Durable and robust reference electrode in a compact electrochemical measuring unit for planar electrodes on flat supports

Abstract

Long-life reference electrode in a potential measuring unit for flat supports, in which at least one working electrode and at least one reference electrode are applied to a flat support made of an electrical insulating material, these each having an external contact lead via a covered conductive layer, characterized in that a flat porous layer is located around the reference electrode, a housing is tightly placed on this porous layer, the porous layer having contact with the environment to ensure ionic conductivity, and the housing is tightly filled with a gel.

Description

[0001] The present invention relates to reference electrodes on flat support materials used for electrochemical measurements (e.g. potentiometry, voltammetry). State of the art

[0002] Systems manufactured using known layer deposition techniques, such as thick-film technology, include, in particular, planar reference electrodes based predominantly on silver-silver chloride. In the manufacture of such planar electrodes, a silver layer is first applied, often using thick-film technology.

[0003] Reference electrodes of the second type are special electrodes whose potential depends indirectly on the concentration of the surrounding electrolyte solution. Their potential is independent of the electrolyte concentration. This is achieved by a saturated solution of a sparingly soluble salt and a readily soluble alkali salt with the same anion, which keeps the potential stable. An important reference electrode is the silver-silver chloride electrode, which is very frequently used in potentiometry.

[0004] US 000005384031 A describes an embodiment in which the metal is coated with a poorly soluble salt and then coated with a hydrophilic medium, such as polyvinyl alcohol. Contact with the measuring medium is established via this layer, with the majority of this hydrophilic layer being sealed with a water-impermeable layer. A disadvantage of this solution is that the electrodes have only a limited service life in the measuring medium.

[0005] Another approach involves covering a reference electrode based on the solubility equilibrium of silver and silver halide with a halide-containing paste as a covering reservoir. This method allows for an extension of the measurement time. However, the long diffusion path requires a considerable amount of time until the sensors are ready for the first measurement, which is not optimal for the user.

[0006] EP 000000682247 A1 discloses a possibility for extending the lifetime of a second type of reference electrode on planar silver layers as electrodes.

[0007] There, an unspecified polymer matrix for the halide salt is mentioned. In a publication (AWJ Cranny, JK Atkinson, Meas. Sci. Technol. 9 (1998) 1557-1565), the corresponding inventors show that potassium chloride (KCl) in a polymer matrix (mineral-filled, thermosetting modified silicone) on the silver electrode stabilizes the electrode potential. For this purpose, the mixture of polymer and KCl is applied as a paste to the silver layer and cured at 200 °C for 1 h. This layer is sealed with another layer of the pure polymer, leaving a hydration opening for contact with the measurement solution. Thick-film technology is required to apply these coatings. As the size of the hydration opening increases, the salt loss rate increases and the electrode potential changes. Furthermore, the sealing layer appears to be partially permeable to water.This requires a hydration time of several hours, and a potential drift of up to 6 mV per day occurs during the first 30 days. This results in a potential change of approximately 180 mV, which leads to significant errors in potentiometric measurements of ion concentrations.

[0008] A comparable reference electrode is also known from DE 000069333218 T2. This contains a hydrophilic wick material impregnated with electrolyte and located on a silver-silver chloride electrode, which in turn is covered by a sealing layer.

[0009] A review of other similar attempts to increase the lifetime of reference electrodes is also based on modifications of the polymer matrices and sealing layers (M. Sophocleous, JK Atkinson, Sens. Actuators A 267 (2017) 106-120).

[0010] DE 000010246303 A1 discloses a system as a reference electrode with an extended lifetime, which additionally provides a significantly larger reservoir for the halide salt. This reservoir in the carrier supplies the necessary ions that are lost at the contact surface with the measuring solution, thus achieving saturation of the halide concentration at the reference electrode over a longer period of time.

[0011] EP 000004336177 A1 discloses a polymer reference electrode consisting of a carrier foil, a silver-silver chloride electrode applied thereto, a solid electrolyte material on the electrode, and a permeable polymer layer covering the solid electrolyte. This sensor is intended to provide increased stability, with little or no alkali chloride leakage, but is quickly wettable due to optimized water absorption and diffusion. However, as the corresponding patent specification shows, the potential of this electrode drifts significantly after 4 to 5 days at the latest.

[0012] The solutions known so far are also mechanically vulnerable because the cover layers have thicknesses of significantly less than 1 mm and cannot be used directly for measurements in highly inhomogeneous matrices (e.g. soils) which contain solid components (e.g. sand, stones). Object of the invention

[0013] The object of the invention is to realize a second-type reference electrode with an electrode on a flat support without the use of additional coating technologies. Furthermore, an extended service life with the most stable electrical potential possible over days / months, a robust design, and easy replacement of the internal electrolyte are to be ensured. The reference electrode is intended to provide the reference potential for electrodes located on the support.

[0014] The objective is to realize an electrochemical measurement unit for a ready-to-use, durable, robust, compact and flexible solution for using flat electrodes on flat supports for electrochemical measurements in a wide range of applications, including challenging environments such as inhomogeneous sample matrices. Description of the invention

[0015] A potential measuring unit 1 consists as in Fig.shown essentially consisting of the flat carrier 2 with working electrode 3 and reference electrode 4, a porous layer 5, the housing 7, the carrier plate 6 and the gel 8.

[0016] The flat carrier 2 consists of an electrically insulating material, e.g. aluminum oxide, on which at least one working electrode 3 and at least one reference electrode 4 are applied using thick-film technology or screen printing. The electrodes 3 and 4 are each provided with separate contact leads to the outside. For this purpose, a conductive layer 9 is applied to the substrate and structured. This layer can be made of a precious metal such as platinum, gold or silver. At the ends of these conductive layers 9 are contact pads that enable the sensor to be connected to an external measuring circuit. The individual contact paths to the electrodes are covered with a non-conductive layer and are thus electrically insulated from one another, allowing electrochemical measurements in the medium 10. The reference electrode 4 usually consists of a silver layer that is additionally coated with a silver-silver chloride mixture.A porous layer 5 is applied around the electrode 4. This porous layer encloses the reference electrode 4.

