz, umfasst;
• – ein den Bezugselektrolyten kontaktierendes, elektrisch leitendes Bezugselement;
• – eine in einer Wand des Gehäuses angeordnete Überführung, über die der Bezugselektrolyt mit einem das Gehäuse umgebenden Medium in elektrolytischem Kontakt steht; und
• – einen in dem Gehäuse enthaltenen Elektrolytsalzvorrat; wobei der Elektrolytsalzvorrat mindestens einen aus mindestens einem Salz, insbesondere dem Elektrolytsalz, bestehenden Formkörper, hergestellt nach dem erfindungsgemäßen Verfahren in einer der voranstehend beschriebenen Ausgestaltungen umfasst.

The reference electrode according to the invention comprises:

• A housing in which a reference electrolyte is accommodated, which comprises an electrolyte salt, in particular an alkali metal halide salt;
• - A reference electrolyte contacting the electrically conductive reference element;
• - An arranged in a wall of the housing transfer, via which the reference electrolyte is in electrolytic contact with a medium surrounding the housing; and
• An electrolyte salt reservoir contained in the housing; wherein the electrolyte salt stock comprises at least one shaped body consisting of at least one salt, in particular the electrolyte salt, prepared by the process according to the invention in one of the above-described embodiments.

The molded article can thus be produced by introducing a molten salt formed from the electrolyte salt into a mold and solidifying the molten salt to form the molded article in the mold.

The molded body can be configured as a ring. The reference electrode may also comprise a plurality of rings. The one or more rings may be arranged to enclose at least a portion of the reference member.

In an advantageous embodiment, the shaped body has a longitudinal extent which is greater than a maximum diameter, in particular an outer diameter, of the shaped body extending perpendicularly to this longitudinal extent.

Characterized in that the electrolyte salt reservoir is designed as a molded body having a longitudinal extent which is greater than a perpendicular to this longitudinal extent extending maximum diameter of the shaped body, the salt reservoir can evenly over a substantial proportion of a longitudinal extent of the housing or over a substantial proportion of the longitudinal extent be positioned distributed the reference element. As has been shown, this reduces the damage to the reference element which is to be observed in the case of reference electrodes with a salt reservoir in the form of salt rings. This effect can be explained by the fact that in this embodiment no or only slight gradients of the electrolyte salt concentration occur over the length of the reference element or the housing of the reference electrode.

In contrast, in operation of a reference electrode in which the electrolyte salt reservoir is in the form of at the front of the housing of the reference electrode, i. arranged in the region of the housing, which also includes the transfer, arranged salt rings, deplete the reference electrolyte in a region remote from the salt rings, so that forms along the longitudinal extension of the housing or the reference element, a gradient of the electrolyte salt concentration. This can lead to a local chemical attack of the reference element in areas of low electrolyte salt concentration. This can even go so far that the reference element is completely dissolved in this area, and the front, the diaphragm facing end portion of the reference element tears off.

A further advantage of an electrolyte salt reservoir designed as a shaped body which has a longitudinal extension running parallel to a longitudinal extension of the housing and / or to a longitudinal extent of the reference element, which is greater than a maximum diameter perpendicular to this longitudinal extent, is that the molded body extends beyond the operating time of the reference electrode becomes shorter and shorter, the further it dissolves in the reference electrolyte beginning at its end facing the transfer. Thus, the decrease in the longitudinal extent of the shaped body is a good indicator of the remaining life of the reference electrode. Another advantage of such a shaped body is that it in the production The reference electrode can be easily placed in the housing of the reference electrode.

In one embodiment, the ratio of the longitudinal extent to the maximum diameter of the shaped body is more than 2: 1, preferably at least 5: 1, particularly preferably 10: 1. In an advantageous embodiment, the shaped body is designed at least in sections, preferably over its entire longitudinal extent, as a cylinder, in particular as a rod, as a channel or as a prism.

The shaped body can be arranged within the housing chamber such that its longitudinal extension extends at least over 50%, preferably at least over 70%, particularly preferably at least over 90%, of the length of the reference element immersed in the reference electrolyte, in particular parallel to the reference element.

