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.

Description

[0001] H30090

[0002] 1

[0003] Electrochemical sensor with an alkaline earth-refined electrolyte formulation

[0004] The invention relates to an electrochemical sensor with a reference electrode having a porous diaphragm and an electrolyte formulation with at least one alkaline earth metal ion, as well as an electrolyte formulation for an electrochemical sensor.

[0005] Electrochemical sensors are used to analyze media, especially liquid process media, to determine specific properties of the medium in question, particularly the pH value, the redox potential, and / or the presence of certain ions or types of molecules in the medium. The medium to be analyzed is also synonymously referred to as the measuring fluid or working fluid.

[0006] The properties that can be determined using electrochemical sensors are summarized below under the term "electrochemical measurand." To determine electrochemical measurands, suitable measuring electrodes are used. These electrodes have the property that, upon contact of the measuring electrode with the medium to be examined, either an electrochemical potential is established that depends on the electrochemical measurand to be determined, for example, the pH value or the concentration of a specific ion type. This potential is recorded as a measurement signal, which is why such sensors are also called potentiometric sensors.

[0007] Or, in so-called amperometric sensors, a specific potential difference (voltage) is applied between a measuring electrode and a reference electrode in such a way that the ions or molecules to be measured are oxidized or reduced at the measuring electrode. The current flowing between the measuring electrode and a third electrode, the so-called counter electrode, is measured. This current is proportional to the concentration of the ions or molecules to be determined. Both potentiometric and amperometric electrochemical sensors therefore require a reference half-cell or reference electrode.

[0008] Electrochemical sensors are often designed as single-rod measuring electrodes, with the spaces for the reference half-cell and the measuring electrode arranged concentrically. A single-rod measuring electrode is a combination of a measuring electrode and a reference electrode housed in a single rod. The term "measuring electrode" is also used synonymously with the term "working electrode," and the term "reference electrode" is also used synonymously with the term "reference electrode."

[0009] The measuring electrode can be designed as an ion-selective electrode which comes into direct contact with the medium to be examined, so that the ion-selective electrode and the medium directly form a system whose electrochemical potential depends on the concentration of a specific type of ion in the medium.

[0010] In sensors with a glass membrane, however, the measuring electrode itself already contains a system consisting of a measuring lead electrode in contact with an internal electrolyte, with the internal electrolyte then in contact with the medium being measured via the glass membrane. For example, with pH electrodes, a potential difference dependent on the pH value of the medium is usually measured across the glass membrane, which is located between the medium and the internal electrolyte of the measuring electrode. The internal electrolyte then forms a buffer system and is selected such that an electrical potential difference across the glass membrane is as proportional as possible to the desired electrochemical measurement variable.

[0011] In general, a system with a measuring electrode and, if applicable, the associated internal electrolyte / measuring membrane is referred to below as a measuring half-cell of an electrochemical sensor. The measuring electrode is located in a measuring half-cell chamber that also contains the internal electrolyte. In potentiometric or amperometric sensors, the measuring electrode is usually made of a metal such as platinum, gold, indium, rhodium, or other metals, metal alloys, or metal oxides such as IrO2, RuO2, TiO2, TaO2, ZnO2, or others.

[0012] Another type of electrochemical sensor is the so-called ISFET (ion selective field effect transistor) sensor, in which an ion-sensitive layer is applied to the gate contacting the medium. Depending on the concentration of the species to be determined, this potential forms, which regulates the current flow between the source and drain of a field effect transistor.

[0013] Because electrical potentials can only ever be measured or adjusted as a difference to a reference potential, an electrochemical sensor always requires a measuring electrode and another reference electrode that provides an electrochemical reference potential as a reference value for the measuring signal supplied by the measuring electrode.

[0014] The reference electrode comprises a redox system consisting of a reference electrode in contact with a reference electrolyte. The reference redox system is selected so that a known, preferably stable, and easily reproducible electrochemical potential is established upon contact between the reference electrode and the reference electrolyte. In general, any reference redox system of the type described is referred to below as the reference half-cell of an electrochemical sensor.

