WO2010072601A1

WO2010072601A1 - Measuring probe for electrochemical measurements

Info

Publication number: WO2010072601A1
Application number: PCT/EP2009/067034
Authority: WO
Inventors: Michael Hanko; Katrin Scholz
Original assignee: Endress+Hauser Conducta Gesellschaft Für Mess- Und Regeltechnik Mbh+Co. Kg; VOIGTLÄNDER, Jens
Priority date: 2008-12-22
Filing date: 2009-12-14
Publication date: 2010-07-01
Also published as: DE102008055107A1

Abstract

The invention relates to a pH measuring probe for electrochemical, particularly potentiometric, measurements comprising a component made of an electrically insulating material, particularly glass or plastic, at at least one surface, wherein the measuring probe comprises at least one electrode, which is connected in an electrically conductive manner to an electrically conductive coating applied to the component made of electrically insulating material, and wherein at least one part of the electrically conductive coating is overlaid with at least one electrically insulating layer.

Description

Measuring probe for electrochemical measurements

The invention relates to a measuring probe for electrochemical, in particular potentiometric, measurements.

A potentiometric measuring probe can, for example, be designed as a probe of an ion-selective sensor, a pH sensor being understood here as a special case of an ion-selective sensor for H ₃ θ ⁺ ions, or of a redox sensor. Such measuring probes usually comprise at least one measuring piece and at least one reference half-cell. A half-cell is generally understood to mean an assembly which contains at least one electrode which is in contact with a liquid or gelatinous electrolyte solution, the electrode being electrically conductive with means or devices for detecting the electrical potential of the electrode or for setting a defined potential the electrode can be connected.

A measuring half-line of a measuring probe of an ion-selective sensor comprises an ion-selective layer or membrane. In the example of a pH sensor, this is an H ₃ θ ⁺ ion-selective glass membrane, in the example of an ion-selective sensor for other than H ₃ O ⁺ ions is often an ion-selective polymer membrane. The respective sensor membrane separates an interior of a measuring half-cell housing, which is closed off from the surroundings of the probe, generally at least partially filled with a liquid or gel electrolyte, from the probe surroundings. In measuring operation, the measuring probe is immersed in a mostly liquid measuring medium such that the ion-selective layer or membrane of the measuring half cell comes into contact with the measuring medium. In contact with the measuring medium, a potential dependent on the ion activity to be determined forms on the ion-selective layer or membrane, which potential is applied to measuring electronics via a measuring system arranged in the measuring half cell and which can be determined against a reference potential also applied to the measuring electronics. to determine the corresponding ion activity or the corresponding pH. The lead-off system, also referred to as a lead-off, can be designed as a lead-off electrode, which dips into the electrolyte of the measuring half-cell, or alternatively as a solid lead. A fixed lead comprises a solid, electrically conductive assembly which is electrically conductively connected to the ion selective layer or membrane to drain the membrane potential.

A measuring half-cell of a probe of an ion-selective sensor, for. As a pH sensor, a Halbleäterefement, for example, an ion-selective field effect transistor (! SFET) _{1 may} include. During measuring operation, the JSFET is brought into contact with a measuring medium and its conductivity, which is dependent on the ion activity to be determined in the measuring medium, between source and drain is determined with the aid of measuring electronics. Using a likewise at the Messelektronak adjacent reference potential, the corresponding ion activity or the corresponding pH can be determined.

The reference potential is provided by a reference half-line for a potentiometric probe. The reference half-cell comprises a reference system, for example a silver / silver chloride system. This reference system is generally formed from a liquid or gel electrolyte, for example, a potassium chloride solution or a gel comprising a potassium chloride solution, and a drain electrode immersed in the electrolyte. The lead-out electrode often consists of a silver wire coated with silver chloride. The discharge electrode is electrically conductively connected via a contact point arranged outside the electrolyte with the measuring electronics. This reference system is housed in a completely closed compared to the measuring half-cell housing. In measuring operation, the reference system is in contact with the measuring medium for charge equalization, for example via a diaphragm provided in the housing wall of the reference half-cell.