The reference element may be an electrically conductive metal, e.g. an electrically conductive metal wire, in particular a silver wire include. The metal wire may have, at least in one section, a layer of a sparingly soluble salt, in particular a halide salt, of the metal from which the metal wire is formed. The sparingly soluble salt has the same anion as the electrolyte salt.

The reference element can be formed, for example, from a silver wire having a silver chloride layer at its front end facing the transfer. The electrolyte salt can be, for example, potassium chloride KCl, the shaped body consists in this case of potassium chloride and is preferably free of additives. The reference electrolyte may be a saturated potassium chloride solution, which may optionally contain other additives.

A housing chamber containing the reference electrolyte may be formed in the housing of the reference electrode, wherein the reference element is electrically conductively connected to an electrical contact point arranged outside the housing chamber and protrudes so far into the housing chamber that a portion of the reference element is accommodated in the reference electrolyte accommodated in the housing chamber dips.

In one embodiment of the reference electrode, the housing chamber is cylindrical and the longitudinal extent of the molded body extends parallel to the cylinder axis of the housing chamber. A reference electrode designed in this way can, for example, be combined with a separate measuring electrode to form a potentiometric or amperometric measuring chain.

In another embodiment, the housing chamber may be configured as an annular chamber formed between two mutually concentrically arranged tubular walls, wherein the molded body is disposed within the annular chamber and the longitudinal extent of the molded body extends parallel to one or both tube axes of the tubular walls. In this embodiment, the reference electrode may be part of a combination electrode, wherein the housing chamber of the reference electrode surrounds a rod-shaped measuring electrode annular and the inner tubular wall is preferably simultaneously a wall of the measuring electrode.

In the housing or in the housing chamber, a holding device, in particular a spacer, may be arranged, by means of which the molded body is fixed in a predetermined orientation, in particular a predetermined distance relative to the reference element.

• – Einbringen eines mittels des weiter oben beschriebenen Verfahrens nach einer der weiter oben beschriebenen Ausgestaltungen hergestellten Formkörpers in ein Bezugselektrodengehäuse;
• – Einbringen mindestens eines Abschnitts eines elektrisch leitfähigen Bezugselements in das Bezugselektrodengehäuse, und
• – Einbringen eines Bezugselektrolyten in das Bezugselektrodengehäuse, so dass mindestens ein Abschnitt des Bezugselements und mindestens ein Abschnitt des Formkörpers mit dem Bezugselektrolyten in Kontakt stehen.

The invention also includes a method for producing a reference electrode, in particular a reference electrode according to one of the embodiments described above. This procedure includes the following steps:

• - Introducing a molded article produced by the method described above according to one of the embodiments described above in a reference electrode housing;
• - Introducing at least a portion of an electrically conductive reference element in the reference electrode housing, and
• - Introducing a reference electrolyte in the reference electrode housing, so that at least a portion of the reference element and at least a portion of the molding body are in contact with the reference electrolyte.

The step of introducing the shaped body may precede the production of the shaped body according to the method described above according to one of its embodiments. In this case, the finished molded body is introduced into the reference electrode housing.

In another embodiment, the molten salt for producing the molded body in the reference electrode housing or in a arranged within the reference electrode housing, in particular designed as a losbarer form, cast mold and allowed to solidify in the mold. Alternatively, salt present in the form of solid particles may be introduced into the mold disposed within the reference electrode housing and melted in the mold to produce the molten salt. Subsequently, in this process variant, the molten salt in the mold solidified within the reference electrode housing.

The invention also includes an electrochemical sensor comprising a reference electrode according to one of the above-described embodiments. The electrochemical sensor may comprise a further electrode, in particular an ion-selective electrode, e.g. a pH glass electrode, and a measuring circuit in electrical contact with the reference electrode and the further electrode.