[0015] The reference half-cell comprises a reference half-cell chamber containing the reference electrode and the reference electrolyte. The electrochemical potential of the reference half-cell should change as little as possible due to contact between the reference electrolyte and the medium under investigation.

[0016] Common reference half-cells are based on electrodes of the second type, in which a metal reference electrode coated with a sparingly soluble metal salt is in contact with a reference electrolyte containing a chemically inert salt that is readily soluble in the reference electrolyte and has the same anion as that of the sparingly soluble electrode coating. The reference electrolyte is often a liquid or a rather viscous gel, in particular an aqueous salt solution or an aqueous gel of a specific salt.

[0017] A common redox system used for reference electrodes, as may also be intended for use with the present invention, consists of an Ag wire with a coating of AgCl (silver-silver chloride electrode) which is immersed in a reference electrolyte of aqueous KCl solution.

[0018] Other redox systems that are also suitable for use with the present invention are also based on the use of KCl, such as the calomel redox system (Hg / Hg2Cl2).

[0019] The reference electrolyte is also selected based on its good electrical conductivity, chemical inertness and the fact that its ions have as equal a mobility as possible.

[0020] For potentiometric sensors, such as pH sensors, the reference half-cell and measuring half-cell must be brought into electrolytic contact with the medium under investigation to determine the respective electrochemical parameter. A potential difference is then determined that corresponds to the difference in the electrochemical potentials of the reference half-cell and the measuring half-cell.

[0021] The resulting electrochemical potential between the measuring half-cell and the medium under test, as well as between the reference electrode and the reference electrolyte, depends on many factors. Therefore, when commissioning an electrochemical sensor, it must first be calibrated with solutions or electrolytes of known ionic activity (standardized buffer solutions are usually used for this purpose). The desired electrochemical measurement value can be quantitatively determined from the potential difference determined for an unknown medium. To establish the electrolytic contact between the reference electrolyte and the medium under test required for the measurement, the reference half-cell is in contact with its surroundings via an opening, a diaphragm, or a similar connection.

[0022] The diaphragm is characterized by the fact that it prevents mixing of the KCl solution and the medium under investigation, yet allows charge transport between the KCl solution and the medium in the form of ion migration. Diaphragms with a porous structure are often used for this purpose. One disturbing factor of the diaphragm is its diffusion potential. This potential always arises at the phase boundary between two electrolytes of different concentrations or compositions. The diffusion potential is due to the different migration speeds of the ions through the diaphragm due to the interaction of the ions with the diaphragm surface, which in turn depend on the charge and size of the ion type, i.e., on the charge density and extent of its hydration shell and the diaphragm material.

[0023] With increasing differences in ion concentration between medium and reference electrolyte, the diffusion gradients and thus also the diffusion potentials increase.

[0024] When anions diffuse faster than cations, or vice versa, a potential arises at the boundary between the two solutions. To keep the diffusion potential at the diaphragm of a reference electrode as low as possible, the various ions in the reference electrolyte should have the same ionic mobility. This ideal state is almost achieved with a 3 M KCl solution. In general, the higher the flow velocity of the reference electrolyte through the diaphragm, the lower the diffusion potential.

[0025] However, this is counteracted by the desire for a slow release of the reference electrolyte and the ions from the reference electrolyte chamber. This delays the loss of the reference electrolyte through leakage and the depletion of the reference electrolyte in ions as long as possible, thus increasing the sensor's service life and potential stability. This is achieved by adding thickeners and / or uncharged organic compounds to the reference electrolyte, which in turn results in lower KCl concentrations.

[0026] EP 1 219 959 A1 describes an electrochemical sensor for measuring pH. It comprises a measuring electrode and two reference electrodes with different potential stabilities. The reference electrode with the higher potential stability is used to determine the measured value in the measuring medium, while the reference electrode with the lower potential stability is used to determine the salt concentration in the reference electrolyte. Porous or fine-pored materials are described as suitable for diaphragms, in particular porous ceramic plugs, ground glass joints, cotton fibers, synthetic fibers, metal threads, porous plastic plugs, and wooden pins.