In addition, such measuring probes can be provided with an additional electrode arranged predominantly on a housing outer wall of the measuring probe. This is used, inter alia, for sensor diagnosis, for example for measuring and monitoring the electrical resistance or the electrical impedance of the diaphragm of the reference half cell or the ion-sensitive membrane or layer of the measuring half cell. In addition, such an additional electrode can be used to ground a measuring medium, for example a process solution in which the measuring probe is immersed, or to set it to a defined potential. In addition, the redox potential of the measuring medium can be determined with the aid of such an additional electrode. The additional electrode is electrically conductively connected to a contact point, which is arranged outside the immersion region of the measuring probe in the measuring operation, and which in turn is connected in an electrically conductive manner to the measuring electronics.

The lead electrodes of the measuring half cell and the reference half cell and the additional electrode are often designed as connected to the measuring electronics metal wires or connected by means of metallic wires to the measuring electronics. Such a designed measuring probe is described for example Sn DE 102 43 930 A1 without additional electrode or in US 2003/0132755 A1 with additional electrode.

A disadvantage of a measuring probe according to DE 102 43 930 A1, in which the discharge electrodes are designed as wires, is that these wires must be passed through a connecting wall terminating the measuring probe or closure cap, so that they with the

Measuring electronics can be connected. This means that a liquid and preferably Gas or vapor-tight implementation of the wires through glass or adhesive layers must be provided. A liquid and preferably also gas or vapor-tight execution is to ensure safe only by increased effort, for example by providing additional sealing elements. This makes the production very complicated and complicates the automation of the production of such probes.

A disadvantage of a measuring probe with a wire-shaped additional electrode is that it requires a complex fastening process in the course of the production process of the measuring probe, for example by melting the additional electrode into the housing of the measuring probe, and secondly that it is susceptible to defects.

To overcome these disadvantages, it has been proposed to configure the discharge electrodes or the additional electrode as electrically conductive layers. Thus, DE 10 2005 033 727 A1 describes a potentiometric measuring probe, in particular for pH measurement, in which the discharge electrodes are each designed as a coating of a housing inner wall of the measurement or reference half cell. EP 1 610 120 A1 describes a potentiometric measuring probe with a metallic coating, for example of platinum, applied on the housing outer wall of the measuring probe as an additional electrode.

Disadvantages of these solutions result from the fact that the entire electrically conductive layer acts as an active electrode surface. Thus, in particular in the presence of different electrolyte concentrations or Elektrofytaktävitäten at different areas of the electrode surface concentration elements occur, which can lead to a progressive destruction of the electrode and to a falsification of the measurement results. Such negative-affecting concentration elements can occur, for example, on electrically conductive layers which, like the layers described in EP 1 610 120 A1, are applied to an outer wall of the housing and are in contact with the measuring medium if substances in the measuring medium and / or in adjoining phases, such as a foam layer or adjacent to the measuring medium gas phase, not in chemical equilibrium. In addition, possibly lying on the outer housing wall of the measuring electrode electrically conductive layers can be affected by corrosive substances in their functionality, for example by passivation or corrosion.

Even when formed as electrically conductive layers Ableätelektroden in the housing interior of the probe such undesirable effects, especially in the field of implementation of the

Metal layer through the housing wall towards an outside of the housing

Contact point at which the layers are each electrically conductively connected to the measuring electronics, occur. Thus, although the sealing of the bushing itself is simplified, the layer must be protected from corrosive vapors or other harmful substances by further sealing measures in the region of the contact point lying outside the measuring or reference half-cell housing, for example.

Even if the surface of the electrically conductive layer which acts as an electrode is indeterminate, in particular not constant, over the period of use of the measuring probe, this can also falsify the measurement and diagnostic results, in particular when using the electrically conductive layer as additional electrode for the sensor diagnosis. to lead. A non-constant electrode surface, for example due to a varying immersion depth of the acting as an electrode surface of the electrically conductive layer, wherein the varying immersion depth, for example, by a level change of the measuring medium or the inner electrolyte or the reference electrolyte of the probe may result, or if on the surface of the medium to be contaminated, such as by öiige substances or foam.

In the case of electrically conductive layers on the housing outer wall of the measuring probe, mechanical wear phenomena may also occur, for example for abrasion of the electrically conductive layers in contact with the measuring medium.

