REFERENCE HALF-CELL AND ELECTROCHEMICAL SENSOR WITH THE REFERENCE HALF-CELL

Abstract

A reference half-cell for application in an electrochemical sensor, comprising a housing, in which a chamber containing a reference electrolyte is formed, wherein the reference electrolyte is in contact with a medium surrounding the housing via a liquid junction arranged in a wall of the housing, wherein the liquid junction comprises a porous diaphragm, and wherein the diaphragm has, at least partially, a coating, which comprises at least one metal.

Claims

1. A liquid junction for use in a reference half-cell, comprising: a liquid-permeable solid body comprising an electrically non-conductive first material and an electrically conductive second material, wherein the body is formed primarily of the first material, and wherein the second material is distributed sufficiently uniformly on and within the body, whereby the liquid-permeability of the body is reduced and an electrical resistance of the body is reduced by the distributed second material.

2. The liquid junction of claim 1, wherein the first material is selected from the group consisting of zirconium dioxide, ceramic, glass, and PTFE.

3. The liquid junction of claim 1, wherein the second material is selected from the group consisting of copper, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.

4. The liquid junction of claim 1, the solid body further comprising an electrically conductive third material, wherein the third material is distributed sufficiently uniformly on and within the body, whereby the liquid-permeability of the body is reduced and the electrical resistance of the body is reduced by the distributed third material, and wherein the third material is selected from the group consisting of copper, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.

5. The liquid junction of claim 1, wherein the solid body further comprises an adherence-promoting material composed of molecules having functional thiol groups, the adherence promoting material distributed uniformly on and within the body.

6. The liquid junction of claim 1, wherein the second material has a biocidal effect.

7. A reference half-cell for an electrochemical sensor, comprising: a housing defining a chamber containing a reference electrolyte; and a liquid junction disposed in a wall of the housing, the liquid junction comprising a diaphragm made of porous media, the diaphragm having an outer surface defining a volume, the volume including a plurality of pores within the media, each of the plurality of pores having an inner surface, wherein the diaphragm includes a conductive material distributed on the outer surface and on the inner surfaces of the plurality of pores, such that the conductive material at least partially covers the outer surface and the inner surfaces, wherein the conductive material includes at least one metal, and wherein the reference electrolyte is in contact via the liquid junction with a medium surrounding the housing.

8. The reference half-cell of claim 7, wherein the porous media is selected from the group consisting of zirconium dioxide, ceramic, glass, and PTFE.

9. The reference half-cell of claim 7, wherein the at least one metal is selected from the group consisting of copper, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.

10. The reference half-cell of claim 7, wherein the conductive material forms a coating at least partially covering the outer surface and the inner surfaces.

11. The reference half-cell of claim 10, wherein the coating has a biocidal effect.

12. The reference half-cell of claim 10, wherein the coating has a first layer composed of a first metal and a second layer composed of a second metal.

13. The reference half-cell of claim 7, further comprising: an electrode of a second type contained in the chamber and configured to act as a potential forming system.

14. The reference half-cell of claim 13 wherein the electrode is a silver/silver halide electrode.

15. An electrochemical sensor for measuring at least one measured variable of a measured medium, comprising: at least one reference half-cell, the reference half-cell comprising: a housing defining a chamber containing a reference electrolyte; and a liquid junction disposed in a wall of the housing, the liquid junction comprising a diaphragm made of porous media, the diaphragm having an outer surface defining a volume, the volume including a plurality of pores within the media, each of the plurality of pores having an inner surface, wherein the diaphragm includes a conductive material distributed on the outer surface and on the inner surfaces of the plurality of pores, such that the conductive material at least partially covers the outer surface and the inner surfaces, wherein the conductive material includes at least one metal, and wherein the reference electrolyte is in contact via the liquid junction with a medium surrounding the housing.

16. The electrochemical sensor of claim 15, wherein the material forms a coating at least partially covering the outer surface and the inner surfaces.