The measuring circuit may, depending on the type of electrochemical sensor for detecting a potential difference between the reference electrode and the further electrode (potentiometry) or for setting a desired voltage between the reference electrode and the further electrode and for detecting a current flowing between the further electrode and an additional counter electrode current ( Amperometry). The measuring circuit may be further configured to generate from the detected measured variable, that is to say, for example, the potential difference between the reference electrode and the further electrode or the current strength of the current flowing between the further electrode and the counter electrode, a measuring signal representing the measured variable, if necessary further processing , eg to digitize, and output the measurement signal or a further processed measurement signal to a higher-level unit. The higher-level unit can be a transmitter, a computer, a programmable logic controller or a process control computer.

The electrochemical sensor may in particular be a potentiometric pH sensor, which further comprises at least one pH glass electrode, and a measuring circuit in electrically conductive contact with the reference electrode and the further electrode, which is designed for a potential difference between the pH glass electrode and the To detect reference electrode and output a dependent measurement signal.

In the following the invention will be described in more detail with reference to the embodiments illustrated in the figures.

Show it:

1 a schematic longitudinal sectional view of a reference electrode;

2 a schematic representation of a potentiometric sensor with a measuring electrode and a reference electrode;

3 the production of a shaped body according to a first method;

4a) b) the production of a shaped article according to a second method;

5 an apparatus for producing a plurality of moldings;

6 a first example of an electrolyte salt molded body;

7 A second example of an electrolyte salt molding.

1 shows a schematic longitudinal sectional view through a reference electrode 1 , The reference electrode 1 has a housing 2 of electrically insulating material, for example glass or plastic, in which the reference electrolyte 3 is included. In the wall of the housing 2 is as an overpass 6 Serving diaphragm of an open-pore solid, such as a ceramic or a plastic arranged. In an alternative embodiment, the transfer 6 be configured in the form of a gap. The housing 2 is destined, in its front, the overpass 6 be immersed in a measuring medium so that the reference electrolyte 3 about the overpass 6 is in electrolytic contact with the measuring medium. The electrolytic contact between the measuring medium and the reference electrolyte 3 allows the exchange of ions between the measuring medium and the reference electrolyte 3 , The housing 2 is backside, ie liquid-tightly closed at its end, which is intended for immersion in the measuring medium end region, in the present example by gluing 5 , Alternatively, the housing 2 also be sealed on the back. The reference electrolyte 3 in the present example is a saturated potassium chloride solution which is thickened by the addition of a polymer and has a gel-like nature. Alternatively, an aqueous saturated potassium chloride solution which is not thickened, ie free of polymer additives, can also be used as the reference electrolyte. In the reference electrolyte 3 dives a reference element 4 a, which may be configured for example as a chlorided silver wire. At the reference element 4 turns the potential of the reference electrode 1 which essentially depends on the concentration of the sparingly soluble silver chloride, as stated at the outset, and which therefore remains constant as long as the chloride concentration in the reference electrolyte is high enough. The reference element 4 is on the back by the gluing or fusion of the housing 2 led out and with a contact point 7 outside the case 2 is arranged, connected. About this contact point 7 can a measuring circuit (in 1 not shown) the potential of the reference electrode 1 tap.

In through the housing 2 formed chamber in which the reference electrolyte 3 is a rod-shaped, consisting of potassium chloride moldings 8th contained, which serves as an electrolyte salt supply. In the present example, the shaped body consists of pure potassium chloride. In contrast to known, serving as an electrolyte salt stock compacts contained in known from the prior art reference electrodes, this molded body is 8th free of excipients. The longitudinal extension of the molding 8th extends substantially parallel to the longitudinal axis of the housing 2 and to the longitudinal extent of the reference element 4 ,

In the example described here, the reference electrode is a silver / silver chloride reference electrode. The shaped body consists in the present example of potassium chloride, but can of course alternatively be formed from another chloride salt or from a mixture of several chloride salts. In general, the shaped body can be formed from another suitable salt or salt mixture, in particular from halide salts. The reference electrode can of course also be formed by another electrode of the second kind, in particular a calomel electrode. The material of the shaped body is then preferably adapted to its function as an electrolyte salt supply to the electrode of the second type.