It is therefore the object of the invention to specify a generic measuring probe which overcomes the disadvantages of the prior art. In particular, a high stability of the electrodes, as well as a high accuracy of measurement or reliability of diagnostic measurements should be ensured even under adverse conditions.

This object is achieved by a measuring probe for electrochemical, in particular for potentiometric, measurements comprising a component formed at least on a surface of electrically insulating material, in particular glass or plastic, wherein the measuring probe has at least one electrode with an electrically insulating on the off Material formed surface of the component applied electrically conductive coating is electrically conductively connected, wherein at least a portion of the electrically conductive coating is overcoated with at least one electrically insulating layer.

A component of the measuring probe may, for example, be understood to be a single component, an assembly or a subregion of a component or an assembly of the measuring probe. Under an electrode is here and below an electrically conductive at least on the surface Component of the probe understood, which is in contact with a liquid or gel electrolyte, in particular also a measuring medium.

An electrically conductive connection is to be understood as meaning a connection which allows charge transport between two spatially remote points to be connected. This charge transfer is preferably carried out metallically conductive, but can also be carried out semiconducting or ioπenleitend or in combinations thereof. Such a compound can be produced by attaching the contacting coating to, for example, a junction

Electrode is sputtered or on the provision of conductive interconnected intermediate layers, via compression, melting or with the help of conductive adhesive, etc.

By covering the electrically conductive coating, which merely serves to make contact with the actual electrode, with an electrically insulating layer in at least one subarea, no unwanted reactions can occur in this area, for example due to the formation of concentration elements or falsification of measured values or diagnostic parameters due to contamination in the area of an interface , For example, between the measuring medium and an adjacent gas phase or an inner electrolyte of the probe and an adjacent gas phase occur. In addition, the electrically insulating layer serves to protect the underlying electrically conductive coating from mechanical wear, for example abrasion.

In one development, the measuring probe has a housing, wherein the component formed from electrically insulating material forms at least a part of the housing.

In one embodiment, the electrically conductive coating is applied at least partially on an outer wall of the housing of the measuring probe. In this case, the electrically conductive coating can be used to make contact with an additional electrode which is applied to the outside of the housing and which is brought into contact with the measuring medium during measuring operation.

In a further embodiment, the housing of the measuring probe comprises an immersion region for immersion in a measuring medium for performing electrochemical, in particular potentaeometric, measurements, wherein the at least one electrically conductive coating is conductively connected to an electrical contact point, which is arranged outside the immersion region. The immersion region is understood to be that region of the measuring probe which comes into contact with the measuring medium during a measurement.

In one development, the electrode is arranged on the outside of the housing of the measuring probe with its entire surface within the immersion region. The conductive coating used for Contacting the electrode is used, extends according to the part in the immersion area and partly in a non-immersion area belonging to the surface area of the housing outer wall. The electrically insulating layer preferably extends both over the part not in the immersion region! the electrically conductive coating as well as in the immersion compartment, so that the electrically conductive coating in the region of the interface between the Meßmedtum and the adjacent gas phase is also electrically and chemically isolated.

In a further embodiment, the electrically conductive coating comprises a metal, in particular platinum, silver, copper or gold, a composite material with metal pigments, in particular platinum, silver, copper or gold pigments, for example silver-conductive ink, an electrically conductive polymer, in particular the group comprising polyacetylenes,

Polyphenylenes, Polyphenyienvänylene, Polyfurane Polyaniiine, Polypyrrole, Polythiophene, poly (3,4-ethylenedioxythiophene, PEDOT) and their derivatives, doped carbon, carbon nanotubes, doped carbon nanotubes, an electrically conductive ceramic, or an electrically conductive oxide , in particular an electrically conductive oxide such as antimony tin oxide (SnO ₂ : Sb),

Indium zinc oxide (ZnO: In), aluminum zinc oxide (ZnO: Al), fluorine tin oxide (SnO ₂ : F) or indium tin oxide

(ITO), These conductive oxides have the added advantage of being visible

Spectral range are transparent. They therefore allow a visual inspection of the region of the measuring probe located below the coating, for example of a housing interior, even if the electrically conductive coating is applied over a large area to the component of the measuring probe.