17. The electrochemical sensor of claim 15, further comprising: at least one sensorially active component in contact with the measured medium during measuring having a property changing as a function of a measured variable to be determined; and a measuring circuit interacting with the reference half-cell and the at least one sensorially active component and embodied to produce a measurement signal dependent on the property of the sensorially active component representing the measured variable.

18. The electrochemical sensor of claim 17, wherein the sensorially active component comprises one of: a metal redox electrode, a non-metallic redox electrode, a surface of a metal-metal oxide electrode, an ion-selective field effect transistor, or an ion-selective glass membrane.

19. The electrochemical sensor of claim 15, further comprising: a working electrode and a counter electrode, which, with the reference half-cell, form a three-electrode arrangement; and a control circuit that is embodied to set a predetermined voltage between the counter electrode and the reference half-cell and to register the electrical current flowing through the measured medium between the counter electrode and the working electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The invention will now be described in greater detail based on the examples of embodiments illustrated in the drawing, the figures of which show as follows:

[0040] FIG. 1 is a schematic representation of a reference half-cell with a diaphragm, which has a coating comprising at least one metal;

[0041] FIG. 2 is a schematic representation of a pH measuring chain in the form of a combination electrode with a reference half-cell, whose diaphragm has a coating comprising at least one metal;

[0042] FIG. 3 is a graph, in which is presented for proof of a biocidal effect of a silver coating of a ceramic diaphragm of a reference electrode, the measurement voltage curves of pH combination electrodes with diaphragms coated in such a manner and, for comparison, the measurement voltage curves of pH combination electrodes with conventional diaphragms in measurement operation in a fermenter;

[0043] FIG. 4 is a graph for illustrating the stirring dependence of the potential output from different reference half-cells as a function of the conductivity of a measured medium;

[0044] FIG. 5 is a graph, in which is presented the lessening of stirring dependence of various reference electrodes with coated ceramic diaphragm in comparison to a conventional ceramic diaphragm;

[0045] FIG. 6 is a graph, in which are presented experimentally ascertained flow and experimentally ascertained diaphragm resistance for different coated ceramic diaphragms.

DETAILED DESCRIPTION

[0046] FIG. 1 shows a schematic representation of a reference half-cell 1 with a rod-shaped, essentially cylindrical housing 3, in which a chamber is formed filled with a reference electrolyte 5. The housing 3 can be formed of an electrically insulating material, for example, glass or a synthetic material, such as a plastic. Reference electrolyte 5 is, for example, an aqueous 3 molar KCl solution or some other alkali metal halide solution. Reference electrolyte 5 is, via a liquid junction 9 formed by a porous diaphragm 10 arranged in the wall of the housing 3, in ionically conducting contact with a measured medium 13 in which the reference half-cell 1 is at least sectionally immersed for performing electrochemical measurements, for example, potentiometric or amperometric measurements. Diaphragm 10 can be formed, for example, of a ceramic material, a porous glass material or a porous synthetic material, e.g. the plastic, Teflon. Extending into the reference electrolyte 5 is a potential sensing element 7 in the form, for example, of a chlorided silver wire. The potential sensing element 7 forms, with the reference electrolyte 5 in contact via the liquid junction 9 with the measured medium 13, a reference electrode of second type, in the present example an Ag/AgCl, reference electrode, which outputs via the connection 11 of the potential sensing element 7 a stable potential independent of the pH value or other ion concentrations of the measured medium.

[0047] Diaphragm 10 includes, for lessening the stirring-, or liquid flow, dependence of the reference half-cell potential tappable at the connection 11 and for lessening of the outflow of reference electrolyte from the housing 3 into the measured medium 13, a coating, which comprises at least one metal. The coating can be a purely metal coating of one or more metal layers with metallic properties. If the coating is a plurality of individual layers, different individual layers can be of different metals. Advantageously, the coating has especially a certain electrical conductivity.

