ELECTROCHEMICAL SENSOR HAVING AN ALKALINE EARTH-MODIFIED ELECTROLYTE FORMULATION

Abstract

The invention relates to an electrochemical sensor having a porous diaphragm and an electrolyte formulation that comprises at least one solvent, at least one electrolyte from the group of alkali metal salts and at least one alkaline earth metal ion, and to an electrolyte formulation.

Claims

1. An electrochemical sensor for measuring a working fluid, having a reference electrode (3) arranged in a first volume having a first electrically conductive fluid (6), and at least one working electrode (2), wherein the first volume is delimited at least by a porous diaphragm (10) with an open porosity of less than 35%, and wherein the first electrically conductive fluid (6) is an electrolyte formulation which comprises at least one solvent, at least one electrolyte from the group of alkali metal salts and at least one alkaline earth metal ion in a concentration in the range of 30 to 95 mmol/kg.

2. The sensor according to claim 1, wherein the diaphragm (10) has an open porosity of less than 30%.

3. The sensor according to claim 1, wherein the diaphragm (10) has an open porosity of less than 25%.

4. The sensor according to claim 1, wherein the diaphragm (10) has an open porosity in the range of 18-22%.

5. The sensor according to claim 1, wherein the alkaline earth metal ion is a magnesium ion.

6. The sensor according to claim 1, wherein the alkaline earth metal ion is a calcium ion.

7. The sensor according to claim 1, wherein the concentration of the alkaline earth metal ion in the electrolyte formulation is in the range of 30-85 mmol/kg.

8. The sensor according to claim 7, wherein the concentration of the alkaline earth metal ion is in the range of 40-75 mmol/kg.

9. The sensor according to claim 8, wherein the concentration of the alkaline earth metal ion is 55 mmol/kg.

10. The sensor according to claim 1, wherein the concentration of the alkaline earth metal ion is in the range of 20-50 mmol/kg.

11. The sensor according to claim 1, wherein the alkaline earth metal ion is present as part of an inorganic salt of the alkaline earth metal.

12. The sensor according to claim 11, wherein the inorganic salt of the alkaline earth metal is a chloride salt.

13. The sensor according to claim 1, wherein the electrolyte of the electrolyte formulation is potassium chloride.

14. The sensor according to claim 1, wherein the electrolyte formulation has a water content of at least 20 wt %.

15. The sensor according to claim 1, wherein the electrolyte formulation comprises at least one evaporation-inhibiting agent and/or at least one thickening agent.

16. The sensor according to claim 1, wherein the diaphragm (10) is a ceramic diaphragm.

17. The sensor according to claim 16, wherein the ceramic diaphragm comprises yttrium oxide-doped zirconium dioxide.

18. The sensor according to claim 17, wherein the yttrium oxide doping is 8 mol %.

19. The sensor according to claim 1, wherein the sensor is a potentiometric sensor selected from the group consisting of a pH sensor, redox sensor and ion-selective sensor.

20. The sensor according to claim 1, wherein the sensor is a combination electrode.

21. An electrolyte formulation for an electrochemical sensor with a porous diaphragm, comprising at least one solvent, at least one electrolyte from the group of alkali metal salts and at least one alkaline earth metal ion in a concentration in the range of 20-95 mmol/kg.

Description

[0049] The invention is explained in more detail with reference to the figures, in which:

[0050] FIG. 1 is a schematic representation of a longitudinal section through a combination electrode according to an embodiment of the invention.

[0051] FIG. 2 is a graph of the interrelationship between diaphragm porosity and diffusion potential.

[0052] FIG. 3 is a graph of the dampening effect of alkaline earth metal ions on the diffusion potential.

[0053] For measuring a working fluid, a combination electrode 1 according to the representation in FIG. 1 has a working electrode 2, a reference electrode 3, a diaphragm 10 and a first electrically conductive fluid 6. The first electrically conductive fluid 6 is in contact with the reference electrode 3 and the diaphragm 10 such that the diaphragm 10 is connected to the reference electrode 3 in an electrically conductive manner via the first electrically conductive fluid 6. The first electrically conductive fluid 6 is also referred to as the reference electrolyte.

[0054] The combination electrode 1 can be a pH combination electrode and/or a redox sensor. The pH combination electrode can be a pH glass electrode.

[0055] The combination electrode 1 has an outer tube 4 and an inner tube 5 which is arranged within the outer tube 4. The outer tube 4 is also synonymously referred to here as external tube, and the inner tube 5 is also synonymously referred to here as internal tube.

[0056] The volume between the two tubes is configured as a reference space in which the first electrically conductive fluid 6 is arranged. The outer tube 4 has a first outer tube longitudinal end 15 and a second outer tube longitudinal end 16. A first inner tube longitudinal end 17 is arranged in the region of the first outer tube longitudinal end 15. A second inner tube longitudinal end 18 is arranged in the region of the second outer tube longitudinal end 16.

[0057] In the inner tube 5, an inner tube space 19 is formed in which a second electrically conductive fluid 7 (also referred to as inner electrolyte) is arranged. In addition, the inner tube 5 according to FIG. 1 has a storage vessel 8 at its first longitudinal end 17, the volume of which is fluidically connected to the inner tube space 19 and has a larger diameter than the inner tube 5 so that a larger amount of the second electrically conductive fluid 7 can be provided.