From a production-technical point of view, the electrically conductive oxides in conjunction with an electrically insulating housing of Gias represent particularly advantageous materials for the electrically conductive coating, since processes known from the field of plasma picture screen technology are known to apply electrically conductive oxide layers of very good stability to glass substrates. As a rule, these coatings can be applied without an adhesion promoter layer between the glass substrate and the electrically conductive oxide layer. In contrast, when using metal layers as the electrically conductive coating on a glass substrate is often the

Provision of a primer layer advised.

The layer thickness of the electrically conductive coating is, for example, in the range between 5 nm and 500 μm, in particular between 50 nm and 5 μm, in particular between 50 and 700 nm.

In a further embodiment, the insulating layer comprises an oxide applied by sputtering in particular or a plastic, in particular perfluoropolymers, polytetrafluoroethylene (Teflon), polyetheretherketones (PEEK), poly-sulfones (PSU), polyvinyl chloride (PVC) ₁ polyethersulfones (PES) or an insulating ceramic. The said oxide can be selected in a development from the group consisting of SiO ₂ , silicates, silicate glasses, oxide glasses and Al ₂ O ₃ .

The layer thickness of the insulating layer can be arbitrarily large in principle, it is in particular 50 nm to 500 .mu.m, in particular 50 to 5 .mu.m, in particular 50 to 700 nm.

In a further embodiment, the electrically insulating layer is at least partially overlaid with at least one further electrically conductive layer. This additional electrically conductive layer may serve the electromagnetic shielding of the underlying, separated by the electrically insulating layer of the additional electrically conductive layer electrically conductive coating for contacting the electrode. In this case it is placed on a defined potential, in particular on ground. The additional electrically conductive layer may alternatively also serve for contacting a further electrode.

In a further development of this embodiment, a layer stack, a so-called multilayer, may be provided instead of a single electrically conductive coating overcoated with an insulating layer or a layer package with a first electrically conductive reinforcement overlaid with an electrically insulating layer and a further electrically insulating layer arranged thereon comprising any number of electrically conductive layers, each separated at least by an insulating layer. The various electrically conductive layers can serve, for example, for contacting a plurality of different electrodes. The outermost electrically conductive layer can serve as electromagnetic shielding.

The further electrically conductive coating or, in the case of a multilayer, the further conductive layers is or are connected, for example, to a respective contact point which is arranged outside the immersion region of the measuring probe and via which a defined potential or ground is applied to the electrically conductive layer can be tapped or a potential or another electrochemical characteristic can be tapped.

As materials for the further electrically conductive layer or the further electrically conductive layers, in principle all materials mentioned above are suitable, which are also suitable for forming the electrically conductive coating for contacting the electrode. Because of their transparency in the visible spectral range, the above-mentioned electrically conductive oxides are particularly suitable as a material for the further electrically conductive layer. Optionally, the further, arranged on the first insulating layer, electrically conductive layer or the uppermost electrically conductive layer of MultÜayers be covered with at least one further insulating coating, which they both electrically and chemically compared to the measuring medium or substances in the the measuring medium adjacent gas phase isolated, or protects the underlying layer package from mechanical wear, such as abrasion. This optional insulating coating can consist, for example, of an oxide applied by means of sputtering or a plastic, in particular from the group comprising perfluoropolymers, polytetrafluoroethylene (Teflon), polyetheretherketones (PEEK), polysulphones (PSU), polyvinyl chloride (PVC) and polyethersulphones (PES). The said oxide may be selected in a Weiterbiidung from the group comprising ₂ S1O, silicates, Siltkatgiäser, oxidic glasses and AI2O3.

The layer thicknesses of the further electrically conductive layers and the further insulating layers can be selected in the same order of magnitude as the layer thicknesses of the electrically conductive coating for contacting the electrode or the insulating layer arranged on this electrically conductive coating.

In one possible embodiment, the electrode is formed at least by an electrically conductive layer comprising a metal, in particular platinum, silver or gold, which is applied to a further part of the electrically conductive coating applied to the assembly or component. The metal layer can be covered with additional layers. For example, to form a deflection electrode of a reference half-cell of the measuring probe, the metal layer may be formed of silver and may be coated with another layer of a sparingly soluble silver salt, e.g. Silver chloride, to be overcoated.