[0048] The coating effects, as already described above, a narrowing of the pores of the diaphragm and, so, lessens the flow of reference electrolyte out through the diaphragm 10. As can be shown based on measurements further discussed below, the diaphragm resistance is not significantly increased, in spite of the pore narrowing. If suitable biocidally acting metals are used for the coating, such as, for example, silver or copper, the coating can simultaneously serve to prevent growth of microorganisms in and on the diaphragm 10 and thereby avoid related disturbances of the reference half-cell, for example, a drifting of the potential or even a complete failure of the reference half-cell 1.

[0049] Metals suitable for the coating include, for example, silver, gold, platinum, copper, iridium, osmium, palladium, rhodium and ruthenium. Metals with biocidal effect include, for example, silver or copper.

[0050] FIG. 2 shows, schematically, a potentiometric pH combination electrode 100 as an example of an electrochemical sensor with a reference half-cell. Combination electrode 100 includes a reference half-cell 101 and a measuring half-cell 115, which are formed in a shared, rod-shaped housing of an insulating material, for example, glass. The housing includes an inner tube 123, in which measuring half-cell 115 is formed. The inner tube 123 is closed on its front end by a pH-sensitive, glass membrane 117, on which forms, in contact with the measured medium 113, a potential dependent on the pH value of the measured medium 113. Contained in the inner tube 123 is an inner electrolyte 119, for example, a pH buffer solution, into which a potential sensing element 121 extends. The potential sensing element 121 can be formed, for example, of a chlorided silver wire.

[0051] The reference half-cell 101 is arranged in an annular chamber, which is formed between the inner tube 123 and an outer tube 103 surrounding the inner tube 123. Outer tube 103 is connected at its front end in the region of the membrane 117 with the inner tube 123. Contained in the annular chamber is a reference electrolyte 105, for example, a 3 molar KCl solution, into which extends a potential sensing element 107 formed by a chlorided silver wire. Arranged in the outer tube 103 as liquid junction between the reference electrolytes 105 and a measured medium 113, into which the combination electrode 100 extends in measurement operation, is a diaphragm 110, which has a coating, especially a metallic coating, comprising at least one metal. The coating can be embodied in the same manner as the coating of the diaphragm 10 described in connection with the reference half-cell 1 of FIG. 1.

[0052] On its back end facing away from the measured medium 113, the combination electrode 100 is provided with a suitable plug 129, which seals the inner tube 123 and the annular chamber formed between inner tube 123 and outer tube 103 to liquid. The potential sensing elements 121, 107 of the measuring- and reference half-cells are led through the plug 129 by means of cable guides 111, 125. Potential sensing elements 121, 107 are connected with a measuring circuit 127, which registers the potential difference tappable on the potential sensing elements 107, 121 and representing the pH value of the measured medium 113. In given cases, measuring circuit 127 transforms and/or amplifies the potential difference to be output as measurement signal of the combination electrode 100. In such case, the measuring half-cell 115 represents a first galvanic half element, which is in contact with the measured medium 113 via the pH-sensitive glass membrane 117, while the reference half-cell 101 forms a second galvanic half element. The potential difference registerable between the potential sensing elements 107, 121 corresponds thus to the galvanic cell voltage of the combination electrode 100. The galvanic cell voltage depends on the pH value of the measured medium 113.

[0053] Measuring circuit 127 can be embodied as an electrical or electronic circuit arranged in a plug head (not shown) of the combination electrode 100. In such case, the plug head of the combination electrode 100 can be connected by means of a cable connection, or wirelessly, for communication with a superordinated unit, for example, a measurement transmitter, a computer or a process control station. Measuring circuit 127 is correspondingly embodied to output the measurement signal to the superordinated unit, which processes the measurement signal, outputs such via a user interface, stores and/or forwards such to another superordinated unit, e.g. a process control station.

[0054] Different method variants for forming an at least one metal-comprising, especially metallic, coating of a diaphragm will now be described. Such variants are suitable, for instance, for manufacturing a diaphragm for a reference half-cell of FIG. 1 or for a reference half-cell embodied as a component of a combination electrode of FIG. 2. As explained above, a reference half-cell of the form shown in FIG. 1 can be applied advantageously in a large number of different electrochemical sensors, especially in potentiometric sensors or in amperometric sensors.