[0027] EP 1 636 149 B1 describes an electrochemical sensor with a diaphragm exhibiting an open porosity between 20 and 38%. The diaphragm material, a porous ceramic, is impregnated with a lyogel precursor material during its production, which then converts into a lyogel. By at least partially filling the cavities of the porous ceramic with lyogel, a porous ceramic with reduced pores is created. However, these reduced pores result in particularly high diffusion potentials, especially in the case of a medium with a low ion concentration.

[0028] The task is to provide an electrochemical sensor with an improved reference half-cell.

[0029] This object is achieved by an electrochemical sensor according to claim 1 and an electrolyte solution according to claim 21. Further advantageous embodiments and refinements of the invention emerge from the subclaims, the figures, and the exemplary embodiments. The embodiments of the invention can be advantageously combined with one another.

[0030] A first aspect of the invention relates to an electrochemical sensor for measuring a working fluid, comprising a reference electrode arranged in a first volume comprising a first electrically conductive fluid and at least one working electrode, wherein the first volume is delimited by at least one porous diaphragm with an open porosity of less than 35%, and the first electrically conductive fluid is an electrolyte formulation comprising at least one solvent, at least one electrolyte from the group of salts of alkali metals and at least one alkaline earth metal ion in a concentration in the range of 20 to 95 mmol / kg.

[0031] The first electrically conductive fluid can also be called the reference electrolyte.

[0032] Surprisingly, it has been shown that ZrC-based diaphragms exhibit a high adsorptive affinity for alkaline earth ions, so that the presence of this ion species in the reference electrolyte in concentrations ranging from 20 to 95 mmol / kg significantly dampens the diffusion potential. Doping the reference electrolyte with alkaline earth metal ions advantageously guarantees a permanent and steady concentration of diffusion potential-damping cations at the diaphragm location of an electrochemical sensor. This reduces measurement errors caused by diffusion potentials in the corresponding sensors.

[0033] The open porosity of the diaphragm is determined by means of mercury porosimetry. Common diaphragms have a porosity of 35% or more. In contrast, the diaphragm of the sensor according to the invention has an open porosity of less than 35%. The diaphragm preferably has an open porosity of less than 30%, more preferably less than 25%, and particularly preferably in the range of 18-22%. Reducing the open porosity of the diaphragm has, among other advantages, that the duration of possible dry storage of the sensor, which is limited, among other things, by the continuous outflow of the reference electrolyte from the diaphragm, is extended. However, reducing the open porosity of the diaphragm influences the diffusion potentials at the diaphragm that occur during use of the sensor, particularly with working fluids with low conductivity, so that this measure significantly reduces the operational area of ​​application and / orthe specification of the sensor would have to be restricted, especially in the case of dilute solutions as working fluids.

[0034] The inventive combination of reducing the open porosity with the use of at least one alkaline earth metal ion in the reference electrolyte solution creates a synergistic effect, as both enable an expansion of the sensor's specifications. For example, the sensor's operational field of application can be extended to solutions (working fluids) with lower ion concentrations of up to 0.5 mS / cm. While the lower porosity of the diaphragm increases the diffusion potential, the alkaline earth metal ions in the reference electrolyte dampen the diffusion potential. As already described, the lower porosity of the diaphragm reduces the outflow of ions from the diaphragm. This also extends the operating life and dry storage time of the sensor.It is also possible to reduce the volume of the reference electrolyte and to make the sensors smaller without sacrificing performance comparable to conventional sensors.

[0035] As the open porosity values ​​decrease, the permeability of the diaphragm decreases. Therefore, the diaphragm preferably has an open porosity of at least 5%, more preferably at least 10%. Thus, the diaphragm preferably has an open porosity of at least 5% and below 35%, more preferably at least 5% and below 30%, more preferably at least 5% and below 25%. Alternatively, the diaphragma preferably has an open porosity of at least 10% and below 35%, more preferably at least 10% and below 30%, more preferably at least 10% and below 25%. As already described, the diaphragm particularly preferably has an open porosity in the range of 18-22%.