In an alternative embodiment, the electrode is formed by a shaped body, in particular a ring or a pin, of a material which comprises a metallic conductor, in particular a metal, an ion conductor and / or a semiconductor. In the case of a housing with a substantially circular cross section, an electrode in the form of a metal ring which can be glued or pressed onto the housing is particularly advantageous. If the housing is made of Gias, the metal ring can also be fixed by melting on the housing wall.

In a further embodiment, the housing has a first housing part, which delimits a first housing interior, and a second housing, which delimits a second housing interior, wherein the first and the second housing interior are liquid-tightly separated from each other, and wherein the first housing part is part of a measuring half-cell and the second housing part is part of a reference half-cell. The two housing parts can be connected to each other in such a way that they form a combination electrode, for example for measuring an ionic activity or a pH.

In such a configuration of the measuring probe as a single-rod measuring chain, the first and / or the second housing part or at least one further third housing part is in contact with the measuring medium with its housing outside in an immersion region.

In a further development, the measuring half-cell comprises the first, in particular tubular, housing part, a measuring diaphragm, which is arranged in particular on a first end portion of the first Gehäuseteüs and the first housing part closes liquid-tight, a first electrolyte, in particular comprising a buffer solution, a first Ableitelektrode with the first electrolyte is in contact to derive a first electrical potential.

The reference half-line in this development comprises the second, in particular tubular, housing part, at least one second electrolyte, in particular a potassium chloride solution or a

Gel containing potassium chloride solution, which is in contact with a, the second housing part at least in a partial area surrounding medium, a second discharge electrode, which is in contact with the at least one second electrolyte to derive a second electrical potential.

In this case, the second housing part may surround the first tubular housing part at least in sections and be connected to the first tubular housing part adjacent to the first end section, so that an annular housing interior is formed between the first tubular housing part and the second tubular housing part, in which the second

Electrolyte is added. This annular chamber forms the housing of the Referenzhaibzelle, while the first tubular housing part forms the housing of the measuring half cell.

In an alternative development, the measuring half-cell can be equipped with a fixed discharge. In this case, the measuring half cell comprises the first, in particular tubular, housing part, a measuring diaphragm, which is preferably arranged at a first end portion of the first tubular housing part and the first housing part liquid-tight, a solid electrically conductive assembly which is electrically conductively connected to the measuring diaphragm to derive a first electrical potential. The reference half-line comprises the second, in particular tubular, housing part, at least one electrolyte, in particular a potassium chloride solution or a gel comprising a potassium chloride solution, which is in contact with a medium surrounding the second housing part at least in a partial region, an electrode which is in contact with the at least one second electrolyte to dissipate a second electrical potential.

In this configuration of the measuring probe as a single-rod measuring chain, for example the Abieitelektroden or the solid dissipation forming solid electrically conductive assembly mitteis an electrically conductive coating, which is covered with at least one other, in particular an electrically insulating layer, contacted and with a in a connection-side region of Measuring probe arranged contact point for connection to a measuring electronics are electrically conductively connected.

In a further refinement, the at least one electrically conductive coating which is at least partially coated with at least one electrically insulating layer and the electrode connected therewith in an electrically conductive manner is applied to an outer wall of the first and / or the second and / or another housing part. This brings about a simplification for the production of the electrically conductive coating. If the electrically conductive coating is applied, for example, by means of sputtering or vapor deposition, the outside is more easily accessible for a corresponding tool, while the splitting or vapor deposition of a coating in a housing interior, depending on the housing size, can be very complicated.

If the outer wall of the first, second or further housing part simultaneously forms the outer wall of the measuring probe that is in contact with the measuring medium during measurement operation, the electrode in this embodiment can be used as an additional electrode. If the outer wall of the first, second or further housing part forms an inner wall of a housing part of the measuring probe which forms the housing of the measuring half cell or of the reference half cell, then the electrode can serve as a deflection electrode of the measuring or reference half line.