[0055] Common to all of the here described method variants is that a porous substrate, for example, a diaphragm already held in a housing wall of a reference half-cell blank, or a porous stock, from which later a diaphragm is to be manufactured, is supplied with a solution, from which a metal-comprising coating, especially a metallic coating, will be deposited on the surface of the diaphragm. The terminology, surface of the diaphragm refers here to the surface of the porous structure, thus also to the inner walls of the pores of the diaphragm.

[0056] In a first method variant, an uncoated diaphragm can be provided, which is already connected by material bonding with a housing wall of a tubular blank, from which later a reference half-cell of FIG. 1 or a reference half-cell of a combination electrode of FIG. 2 will be produced. In order to supply the diaphragm with the solution, from which a metal coating will be deposited on the surface of the diaphragm, the blank can be so immersed into the solution, that the diaphragm is wetted thereby. In order to assure that the solution also penetrates into the pores of the diaphragm, so that the coating also is formed at least on the pore walls of the larger pores, the solution can be drawn through the pores of the diaphragm into the housing interior of the blank by producing a negative pressure in the tubular housing of the blank.

[0057] With this method variant, an option is to provide not only the diaphragm, but, instead, also the entire liquid immersed, outer surface of the reference half-cell housing with a metal coating. This can be desired, when a biocidal effect of the coating is important for the planned application of the reference half-cell. If the biocidal coating covers a surface region of the reference half-cell beyond that of the diaphragm, then biofouling, i.e. the growth of an undesired biofilm on the outer surface of the reference half-cell, can be more effectively suppressed.

[0058] In another method variant, for example, cylindrical, porous stock is supplied with the liquid, especially immersed into such, and the metal coating deposited from this solution onto the surface of the stock. The coated stock can then be divided into individual segments, each of which can then be welded as diaphragm into its own rod-shaped housing blank to form a reference half-cell of FIG. 1 or FIG. 2. This method variant is suited preferably for batch production of reference half-cells, in the case of which a coating of only the diaphragm is desired. Especially, in this method, by means of a single coating step, a plurality of coated diaphragms can be produced, which then can be mounted in a corresponding number of reference half-cell blanks by welding or adhesion.

[0059] Also in this method variant, it is advantageous to draw the liquid containing the metal to be deposited into the pores of the stock by applying a negative pressure, in order to assure that the metal is deposited also within the pores. Alternatively, a better wetting of the pore interiors can be achieved in the case of both method variants also by means of an ultrasonic bath.

[0060] Suited as material for a ceramic substrate for manufacturing a reference half-cell with a coated ceramic diaphragm, independently of whether it is coated in the form of ceramic stock or in the form of a ceramic diaphragm already bonded in a wall of a reference half-cell blank, is, for example, CaO-stabilized ZrO.sub.2 with a pore diameter of 450 nm. Another suitable material is Y.sub.2O.sub.3-stabilized ZrO.sub.2 with a pore diameter of 1.8 m.

[0061] Before applying the coating, the surface of the diaphragm, or of the stock, can be thoroughly cleaned with a cleaning liquid, for example, with an organic solvent, such as acetone. In an optional, additional step, the surface can, after removal of the cleaning liquid, be cleaned and/or activated with piranha solution for the subsequent coating. Piranha solution is produced by mixing 98% sulfuric acid with 30% hydrogen peroxide solution in a volume ratio between 1:1 and 3:1. The piranha solution is subsequently removed by thorough re-rinsing with water. Then, a renewed cleaning step can occur, in which the diaphragm, or the porous stock, is rinsed with ethanol, in order to remove residual water possibly still in the pores. Following a terminal, drying step, the diaphragm, or the stock, is supplied with the coating solution.