[0036] Preferably, the alkaline earth metal ion is a magnesium ion. Likewise, the alkaline earth metal ion is a calcium ion. Both alkaline earth metal ions may be present. Other alkaline earth metal ions may also be present in the electrolyte solution.

[0037] Preferably, the concentration of the alkaline earth metal ion in the electrolyte formulation is in the range of 30–85 mmol / kg. More preferably, the concentration of the alkaline earth metal ion in the electrolyte formulation is in the range of 40–75 mmol / kg, and particularly preferably 55 mmol / kg.

[0038] In a further preferred embodiment, the concentration of the alkaline earth metal ion in the electrolyte formulation is in the range of 20 - 50 mmol / kg.

[0039] Preferably, the alkaline earth metal ion is present in the electrolyte solution as part of an inorganic salt of the alkaline earth metal. The inorganic salt of the alkaline earth metal is preferably a chloride salt. It is preferably magnesium chloride and / or calcium chloride or their hydrates.

[0040] The electrolyte formulation comprises at least one, in particular precisely one, electrolyte from the group of alkali metal salts. The electrolyte is preferably potassium chloride. Potassium chloride is easy to provide because it is frequently used in a reference electrolyte solution. The damping effect of alkaline earth metal ions according to the invention, in conjunction with the reduced diaphragm porosity according to the invention, is particularly suitable for solutions containing potassium chloride, since lower potassium loss (potassium leakage) from the sensor into the working fluid enables the sensor to be used in fermenters and stirred incubators or culture vessels for cell cultivation. For example, mammalian cells do not respond well to too much potassium. This can be particularly significant in small culture volumes due to potassium leakage from the sensor.The sensor according to the invention advantageously has a low potassium footprint and is thus particularly suitable for use in mammalian cell cultures. This is particularly important in the biotechnological pharmaceutical industry. Furthermore, a comparatively lower chloride loss (chloride efflux) from the sensor leads to lower drift (i.e., increased drift stability) of the reference electrode and higher measurement quality during the sensor's operational life.

[0041] In a preferred embodiment, the electrolyte formulation comprises potassium chloride and magnesium chloride. In another preferred embodiment, the electrolyte formulation comprises potassium chloride and calcium chloride.

[0042] The solvent in the electrolyte formulation is, in particular, water. The electrolyte formulation preferably has a water content of at least 20% by weight.

[0043] The electrolyte formulation preferably contains at least one evaporation-inhibiting agent and / or at least one thickening agent. The evaporation-inhibiting agent is, for example, glycerol. The thickening agent is, for example, xanthan gum.

[0044] The diaphragm of the sensor according to the invention is preferably a ceramic diaphragm. The ceramic diaphragm can, for example, comprise a porous ceramic made of porcelain, aluminum oxide, spinel, forsterite, and / or zirconium dioxide (also referred to as zirconia). The use of these ceramics as diaphragms is known. Zirconium dioxide can contain calcium oxide, magnesium oxide, or yttrium oxide as a stabilizing additive.

[0045] The ceramic diaphragm preferably comprises yttrium oxide-doped zirconium dioxide. The yttrium oxide doping is particularly preferably 8 mol%. Zirconium dioxide with an yttrium oxide doping of 8 mol% is particularly suitable for glass electrodes due to its hardness, coefficient of thermal expansion, and corrosion resistance. The stabilizing effect of yttrium oxide in the zirconium dioxide structure is based on the increased formation of cubic crystal phases compared to the thermodynamically more unstable tetrahedral and monoclinic material phases. Other stabilizing oxide components that promote the formation of crystalline cubic phases are calcium oxide and magnesium oxide.