The invention also encompasses a device for carrying out potentiometric measurements comprising: a measuring probe according to one of the embodiments or developments described above and a higher-order unit, in particular a measuring transducer, wherein the superordinate unit is provided with the electrically conductive coating applied to the component formed from electrically insulating material Verbindungsseiemente, in particular via elements for Signalwandiung and / or for signal transmission and / or power transmission, is connected. Such elements may include individual electronic components, such as a microcontroller, but also entire circuits or circuit parts.

In one embodiment, the device for carrying out potentiometric measurements comprises means for setting a defined potential at the at least one electrode electrically conductively connected to the electrically conductive coating and / or means for determining an electrode electrically conductively connected between the electrically conductive coating and another Part of the device prevailing electrical or electrochemical characteristic, in particular a potential difference, a resistance or an impedance. The latter allows the monitoring of the measuring probe, in particular with regard to the functionality of individual half-cells or, if the measuring probe is configured as a single-rod measuring chain, of the entire combination electrode. With the aid of the means for adjusting a defined potential at the at least one electrode electrically conductively connected to the electrically conductive coating, a measuring medium in which the measuring probe is immersed with the additional electrode can be earthed or grounded in the case where the electrode forms an additional electrode a defined potential. In this way, the measuring probe can be connected symmetrically, as described for example in the patent EP 1248101 B1.

The means for detecting a potential difference between the electrode electrically conductively connected to the electrically conductive coating and another part of the device, in particular a further electrode, may comprise an input of a measuring amplifier or of an impedance converter, which is adapted to a potential difference between the with the electrically conductive coating connected electrode connected electrode and another part of the device, in particular a further electrode, as a voltage or current signal! output and the amplified or converted voltage signal for analog / digital conversion, display and / or processing to the parent unit passes.

The invention will now be explained in more detail with reference to the embodiments illustrated in the drawings. Show it:

1 is a schematic longitudinal sectional view of a combination electrode for determining a pH of a measuring medium.

2 is a schematic longitudinal sectional view of a combination electrode for determining a pH of a measuring medium according to a first embodiment; Fig. 3 is an electrically conductive coating which is electrically conductive with an electrode of

Combination electrode acc. Fig. 2 is connected;

4 is a schematic longitudinal sectional view of a combination electrode for determining a pH of a measuring medium according to a variant of the first

Embodiment;

5 is a schematic longitudinal sectional view of a combination electrode for determining a pH of a measuring medium according to a second embodiment;

Fig. 6 is an electrically conductive coating which is electrically conductive with an electrode of

Combination electrode acc. Fig. 5 is connected;

7 shows a schematic illustration of a device for carrying out potentiometric measurements with a measuring probe according to FIG. 2.

In Fig. 1, the basic structure of a conventional designed as a single rod measuring probe 1 is shown first. The measuring probe 1 has an electrically insulating housing, for example of Gias, which has, as a first housing part, an electrically insulating inner tube 3 which is closed off at one end by a pH-sensitive glass membrane 5. The inner tube 3 is surrounded by a second housing part designed as an electrically insulating shaft tube 7, wherein the shaft tube 7 is connected to the inner tube 3 in its end region facing the glass membrane 5, so that an annular space extending around the inner tube 3, from the housing interior of the inner tube 3 completely liquid-tight separated and electrically isolated chamber is formed.

The enclosed by the glass membrane 5 and the inner tube 3 Gehäυseinnenraum the inner tube 3 is filled with a pH-buffering solution 13 of known pH, into which a discharge electrode 11, which is formed for example of a chlorided silver wire dips. The measuring half-cell of the measuring probe 1 thus formed is electrically conductively connected to a measuring electrode (not shown) via a contact point 15 which is connected in an electrically conductive manner to the deflecting electrode 11. The Riπgkammer enclosed by the housing outer wall of the inner tube 3 and the housing inner wall of the shaft tube 7 is filled with a Referenzelektroiyt 17, for example, a 3 molar aqueous potassium chloride solution. In the reference electrolyte 17, a Ableiteiektrode 19 dips, which may be formed as the lead electrode 11 as silver chloride coated with silver wire. In the shaft tube 7, an opening 21, for example, a diaphragm, is provided, which is an exchange of charge carriers between the through the shaft tube. 7 limited annular chamber and the environment allows. The reference half-line of the measuring probe 1 thus formed is electrically conductively connected to the measuring electronics via a contact point 23 connected to the deflecting electrode 19, which comprises, inter alia, means for determining the potential difference between the potential of the measuring half cell and the potential of the reference half cell. The measuring electronics can, for example, be housed at least in part in a plug-in head attached to the measuring probe 1 itself or in a measuring transducer connected to the measuring probe 1. In measuring operation, the measuring probe 1 is immersed in a measuring medium 25. The region of the housing outer wall of the measuring probe 1, which comes into contact with the measuring medium 25, is referred to as the immersion region.