[0062] A first option for depositing a metal-comprising coating is to supply the diaphragm with a metal salt solution and then to deposit, by a thermal and/or chemical treatment, the metal in the form of a layer on the diaphragm surface. For this, suited as metal salt solution for producing a platinum layer is, for example, hexachloroplatinic acid. This treatment can, in given cases, be repeated multiple times, in order to achieve a sufficient surface coating, or a sufficient coating thickness.

[0063] A second opportunity for depositing a metal coating is a galvanic deposition, in the case of which the cathode is in contact with a side of the porous substrate to be coated and is immersed in a metal salt solution accommodated in a container. The anode can advantageously be present in the container, for example, secured in or on a container wall. For galvanically producing a silver metal layer, the metal salt solution can be, for example, a silver nitrate solution and the anode can be a silver anode. Between the cathode and the anode is applied a voltage required for galvanic deposition of silver on the cathode. The silver deposited on the cathode grows from the contact between cathode and substrate through the pores of the diaphragm and forms, thus, a coating of the porous substrate within and outside of the pores.

[0064] A third option for depositing a metal coating on the substrate is an electrically currentless deposition, for example, for producing a platinum layer by means of a solution comprising hexachloroplatinic acid and ascorbic acid. In given cases, the substrate can be supplied multiple times, in sequence, with the solution, in order to assure a sufficient thickness of the coating.

[0065] A fourth option for depositing a metal coating on the substrate comprises applying a firing lacquer and subsequent firing of the lacquer for producing a metal layer. Such firing lacquers are commercially obtainable and are conventionally used to produce metal coatings on porcelain, e.g. for decoration of porcelain dishware. In order to assure that the coating also forms within the pores in the case of this method, the firing lacquer can be sucked through the porous structure. Firing leads to a metallic, conductive coating. In given cases, the applying and firing of the lacquer can be repeated multiple times.

[0066] Options include also combinations of these methods, in order, for example, to produce a coating of a plurality of individual layers, especially also of a plurality of individual layers, which comprise different metals. In this way, it is possible, with targeting, to establish a certain pore size or to provide a combination of a desired pore size and a desired maximal diaphragm resistance.

[0067] A biocidally acting coating can, moreover, be produced according to the following method: The cleaned and dried substrate, for example, a porous ceramic substrate, especially a ZrO.sub.2-ceramic, is supplied in a first step with a 2% solution of (3-mercaptopropyl)trimethoxy silane. In order to supply the surface of the substrate as completely as possible with the solution, also within the pores, the solution can, for example, be drawn through the substrate by suction. Thereafter, a rinse step with toluene can be performed. Then, the substrate is dried. The, substrate, silanized in this way, is, thereafter, stored for a certain time, preferably at least 24 hours, in order to achieve a degree of cross linking of the silane on the surface. Then, the substrate is, first of all, immersed in a 2% silver nitrate, dimethyl sulfoxide solution (AgNO.sub.3/DMSO solution) and the solution drawn through the substrate by suction, in order to achieve an as complete as possible supplying of the substrate surface, including the pores, with the solution. Immediately, then, the substrate is immersed in a 0.1 molar lithium borohydride, dimethyl sulfoxide solution (LiBH.sub.4/DMSO solution) and the solution drawn through the substrate by suction. The lithium borohydride reduces silver ions on or in the silane layer to elemental silver, which bonds with the thiol-functionalities of the silane layer. The bonding of the silver coating via the thiol groups of the silane layer to the thereunder lying substrate improves adhesion of the silver coating significantly compared with a direct bonding of the silver coating to the substrate surface. In this way, a significantly longer storability of the silver coating can be assured in comparison with a silver coating applied directly on the substrate, also in the case of difficult environmental conditions in measurement operation with the modified reference half-cell. For achieving a desired layer thickness on the substrate, it can, when required, be supplied multiple times alternately with the AgNO.sub.3/DMSO solution and the LiBH.sub.4/DMSO solution.

[0068] A series of experimental investigations with reference half-cells, respectively, electrochemical sensors, having a ceramic diaphragm coated with at least one metal will now be described in greater detail. The invention is, however, equally applicable for reference half-cells and sensors with porous diaphragms of other materials, especially for reference half-cells and sensors with synthetic material- or glass diaphragms.