[0046] Preferably, the sensor is a potentiometric sensor selected from the group consisting of a pH sensor, a redox sensor, and an ion-selective sensor. However, the reference half-cell can also be used for other electrochemical sensors, such as amperometric or ISFET sensors.

[0047] The sensor is preferably a combination electrode. Combination electrodes are particularly advantageous for use as pH sensors.

[0048] A second aspect of the invention relates to an electrolyte formulation for an electrochemical sensor with a porous diaphragm, comprising at least one solvent, at least one electrolyte from the group of salts of alkali metals and at least one alkaline earth metal ion in a concentration in the range of 20 - 95 mmol / kg.

[0049] The advantages of the electrolyte formulation according to the invention correspond to the advantages of the electrochemical sensor according to the invention.

[0050] The invention is explained in more detail with reference to the figures.

[0051] Figure 1 is a schematic representation of a longitudinal section through a combination measuring chain according to an embodiment of the invention.

[0052] Figure 2 is a diagram showing the relationship between diaphragm porosity and diffusion potential.

[0053] Figure 3 shows a diagram illustrating the damping effect of alkaline earth metal ions on the diffusion potential. A combination electrode 1 as shown in Figure 1 comprises a working electrode 2, a reference electrode 3, a diaphragm 10, and a first electrically conductive fluid 6 for measuring a working fluid. The first electrically conductive fluid 6 is in contact with the reference electrode 3 and the diaphragm 10, so that the diaphragm 10 is electrically conductively connected to the reference electrode 3 via the first electrically conductive fluid 6. The first electrically conductive fluid 6 is also referred to as the reference electrolyte.

[0054] 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 arranged within the outer tube 4. The outer tube 4 is referred to here as the outer tube, and the inner tube 5 is referred to here as the inner tube.

[0056] The volume between the two tubes is designed as a reference space 20, 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] An inner tube space 19 is formed in the inner tube 5, in which a second electrically conductive fluid 7 (also referred to as the internal electrolyte) is arranged. Furthermore, the inner tube 5, as shown in Fig. 1, has a storage vessel 8 at its first longitudinal end 17, the volume of which is fluidly connected to the inner tube space 19 and has a larger diameter than the inner tube 5, so that a larger quantity of the second electrically conductive fluid 7 can be provided.

[0058] The combination electrode 1 has an opening of the reference chamber 20 in the form of an annular gap in the region of the first outer tube longitudinal end 15 and the first inner tube longitudinal end 17. The diaphragm 10 is arranged in the opening and seals the opening. The thickness of the diaphragm 10 is specified in relation to the extent between the first inner tube longitudinal end 17 and the second inner tube longitudinal end 18. The thickness of the diaphragm 10 can be from 0.1 mm to 1.0 mm, in particular from 0.35 mm to 0.7 mm. A liquid 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 in that it prevents mixing of the first electrically conductive fluid 6 and the working fluid, but enables charge transport between the first electrically conductive fluid 6 and the working fluid. In order to perform a measurement during 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 longitudinal end 17 of the inner tube. 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 insulated from one another.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] Diaphragm 10 is made of zirconium dioxide with an 8 mol% yttrium oxide doping. 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 be used to limit the diffusion of ions from the first electrically conductive fluid 6, although this increases the diffusion potential. The relationship 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 dampening effect of tap water compared to 0.015 M NaCl is also clear. Control measurements have surprisingly shown that this dampening 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 at a specific concentration. Various compositions according to the invention are shown as examples in Table 1 as formulations 0, 1, 2, 3, and 4. The values ​​are given in grams unless otherwise stated in the table.