[0017] It can be made of filter paper. The thickness ranges between 1 µm and 500 µm. A thickness of 150 µm is typically used. The pore size of the porous layer is between 0.1 µm and 30 µm; 2 µm to 30 µm is typically used. Above this is the housing 7, which sits tightly on the porous layer 5. The housing is made of plastic. Its external dimensions are 25 mm. An internal volume is cut out of the housing to provide an electrolyte reservoir. This is a cylindrical recess with a volume of approximately 0.7 ml (15 mm diameter, 4 mm deep). This reservoir in the housing 7 is completely filled with an electrolyte gel 8.

[0018] The gel can be a matrix based on a hydrophilic polymer. Typically, a mixture of sodium polyacrylate (NaPAc), KCl, and water (0.5 g NaPAc, 4.5 g KCl, 10 ml water) is used. Electrical contact with the measurement solution is ensured via the porous layer 5, acting as a so-called salt bridge. This layer can consist of filter paper with different pore sizes, or, for example, filter or dialysis membranes made of various materials. This allows the leakage of the internal electrolyte to be controlled, with the electrical resistance at approximately 10 kOhm to 30 kOhm allowing interference-free potential measurement. The leakage rate of the internal electrolyte is reduced by reducing the thickness of the porous layer 5 and its pore size, thus increasing the service life of the reference electrode.However, this also increases the electrical resistance to the measuring solution, which may require more sensitive measuring instruments for potential determination. Depending on the requirements for measurement accuracy, service life, and measuring medium, an application-specific adaptation can be made by selecting the porous layer 5 (thickness, pore size, material).

[0019] It is essential that the porous layer realizes "ionic conductivity," meaning that the ions penetrate to the outside through diffusion (and not through convection or flow transport). The housing 7 is usually screwed to the carrier plate 6.

[0020] This allows the following key features to be achieved: 1. Immediate operational capability

[0021] The reference electrode requires no hydration time and is ready for use immediately after assembling the potential measurement unit. This saves time and significantly simplifies practical use. 2. Long shelf life

[0022] The reference electrode can be stored dry at room temperature under normal laboratory conditions for at least 9 months without losing its functionality. Even after this storage period, it immediately provides a stable reference potential, underscoring its reliability and longevity. 3. Robustness

[0023] The potential measurement unit is robust enough to be used in highly inhomogeneous sample matrices such as soil. This robustness significantly expands the application spectrum of the measurement setup. 4. Compact design

[0024] The combination of working and reference electrode on a single flat carrier results in a very compact and space-saving measuring arrangement. 5. Reusability and adaptability

[0025] The design of the potential measurement unit allows for simple and cost-effective adjustment and replacement of the salt bridge (porous layer or sealing ring) and the internal electrolyte within the enclosure's interior volume. This enables multiple use of the unit and reduces operating costs through easy maintenance and adaptation to different measurement requirements. The volume of the internal electrolyte can also be varied by using differently sized recesses in the enclosure, ensuring the longest possible service life while minimizing space requirements (recess volume) for different measurement requirements. List of reference symbols 1 potential measuring unit 2 flat supports 3 Working electrode 4 Reference electrode 5 porous layer 6 Carrier plate 7 Enclosure 8 Gel 9 conductive layer 10 Measuring medium QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 000005384031 A

[0004] EP 000000682247 A1

[0006] DE 000069333218 T2

[0008] DE 000010246303 A1

[0010] EP 000004336177 A1

[0011] Cited non-patent literature

[0000] A.W.J. Cranny, J.K. Atkinson, Meas. Sci. Technol. 9 (1998) 1557-1565

[0007] M. Sophocleous, J.K. Atkinson, Sens. Actuators A 267 (2017) 106-120

[0009]

Claims

[1] Long-life reference electrode in a potential measuring unit for flat supports, in which at least one working electrode and at least one reference electrode are applied to a flat support made of an electrical insulating material, each of which has an external contact lead via a covered conductive layer, characterized by that there is a flat porous layer around the reference electrode, a housing is tightly placed on this porous layer, the porous layer has contact with the environment to ensure ionic conductivity, and the housing is tightly filled with a gel. [2] Long-life reference electrode in a potential measuring unit for flat carriers according to claim 1, characterized by that the flat support is made of electrical insulating material made of aluminum oxide or a polymer. [3] Long-life reference electrode in a potential measuring unit for flat carriers according to claim 1-2, characterized bythat the reference electrode consists of a silver-silver chloride mixture. [4] Long-life reference electrode in a potential measuring unit for flat carriers according to claims 1-3, characterized by that the porous layer is a filter paper or a polymer with a thickness of 1 to 500 µm and a pore size of 0.1 to 30 µm. [5] Long-life reference electrode in a potential measuring unit for flat carriers according to claims 1-4, characterized by that the housing is made of plastic, screwed to the support and / or connected by means of a clamping device. [6] Long-life reference electrode in a potential measuring unit for flat carriers according to claims 1-5, characterized by that the gel is a halide salt in a polymer matrix or a water-binding polymer with one or more chloride salts. [7] Long-life reference electrode in a potential measuring unit for flat carriers according to claims 1-6, characterized bythat the reference electrode contains a metal layer on which there is an additional layer of the salt or oxide of the metal. [8] Long-life reference electrode in a potential measuring unit for flat carriers according to claims 1-7, characterized by that the metal layer is made of silver.

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