[0069] Tests showed that Saccheromyces cerevisiae applied as test organism did not grow on ceramic diaphragms modified with a silver coating as above described. In an additional experiment, pH combination electrodes, whose diaphragms had a silver coating produced according to the above described method and bonded to the ceramic surface via a silane layer with thiol functionalities, were subjected to long term service in a fermenter. For this, the combination electrodes were applied in a fermenter, which contained the green algae, tetraspora lubrica, as test organism. It is a known property of this algae that it grows vigorously on surfaces.

[0070] The graph of FIG. 3 shows the measurement signals of four such pH combination electrodes (H1, H2, H4, H5) in fermentation operation plotted as a function of time. For comparison, moreover, the measurement signals of two comparison combination electrodes (R1, R2) with a conventional ceramic diaphragm are plotted.

[0071] The evident continuous rise of the measurement signal results from the progress of the non-regulated fermentation process, in the case of which the pH value of the measured medium becomes continuously more alkaline due to the metabolism of the green algae. The short term, greater increases superimposed on the continuously rising pH value and the, in each case, adjoining short term decline of the measurement signal results from simulation of a day/night cycle performed in the experiment. The measured medium was alternately illuminated and, thereafter, shaded. In the case of illumination, the CO.sub.2 content of the measured medium is reduced by the metabolism of the green algae, which leads to a rise of the pH value, while, in the case of darkness, the CO.sub.2 content goes up again, which effects a decline of the pH value of the measured medium.

[0072] It is evident from FIG. 3 that the comparison combination electrodes with the conventional ceramic diaphragms reflect the pH value curve correctly at the beginning of the measurements, but, for instance, from the fifth day/night-cycle, the short term, pH value fluctuations of the measured medium are no longer precisely registered. In contrast therewith, all the combination electrodes with modified diaphragms show, over the entire duration of the experiment, an unchanged, high accuracy of measurement. This is attributable to the fact that, on the conventional ceramic diaphragms in the course of fermentation operation, growth of the algae contained in the measured medium occurs, while, due to the biocidal effect of the silver coating on the ceramic diaphragms of the modified combination electrodes, no, or only a negligible, growth occurs.

[0073] As already mentioned, metal-comprising coating of a ceramic diaphragm in a sensor leads also to a lessening of the liquid flow- or stirring dependence of the sensor signal in media of lesser conductivity and to a reduction of out flow of reference electrolyte through the diaphragm, without significant increasing of the diaphragm resistance. In the following, some measurement results demonstrating this will now be presented.

[0074] FIG. 4 shows a graph illustrating the stirring dependence of the potential output by different test reference half-cells as a function of the conductivity of a measured medium. Used for the measurements for the graph were different test reference half-cells having a ceramic diaphragm with a metal coating. The test reference half-cells were produced by supplying various ceramic substrates of a CaO-stabilized, ZrO.sub.2-ceramic with a pore diameter of 450 nm and a flow rate of 4.85 ml/d, as well as a resistance of 152 ohm/mm (material M1) with a firing lacquer containing the metal to be provided on the ceramic substrate. By subsequent firing, the metal coating was formed. From the coated ceramic substrates, segments were separated and welded into different reference half-cell housings. The reference half-cells were filled with a 3 molar KCl solution and contained a potential sensing element of chlorided silver wire. The ceramic diaphragm of a first test reference half-cell was modified in this way with a gold coating (open squares), that of an additional test reference half-cell with a platinum coating (open triangles), that of a third test reference half-cell with a first layer of gold and a thereover arranged, platinum layer (open diamonds) and the ceramic diaphragm of a fourth test reference half-cell with two platinum layers arranged on top of one another (crosses). A comparison measurement was made with a conventional reference half-cell having an uncoated ceramic diaphragm of the ceramic material also used for the test reference half-cells (filled out diamonds).