The reference electrode 1 may be part of a potentiometric measuring arrangement, which further comprises a measuring electrode and a measuring circuit. The measuring circuit of the potentiometric measuring arrangement is designed to detect and output a potential difference between the measuring electrode and the reference electrode. The measuring electrode may comprise, for example, an ion-selective membrane or a pH-sensitive glass membrane, at which a potential dependent on a concentration of the analyte in the measuring medium is established in contact with the measuring medium. The measuring electrode can not in one with the housing 2 the reference electrode 1 be arranged connected housing.

The reference electrode 1 may be part of an amperometric measuring arrangement in another embodiment, in addition to the reference electrode 1 a working electrode and a counter electrode and a measuring circuit electrically connected to the reference electrode, the working electrode and the counter electrode. For measurement, the reference electrode, the working electrode and the counter electrode are brought into contact with the measuring medium. In the amperometric measuring arrangement, the measuring circuit can comprise a potentiometric control circuit which is designed to predetermine a setpoint voltage between the working electrode and the non-current-carrying reference electrode, to detect the current intensity of the current flowing through the measuring medium when the setpoint voltage is set, and a measurement signal dependent thereon to generate and output.

During operation of the reference electrode 1 gets through the overpass 6 gradually discharged from the solution chloride. At the same time solid potassium chloride goes out of the molding 8th in solution, leaving the reference electrolyte 3 not depleted of chloride. In this way, the potential of the reference electrode remains 1 stable. The resolution of the rod-shaped molding 8th starts at the end of the rod, that of the overpass 6 is facing. From this end, the molding dissolves 8th gradually on, giving it over the life of the reference electrode 1 getting shorter and shorter. This is the length of the rod-shaped molding 8th also an indicator of the remaining life of the reference electrode 1 , because after complete dissolution of the molding 8th an increased drift of the reference electrode potential is to be expected. This increased drift not only requires more frequent calibration but also results in the potential of the reference electrode 1 After the foreseeable future, it is so far away from the original reference potential that a calibration is no longer possible and the reference electrode must therefore reach its end of life and be replaced.

During the gradual dissolution of the molding 8th The remains in an area between the overpass 6 facing the end of the molding 8th and the overpass 6 present reference electrolyte 3 essentially saturated with chlorine. Also in one of the overpass 6 removed back area of the housing 2 , especially in an area between the transfer 6 facing the end of the molding 8th and the level of the reference electrolyte 3 or the back-side bonding 5 the reference electrolyte is depleted 3 not to chloride, because the molding 8th extends far into this area. In this way, concentration gradients over the length of the housing 2 or the reference element 4 leading to damage of the reference element 4 could lead to avoided.

2 shows a schematic representation of a potentiometric Einstabmesskette 10 designed pH sensor, which is a measuring electrode 12 and a reference electrode 11 includes. The measuring electrode 12 is in a tubular housing part 13 made of an insulating material, such as glass or plastic, taken at one end by a pH-sensitive glass membrane 14 is completed. In the housing part 13 is as an inner electrolyte 15 a potassium chloride-containing buffer solution was added, in the one discharge element 16 that can be made of a metal wire dips. In the present example, the diverting element 16 a chlorided silver wire. The reference electrode 11 is concentric around the measuring electrode 12 arranged around. It comprises a through an outer, tubular housing part 17 made of an insulating material and the tubular housing part 13 formed housing. The housing part 17 is at its the pH-sensitive glass membrane 14 facing end with the tubular housing part 13 the measuring electrode 12 connected liquid-tight. In the annular housing chamber thus formed is a reference electrolyte 20 included in the the reference element 19 dips. The reference element 19 may be formed by a chlorided silver wire. The reference electrolyte 20 is in operation of the pH sensor via a passage opening in the outer tubular housing part 17 trained overpass 21 with a the combination electrode in its front end area, the overpass 21 and the pH-sensitive glass membrane 14 includes, surrounding measuring medium in contact. At his the glass membrane 14 opposite back end is the housing of the combination electrode by a bond 22 locked. The reference element 19 and the diverting element 16 are each about a arranged outside the housing pad 23 . 24 with a measuring circuit 25 connected. The measuring circuit 25 is designed to have a potential difference between the measuring electrode 12 and the reference electrode 11 to capture and output a measuring signal representing the potential difference. The measuring circuit can be connected to a higher-level unit, for example a measuring transducer, to which it outputs the measuring signal.