[0063] Another example of the damping effect of alkaline earth metal ions can be found in Fig. 3. Here, the potential difference is plotted against the diffusion potential with reference to sodium chloride as an electrolyte contact solution (first electrically conductive fluid 6). The diffusion potential measurement was performed by potentiometric differential measurement using a Protos® transmitter from Knick Elektronisch Messgeräte GmbH & Co KG. The potentiometric measurements are performed by comparing two local measurement positions. The first location 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 (iea diaphragm with an open porosity of 35%) made of fully stabilized zirconium dioxide ceramic, with a 3 M KCl solution as the 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 zirconium dioxide ceramic. Both glass-shaft half-electrode systems are electrolytically held in ionic contact via an aqueous contact solution with different conductivities, and the measurement is carried out with constant agitation at 100 rpm. With increasing dilution of the electrolyte contact solutions, the potential difference between the two local measurement positions increases due to diffusion, which results in a deviation in the reference potential measurement stability.Diffusion is essentially regulated by the diaphragm porosity, viscosity and co-diffusion processes of the formulated salt electrolyte mixture.

[0064] The diagram also clearly shows that, compared to a solution without alkaline earth metal ions (dashed line), a dampening effect on the diffusion potential occurs 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). Additionally, a dampening effect of calcium ions is also shown (solid line with circles: 84 mmol / kg). The values ​​in Fig. 3 are rounded.

[0065] List of reference symbols

[0066] 1 combination electrode

[0067] 2 working electrode

[0068] 3 Reference electrode

[0069] 4 outer tube

[0070] 5 inner tube

[0071] 6 first electrically conductive fluid

[0072] 7 second electrically conductive fluid

[0073] 8 storage container

[0074] 9 Glass membrane

[0075] 10 Diaphragm

[0076] 12 headboard

[0077] 13 Casting compound

[0078] 14 Sealing ring

[0079] 15 first outer pipe longitudinal end

[0080] 16 second outer tube longitudinal end

[0081] 17 first inner tube longitudinal end

[0082] 18 second inner tube longitudinal end

[0083] 19 Inner tube space

[0084] 20 Reference room

Claims

Patent claims Electrochemical sensor for measuring a working fluid, with a reference electrode (3) arranged in a first volume containing a first electrically conductive fluid (6) and at least one working electrode (2), in which the first volume is delimited by at least one porous diaphragm (10) with an open porosity of less than 35%, and 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. Sensor according to claim 1, in which the diaphragm (10) has an open porosity of less than 30%. Sensor according to claim 1 or 2, in which the diaphragm (10) has an open porosity of less than 25%. Sensor according to one of the preceding claims, wherein the diaphragm (10) has an open porosity in the range of 18 - 22%.Sensor according to one of the preceding claims, wherein the alkaline earth metal ion is a magnesium ion. Sensor according to one of the preceding claims, wherein the alkaline earth metal ion is a calcium ion. Sensor according to one of the preceding claims, wherein the concentration of the alkaline earth metal ion in the electrolyte formulation is in the range of 30-85 mmol / kg. Sensor according to claim 7, wherein the concentration of the alkaline earth metal ion is in the range of 40-75 mmol / kg. Sensor according to claim 8, wherein the concentration of the alkaline earth metal ion is 55 mmol / kg. Sensor according to one of claims 1-6, wherein the concentration of the alkaline earth metal ion is in the range of 20-50 mmol / kg. Sensor according to one of the preceding claims, wherein the alkaline earth metal ion is present as part of an inorganic salt of the alkaline earth metal. Sensor according to claim 11, wherein the inorganic salt of the alkaline earth metal is a chloride salt. Sensor according to one of the preceding claims, wherein the electrolyte of the electrolyte formulation is potassium chloride. Sensor according to one of the preceding claims, wherein the electrolyte formulation has a water content of at least 20 wt%. Sensor according to one of the preceding claims, wherein the electrolyte formulation comprises at least one evaporation-inhibiting agent and / or at least one thickener. Sensor according to one of the preceding claims, wherein the diaphragm (10) is a ceramic diaphragm.The sensor of claim 16, wherein the ceramic diaphragm comprises yttria-doped zirconium dioxide. The sensor of claim 17, wherein the yttria doping is 8 mol%. The sensor of any preceding claim, which is a potentiometric sensor selected from the group consisting of a pH sensor, a redox sensor, and an ion-selective sensor. Sensor according to one of the preceding claims, which is a combination electrode. 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.

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