The measuring probe 1 has, in an end section opposite the immersion region, one or more end parts which enclose the electrolyte or buffer solution in the reference or measuring half-line. The diverting electrodes 1 1, 19 are passed through the end part (s) and are electrically conductively connected to the contact points 15, 23 arranged outside the half-cells.

On the outer wall of the shaft tube 7, which simultaneously forms an outer wall of the probe housing, an electrically conductive coating 27, for example made of platinum, is applied in the form of a longitudinal strip extending from the immersion region of the measuring probe 1 to a contact point 29 arranged outside the immersion region. The partially immersed in the measuring medium 25 during measurement operation electrically conductive coating 27 can be used as an additional electrode, for example as a redox electrode, by the contact point 29 is connected to the measuring electronics.

As described at the outset, concentration elements can occur in the measurement operation due to different electrolyte concentrations at different regions of the electrode surface, which can lead to a progressive destruction of the conductive coating 27 acting as an electrode and to a falsification of the measurement results. In addition, corrosive substances can affect the electrically conductive coating 27 in its functionality, for example by passivation or corrosion.

A change in the depth of immersion of the measuring probe 1 in the measuring medium 25 or the presence of contamination at the interface between the measuring medium 25 and the adjoining gas phase 31 may also lead to a falsification of the measurement or diagnostic results. In addition, solid particles present in the measuring medium 25 can stress the coating 27 mechanically and lead to abrasion, which in turn can impair the conductivity of the coating 27. 2 shows a generic measuring probe 201 for measuring the pH value with an additional electrode which is equipped in such a way that these sources of error are avoided. Like the measuring probe shown in FIG. 1, the measuring probe 201 has an electrically insulating housing, which comprises a first housing part for forming the measuring half cell, which consists of an electrically insulating inner tube 203 of the measuring probe 201 and a dome-shaped glass membrane 205 terminating the inner tube 203 at one end is formed. The inner tube 203 is filled with a pH buffering solution 213 of known pH, for example, with a potassium chloride aqueous acetic acid / acetate buffer solution, into which a dip electrode 211 formed by a chlorided silver wire is inserted, through which the measurement half is terminated final conclusion part is passed, and outside the measuring half-cell housing via a contact point 215 can be electrically connected to a measuring electronics.

As in the case of the measuring probe shown in FIG. 1, the reference half cell is formed by an electrically insulating shaft tube 207 surrounding the inner tube 203, which is connected in one end region to the inner tube 203 to form an annular chamber. The annular chamber is filled with a reference electrolyte 217, for example a 3-molar aqueous KCl solution, into which a discharge electrode 219, likewise formed by means of a chlorided silver wire, is immersed. Of the

Reference electrolyte 217 may also be incorporated into a gel, for example based on diallyldimethylammonium chloride (DADMAC), which fills the reference half-cell. in the

Shaft tube 207 is an opening 221, for example, a diaphragm, is provided which a

Charge carrier exchange between the reference half cell and the environment of the probe 201 allowed. The Ableitelektrode 219 is by a reference HalbzeNe terminal side final

Passed end part and can be electrically connected via a lying outside the reference half-cell housing contact point 223 with the measuring electronics.