The reference electrolyte 20 is in the present example a thickened by means of a polymer, saturated potassium chloride solution. Within through the outer housing part 17 and the housing part 13 formed annular chamber is a rod-shaped body 18 arranged of potassium chloride, whose longitudinal axis substantially parallel to the tube axis of the housing parts 13 and 17 runs. The length of the molding 18 in the present example is approximately equal to the length of the reference electrolyte 13 dipping section of the reference element 19 , The length of the shaped body is advantageous 18 at least between 50 and 100% of the length of the reference element immersed in the reference electrolyte 19 , In the example shown here is the molding 18 of pure potassium chloride, ie it is, apart from negligible, unavoidable impurities, free of excipients and has a density which corresponds approximately to the total crystal density.

As based on the in 1 illustrated reference electrode 1 described, serves the molding 18 as an electrolyte salt reservoir from which via the transfer 21 from the reference electrolyte 20 exiting chloride ions are replaced. As with the in 1 example shown, the resolution of the molding 18 from his the overpass 21 first end lying as a visually detectable indicator for the remaining life of the sensor 10 serve.

In an advantageous variant, a holder can be arranged in the rear region of the annular chamber, by means of which the molded body 18 in a predetermined position, for example at a certain distance from the reference element 19 , is fixable. In a further variant, it is possible on the wall of the outer tubular housing part 17 to attach a scale, on the basis of which a dissolution rate of the molding 18 can be determined. This allows extrapolation to determine a remaining safe residual life of the reference electrode 11 ,

In 3 is a method of making one for use in the reference electrode according to 1 or in the electrochemical sensor according to 2 suitable shaped body of salt illustrated. In the process variant shown here, one in a crucible 31 contained molten salt 32 in a mold 33 cast. In the present example, there is the molten salt 32 from up to negligible impurities pure potassium chloride without further additives. The molten salt 32 is generated in a first step by pure, solid potassium chloride, for example in the form of a variety of crystalline salt particles, in the crucible 31 is given and melted therein by heating to a temperature above 770 ° C, preferably about 20 ° C above the melting point. Preferably, the heating is carried out at a temperature between 780 and 800 ° C, preferably between 785 and 795 ° C. The heating can be done in an oven. It is for example possible to use a crucible made of an electrically conductive material and to heat it by induction. However, the heating can also be effected by means of other means known per se for heat transfer, in particular by radiation, for example with laser heating, and convection, for example in a muffle furnace. Since molten potassium chloride chemically attacks a large number of metals, ceramic or a high-melting glass, for example quartz glass, is particularly suitable as the crucible material. As electrically conductive crucible material special stainless steels or inert metals such as platinum or tantalum, in question.

The molten salt thus formed 32 will in a second step in the form 33 cast. In the present example, the shape 33 a groove-shaped recess in which the molten salt 32 is recorded. Suitable molding materials have a high heat absorption capacity. Form 33 in the example shown here consists of a temperature-resistant, largely inert to the molten salt material, for example made of stainless steel, an inert metal, ceramic or quartz glass. Alternatively, the mold may also be formed as a loseable mold, for example of a pourable material such as sand or ceramic powder.

In a third process step is in the form 33 poured molten salt 32 stiffened. This can be done either by allowing the mold to stand for some time at room temperature (25 ° C) or another predetermined temperature, which is below the solidification point of the molten salt. On cooling, for example, to room temperature, the salt melt cools 32 From and solidifies into a transparent solid, whose shape through the groove-shaped recess of the mold 33 is predetermined. The cooling can be accelerated in a process variant in that a material of high thermal conductivity is selected as the molding material. At the same time, the mold may have areas of high surface area, such as cooling fins, to provide even better heat dissipation. Additionally or alternatively, the mold can be actively tempered, in particular cooled. It is also possible to pass through a predetermined cooling profile by means of the active temperature control. The active temperature control can be effected by means of a cooling medium flowing around or through the mold (cooling liquid, cooling gas) or else by means of thermoelectric elements (Peltier elements). During cooling, the mold may be replaced by a plate (not in 3 represented), which prevents deformation of the solidified in the trough-shaped recess molding. After solidification of the molten salt to form the molding, the shape 33 be removed.