[0075] The test reference half-cells were placed in a measured medium of lesser conductivity, i.e. a conductivity of less than 10 mS/cm, and the potential tappable on the potential sensing element of the test reference half-cells measured relative to the glass half-cell both in the stirred measured medium as well as also in the resting measured medium. The difference between the potentials measured in the flowing state and those in the resting state is plotted as a function of the conductivity of the measured medium in FIG. 4. This difference is here also referred to, for short, as stirring- or liquid flow dependence of the measurement signal. As evident from FIG. 4, this stirring- or liquid flow dependence is clearly smaller in the case of the test reference half-cells than in the case of the conventional reference half-cell.

[0076] FIG. 5 shows a graph, in which the lessening of stirring dependence (A SD) of the test reference half-cells with coated ceramic diaphragm is presented relative to the conventional reference half-cell with uncoated ceramic diaphragm as a function of the conductivity of the measured medium. The terminology lessening of stirring dependence refers here to the difference between the stirring- or liquid flow dependence of the test reference half-cells and that of the conventional reference half-cell presented in FIG. 4. In FIG. 5, the open diamonds represent the results obtained with the test electrode, whose diaphragm has a gold coating. The closed diamonds give measurement points, which were obtained with the test electrode, whose diaphragm was modified with a gold layer and an overlying platinum layer. Measurement points, which were obtained with the test reference half-cell, whose diaphragm has a platinum coating are presented with open triangles. The closed triangles represent measurement results, which were obtained with the test reference half-cell, whose diaphragm has a coating of two platinum layers lying on top of one another.

[0077] FIG. 6 shows results of flow- and resistance measurements on test ceramic diaphragms with metal coating. Flow was measured by placing reference half-cell tubes filled with 3 molar KCl solution, and each having a test ceramic diaphragm arranged in its wall, in a defined volume of DI-water and supplying each tube with a defined pressure. From the gradual change over time of the conductivity of the DI-water due to the escape of KCl solution from the reference half-cell tubes with accompanying enriching of the DI-water with KCl, flow through each of the test ceramic diaphragms was ascertained.

[0078] The resistance of the test diaphragms was ascertained by measuring conductivity between a first platinum electrode arranged in the interior of the reference half-cell tube and a second platinum electrode arranged in a medium surrounding the reference half-cell tube.

[0079] The measured values illustrated in the graph of FIG. 6 were ascertained for different test ceramic diaphragms. A first group of test diaphragms was made of coated ceramic diaphragms of CaO-stabilized ZrO.sub.2 (M1) with a pore diameter of 450 nm, a flow rate of 4.85 ml/d and a resistance of 152 ohm/mm (values in the uncoated state), with a gold coating (open diamond), a coating of a gold layer and a platinum layer lying thereon (filled out diamond), a coating formed of a single layer of platinum (open triangle), and a coating formed of two platinum layers lying on top of one another (filled out triangle). For comparison, moreover, flow- and resistance measurements were performed on an uncoated ceramic diaphragm of the said material (filled out square).

[0080] A second group of test diaphragms was made of coated ceramic diaphragms of Y.sub.2O.sub.3-stabilized ZrO.sub.2 (M2) with a pore diameter of 1.8 m, a flow rate of 6.7 ml/d and a resistance of 1486 ohm/mm (values in the uncoated state), with a gold coating (open circle), a coating formed of a gold layer and a platinum layer lying thereon (filled out circle), a coating formed a of a single platinum layer (minus sign) and a coating formed of two platinum layers lying on top of one another (plus sign). For comparison, moreover, flow- and resistance measurements were performed on the uncoated ceramic material (open square).

[0081] FIG. 6 shows measured diaphragm resistance R plotted against flow rate in ml/d. Through the coating of the ceramic diaphragms, the flow rate of the material M1 was reduced by up to 30% and the flow rate of material M2 by up to 40%. In such case, a relatively small rise of the diaphragm resistance in the case of the diaphragms formed of the material M1 of up to 6% and in the case of the diaphragms formed of the material M2 of up to 33% was detected.