[0058] The combination electrode 1 has an opening of the reference space 20 in the form of an annular gap in the region of the first outer tube longitudinal end 15 and first inner tube longitudinal end 17. The diaphragm 10 is arranged in the opening and seals the opening. In this case, the thickness of the diaphragm 10 is specified in relation to the extension between the first inner tube longitudinal end 17 and the second inner tube longitudinal end 18. The thickness of the diaphragm 10 can range from 0.1 mm to 1.0 mm, in particular from 0.35 mm to 0.7 mm. A fluid channel (not shown) can be provided from the diaphragm, which leads to the first outer tube longitudinal end 15.

[0059] The diaphragm 10 is characterized by the fact that it prevents an intermixing of the first electrically conductive fluid 6 and the working fluid, but allows for a charge transport between the first electrically conductive fluid 6 and the working fluid. In order to carry out a measurement in an operation of the combination electrode 1, the side of the diaphragm 10 facing away from the first electrically conductive fluid 6 must be contacted with the working fluid. In addition, the combination electrode 1 has a glass membrane 9 in the region of the first inner tube longitudinal end 17. The glass membrane 9 is electrically conductively connected to the working electrode 2 via the second electrically conductive fluid 7. The first electrically conductive fluid 6 and the second electrically conductive fluid 7 are electrically isolated from one another. In order to measure the working fluid, the working fluid is also contacted with the side of the glass membrane 9 facing away from the second electrically conductive fluid 7, and an electrical voltage is measured between the working electrode 2 and the reference electrode 3.

[0060] The diaphragm 10 comprises, as a material, zirconium dioxide with an yttrium oxide doping of 8 mol %. Other porous ceramic materials can also be used.

[0061] The material of the diaphragm 10 has an open porosity of 20%. The porosity of the diaphragm 10 can limit the diffusion of ions from the first electrically conductive fluid 6, however, this increases the diffusion potential. The interrelationship between porosity and diffusion potential at different salinities is shown in FIG. 2. The dashed line corresponds to an open porosity of 35%, the solid line marked with diamonds to an open porosity of 25%, and the solid line marked with triangles to an open porosity of 20%. The damping effect of tap water compared to 0.015 M NaCl is also clear. Control measurements have surprisingly shown that this damping effect can be attributed to traces of calcium ions in the tap water.

[0062] The first electrically conductive fluid 6 has a composition that contains a calcium and/or magnesium ion in a certain concentration. By way of example, various compositions according to the invention are shown in Table 1 as formulation 0, 1, 2, 3 and 4. In this case, the values are given in grams unless otherwise stated in the table.

TABLE-US-00001 Ingredient Form. 0 Form. 1 Form. 2 Form. 3 Form. 4 Xanthan 16.7 16.7 17.2 16.7 16.7 Glycerol 303.9 303.9 313.46 303.9 303.9 MgCl.sub.2 0 2.0 4.1 0 8.0 CaCl.sub.2 0 0 0 8.9 0 KCl 117.3 117.3 121.0 117.3 117.3 H.sub.2O 562.1 517.5 533.8 517.5 560 Alkaline earth (mmol/kg) 0.0 22.0 44.1 83.9 84.0

[0063] Another example of the damping effect of alkaline earth metal ions can be found in FIG. 3. Here, the potential difference is shown versus the diffusion potential with respect to sodium chloride as an electrolyte contact solution (first electrically conductive fluid 6). The diffusion potential measurement was carried out by potentiometric differential measurement using a Protos transmit-ter from Knick Elektronisch Messgerte GmbH & Co KG. The potentiometric measurements are carried out by comparing two local measurement positions. The first local measurement position is called the external reference and consists of a glass shaft half electrode system made of Ag/AgCl/KCl and a 35% pore diaphragm system (i.e. a diaphragm with an open porosity of 35%) made of fully stabilized zirconia ceramic, with a 3 M KCl solution as electrode filling (Hamilton standard reference electrode); the second local measurement position, called the test specimen reference electrode, is also a glass shaft half electrode system consisting of Ag/AgCl/KCl, a thickened salt-electrolyte mixture and a diaphragm made of a porosity-reduced fully stabilized zirconia ceramic. Both glass shaft half-electrode systems are electrolytically kept connected ionically via an aqueous contact solution with different conductivity, and the measurement is carried out at a constant agitation of 100 rpm. With increasing dilution of the electrolyte contact solutions, the potential difference between the two local measurement positions increases due to diffusion, as a result of which a deviation of the reference potential measurement stability is determined. The diffusion is substantially regulated by the diaphragm porosity, viscosity and codiffusion processes of the formulated salt-electrolyte mixture.

[0064] The graph also clearly shows here that, compared to a solution without alkaline earth metal ion (dashed line), there is a damping effect on the diffusion potential when magnesium ions are present (solid line with triangles: 22 mmol/kg), and this effect increases with increasing concentration (solid line with diamonds: 44 mmol/kg, solid line with squares: 84 mmol/kg). In addition, a damping effect of calcium ions is also shown (solid line with circles: 84 mmol/kg). The values in FIG. 3 are given as rounded values.

LIST OF REFERENCE SIGNS

[0065] 1 combination electrode [0066] 2 working electrode [0067] 3 reference electrode [0068] 4 outer tube [0069] 5 inner tube [0070] 6 first electrically conductive fluid [0071] 7 second electrically conductive fluid [0072] 8 storage vessel [0073] 9 glass membrane [0074] 10 diaphragm [0075] 12 head part [0076] 13 casting compound [0077] 14 sealing ring [0078] 15 first outer tube longitudinal end [0079] 16 second outer tube longitudinal end [0080] 17 first inner tube longitudinal end [0081] 18 second inner tube longitudinal end [0082] 19 inner tube space [0083] 20 reference space