In 4a and b is an alternative manufacturing process for producing a shaped body of salt. In this example, in a first step ( 4a ) granular salt 41 in a mold 43 given. In a second step, the salt 41 in the shape 43 melted ( 4b ). In a third step, the molten salt is formed 42 stiffened. In the present example is called salt 41 pure KCl used as a form 43 serves a tube of quartz glass. It is understood that a mold made of another material can also be used as the mold for this process, for example one of those described above in connection with FIG 3 shown method variant of the method according to the invention mentioned materials. The heating to melt the salt 41 and the cooling to solidify the molten salt to the molding can be carried out in an analogous manner as described in reference to 3 described method variant described. After solidification of the molten salt to form the molding, the mold can be removed.

The molded article produced by one of the process variants described above, in particular a molded article produced from pure KCl without further additives, is transparent. Its density is close to the density of the crystalline salt, namely between 90 and 100%, or even between 99 and 100% of the density of the crystalline salt from which the shaped body is formed. In the case of potassium chloride, the density of the molded article thus produced is between 1.8 and 1.98 g / cm ³ , or even between 1.9 and 1.98 g / cm ³ . A density in this area can not be achieved by pressing. With potassium chloride, for example, a density of 1.5 g / cm ^{3 is} achieved by pressing. It has been shown in experiments that this shaped body dissolves much slower in water than a salt compact of the same geometry. This is accompanied by a higher mechanical strength of the solidified from the molten salt molding in comparison to a compact.

In 5 a device for the simultaneous production of a plurality of shaped articles made of salt is shown. It is particularly suitable for use in mass production of moldings and reference electrodes with such moldings. The device comprises a base body 54 in which a multitude of channel-shaped recesses 53 are formed, each forming a mold for producing a shaped body. To produce the plurality of moldings, either solid salt is introduced into all molds and melted in the molds or a molten salt is poured into the molds. In the latter case, the single recess 53 be connected to each other by channels, so that a poured into a recess in one place molten salt gradually into all the recesses 53 spreads. This facilitates automation of the process. For tempering the body 53 For example, for heating and / or cooling in the recesses 53 contained solid or molten salt is at the body 54 a tempering 56 attached, which may comprise, for example, one or more thermoelectric elements or a resistance heater or other means for heating and optionally also for cooling. The main body 53 in this case ideally consists of a material with high thermal conductivity.

The in the recesses 53 present molten salt is allowed to solidify by cooling. During cooling can be a counter plate 55 against the main body 54 be applied to a deformation in the recesses 53 To prevent by solidification of the salt forming moldings.

For the production of in 1 The reference electrode shown is a molded body produced by one of the process variants described herein in the housing 2 introduced the reference electrode. Before or after, a portion of the reference element having a silver chloride layer is formed 4 in the case 2 brought in. In a further step, the reference electrolyte 3 in the case 2 brought in. The housing 2 is closed at the back by gluing or merging, the reference element 4 or conductive with the reference element 4 connected electrical conductor is passed through the resulting bonding or merger point. The sequence of the method steps mentioned here does not matter.

For the production of in 2 illustrated electrochemical sensor reference element, a prepared according to the method described above in one of said variants salt moldings and reference electrolyte in a very analogous manner as described above in the housing part 17 brought in. In the housing part 13 becomes the inner electrolyte and a portion of the discharge element 16 brought in. The housing of the sensor is closed at the back by fusing or gluing, wherein the reference element 19 and the diverting element 16 or an electrically conductive with the reference element 19 connected electrical conductor and / or electrically conductive with the discharge element 16 connected electrical conductor is led out through the resulting bonding or merger point through the housing. The reference element 19 and the diverting element 16 or, if present, the electrical conductors connected to the reference or the diverting element become electrically conductive, for example by means of soldering or other contacting techniques, with the measuring circuit 25 connected.

In a further variant of the method for producing a reference electrode with a salt molded body, it is conceivable to introduce a molten salt directly into the housing of the later reference electrode. This can be done by the molten salt is poured directly into the housing or poured into a formed in the housing, in particular losable, mold, which can optionally be removed after solidification of the molten salt to form a shaped body. In this case, it is advantageous to form the housing from quartz glass and to pre-temper before pouring the molten salt to prevent stress cracks.

In another variant, it is conceivable to introduce the salt in the form of solid particles into the housing of the later reference electrode, which may be formed of quartz glass, for example.

In this case, the salt can be introduced into a chamber of the housing or into a form arranged within the housing. The housing is advantageously also in this process variant of quartz glass. Subsequently, the salt is heated in the housing and melted. This can be done in an oven with the means described above. After melting, the salt melt can be cooled again with the means described above, so that it solidifies within the housing to form a molded body. In the case that the salt particles were previously introduced into a arranged in a chamber of the housing mold, this can optionally be removed after the solidification of the molten salt again.

After the solidification of the shaped body ( 3 . 4 ) or the many shaped bodies ( 5 ) This or these are for protection against environmental influences under vacuum or under a dry atmosphere, especially under inert gas, advantageously airtight packaged. This is particularly advantageous with hygroscopic salts, such as potassium chloride, which tend to pick up water from the atmosphere. Advantageously, therefore, the entire process for producing one or a plurality of moldings can be carried out under a protective gas atmosphere.

In 6 is an example of a molded article 8th shown for use as electrolyte salt reservoir in a reference electrode according to the in 1 or after the in 2 illustrated embodiment is suitable. The molded body 8th is cylindrical with a circular cylinder base area configured. It has a rotational symmetry axis A. The longitudinal extent L of the molding 8th corresponds to the length measured along this rotational symmetry axis A. The molded body 8th furthermore has a diameter D, which in the present example is the diameter of a cross-sectional area of the shaped body, in particular the cylinder base area, perpendicular to the rotational symmetry axis A. The diameter D passes through the intersection S of the cylinder base area. The ratio L: D of the longitudinal extent L of the molding 8th for example, the diameter D is at least 5: 1, preferably at least 10: 1.

There are many other geometrical configurations of such for use in a reference electrode after in 1 or after the in 2 illustrated embodiment suitable shaped body conceivable. For example, the shaped body can be designed as a cylinder with a non-circular base area or as a prism. The molded body can also be configured as a ring.

In 7 is another embodiment of a shaped body 8th represented by electrolyte salt, which is designed as a groove. This embodiment is well suited for use in a reference electrode of a potentiometric combination electrode like the one in FIG 2 shown. The channel may in this case be configured and arranged to form part of the inner tubular housing part 13 , which forms the wall of the measuring electrode, surrounds.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• "Working with Ion-Selective Electrodes", K. Cammann, H. Galster, Springer, 1996 [0004]

Claims (14)

Method for producing a shaped article ( 8th . 18 ) comprising: introducing a salt melt, in particular a salt melt formed from one or more salts ( 32 . 42 ) into a form ( 33 . 43 . 53 ); and - solidification of the molten salt ( 32 . 42 ) to form the, in particular one-piece, shaped body ( 8th . 18 ) in the shape ( 33 . 43 . 53 ). Process according to claim 1, wherein the introduction of the molten salt ( 32 . 42 ) in the form of: - generating the molten salt ( 32 ) by melting a salt present in the form of one or more solid particles, in particular a halide salt, or several salts present in the form of one or more solid particles, in particular a plurality of halide salts; and - pouring the molten salt ( 32 ) in the form ( 33 ). Process according to claim 1, wherein the introduction of the molten salt ( 32 . 42 ) in the form of: introducing one in the form of one or more solid particles ( 41 ) present salt, in particular a halide salt, or more present in the form of one or more solid particles salts, in particular a plurality of halide salts, in the form ( 43 ); and - melting the salt or salts in the mold ( 43 ). Method according to one of claims 1 to 3, further comprising: - removing the mold ( 33 . 43 . 53 ) after solidification of the molten salt to form the shaped body. Method according to one of claims 1 to 4, wherein the mold ( 53 ) in a basic body ( 54 ) is formed in which a plurality of other similar forms ( 53 ), and wherein the molten salt from an automatically pivoting crucible, in particular in a single step, in the forms ( 53 ) is poured. Method according to one of claims 1 to 5, wherein the temperature of the molten salt ( 32 . 42 ) and / or the shape ( 33 . 43 . 53 ), in particular when solidifying, is controlled or regulated according to a predetermined temperature profile. Reference electrode ( 1 . 11 ), comprising: - a housing ( 2 . 17 ), in which a reference electrolyte ( 3 . 20 ) which comprises an electrolyte salt, in particular an alkali metal halide salt; - a reference electrolyte ( 3 . 20 ) contacting, electrically conductive reference element ( 4 . 19 ); - one in a wall of the housing ( 2 . 17 ) arranged transfer ( 6 . 21 ), via which the reference electrolyte ( 3 . 20 ) with a housing ( 2 . 17 ) surrounding medium is in electrolytic contact; and - one in the housing ( 2 . 17 ) containing electrolyte salt; wherein the electrolyte salt stock comprises at least one shaped body consisting of at least one salt, in particular the electrolyte salt ( 8th . 18 ) prepared according to the process of claims 1 to 6. Reference electrode ( 1 . 11 ) according to claim 7, wherein the shaped body ( 8th . 18 ) has a longitudinal extent (L) which is greater than a maximum diameter (D, D _max ), in particular outer diameter, of the shaped body extending perpendicular to this longitudinal extent (L) ( 8th . 18 ). Reference electrode according to claim 7 or 8, wherein in the housing a reference electrolyte ( 3 . 20 ) housing body is formed, and wherein the reference element ( 4 . 19 ) with an outside of the housing chamber arranged electrical contact point ( 7 . 23 ) is electrically conductively connected, and protrudes so far into the housing chamber, that a portion of the reference element ( 4 . 19 ) in the reference electrolyte accommodated in the housing chamber ( 3 . 20 immersed). Reference electrode ( 1 . 11 ) according to one of claims 7 to 9, wherein the shaped body ( 8th . 18 ) is arranged within the housing chamber such that its longitudinal extension is at least over 50%, preferably at least over 70%, particularly preferably at least over 90%, of the length of the reference electrolyte ( 3 . 20 ) dipping portion of the reference element ( 4 . 19 ), in particular parallel to the reference element ( 4 . 19 ), runs. Method for producing a reference electrode ( 1 . 11 ), in particular according to one of claims 7 to 10, comprising: - introducing a shaped body produced by means of the method according to one of claims 1 to 6 ( 8th . 18 ) in a reference electrode housing ( 2 . 17 ); Inserting at least a portion of an electrically conductive reference element ( 4 . 19 ) in the reference electrode housing ( 2 . 17 ); and - introducing a reference electrolyte ( 3 . 20 ) in the reference electrode housing ( 2 . 17 ), so that at least a portion of the reference element ( 4 . 19 ) and at least a portion of the shaped body ( 8th . 18 ) with the reference electrolyte ( 3 . 20 ) stay in contact. A method according to claim 11, wherein a molten salt for producing the shaped body is arranged in the reference electrode housing or in a housing disposed within the reference electrode housing. designed in particular as a losbarer form, poured mold and allowed to solidify in the mold. Electrochemical sensor ( 10 ) comprising a reference electrode ( 11 ) according to one of claims 7 to 10. Electrochemical sensor ( 10 ) according to claim 13, further comprising at least one further electrode ( 12 ), in particular an ion-selective electrode, and one with the reference electrode ( 11 ) and the further electrode ( 12 ) in electrically conductive measuring circuit ( 25 ).

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