On the outer wall of the shaft tube 207 is located in an immersion region at the membrane-side end portion of the probe 201, an additional electrode 233, which is melted into the glass of the shaft tube 207 as a metal ring, for example as a platinum ring. The additional electrode 233 is electrically conductively connected to a contact point 229 arranged outside the immersion region via an electrically conductive coating 235 on the outer wall of the shaft tube 207. In the exemplary embodiment illustrated in FIG. 2, the electrically conductive reinforcement 235 is designed in the form of a longitudinal strip running parallel to the probe axis. Equally, however, the electrically conductive coating 235 can also cover a larger area, in particular also the entire cylinder jacket surface of the outer wall of the shaft tube 207 above the additional electrode 233. As the material for the coating 235 are, for example, transparent, electrically conductive oxides, such as indium tin oxide ITO, antimony tin oxide (SnG ^ Sb), indium zinc oxide (ZnO: ln), Aluminum zinc oxide (ZnO: Al) or fluorine tin oxide (SnÜ ₂ : F) in question, but also the aforementioned electrically conductive polymers, metals, doped plastics, etc. Particularly advantageous transparent, electrically conductive oxides can be used if a large-area coating 235 desired is because using these layers, in contrast to metal layers, the probe inside is visible, so that, for example, the level of the reference half-line with electrolyte, defects of the dissipation or reference electrode or other problems with the electrolyte are visually recognizable even with large-area coating 235. For the same reason, such layers are also excellently suitable as electromagnetic sheaths covering the shaft tube 207.

The electrically conductive coating 235 is almost completely overcoated with another layer 237 of electrically insulating and preferably chemically inert material, for example sputtered SiO ₂ , and thus almost completely chemically and electrically insulated from the surroundings of the measuring probe 201 against aggressive media, on the other hand also an electrical insulation of the electrically conductive coating 235. If the measuring probe 201 immersed in measuring mode with its immersion area in a measuring medium 225, the metal ring designed as an additional electrode 233 always completely immersed in the measuring medium 225. On the other hand, the overlaying with an electrically insulating further layer 237 prevents chemical reactions and mechanical abrasion on the electrically conductive coating 235.

In an alternative embodiment, the additional electrode instead of a metal ring as a metallic coating, such as platinum, be configured on the outer wall of the shaft tube. In this case, it is advisable to provide a primer layer of chromium or another transition metal. The contacting of the platinum layer used as an additional electrode via an electrically conductive coating 235, which is electrically insulated from the environment by means of a further insulating layer 239, would be designed in this case analogously as in the embodiment of FIG.

In Fig. 3, the structure of the electrically conductive coating 235 and the further layer 237 on the outer wall of the shaft tube 207 is shown again in detail. 3 a) shows a plan view of the layer system, while FIG. 3 b) shows a cross section through the layer system and the wall of the shaft tube 207 along the line A, and FIG. 3 c) shows a cross section through the layer system and the wall of the shaft tube 207 along the lines B and C shows.

On the shaft tube 207 made of glass, the electrically conductive coating 235, for example, indium tin oxide (ITO), applied. ITO ₁ as well as the other mentioned electrically conductive Oxides, already adheres without adhesion promoter layer very well on glass substrates, but it can be an additional layer, for example, from splintered SiO ₂ , are provided (not shown here). This coating serves in particular as insulation against the diffusion of ions from the material of the shaft tube 207 into the electrically conductive oxide layer. If the electrically conductive coating 235 is formed from a different material, for example from platinum, an additional adhesion promoter layer, for example of titanium, chromium, molybdenum, tantalum or tungsten, may be provided between the glass substrate and the electrically conductive coating 235. The electrically conductive coating 235 is overcoated in a central region over almost its entire length with a further layer 237, for example sputtered SiO ₂ . In a first end region, the electrically conductive coating is covered by a contact point 229 directly adjacent to the further layer 237, which can likewise be configured as a metallic layer, for example of gold. At the contact point 229, a Anschiussdraht be soldered, which connects the contact point 229 electrically conductive with a measuring electronics. The opposite end of the coating 235 formed as a longitudinal strip is covered with a conductive layer, for example made of platinum, which adjoins the SiO ₂ layer 237 laterally, forming the additional electrode 233. Alternatively, the platinum ring of the additional electrode 233 shown in FIG. 2 can be arranged laterally, for example melted, laterally adjacent to the SiO ₂ layer 237.

FIG. 4 shows a variant of the measuring probe shown in FIG. 2 for pH measurement. The reference and Messhaibzeile the probe 201 'is identical to the corresponding halftimes of the probe 201 of FIG. 2 with an inner tube 203', which is closed by a dome-shaped glass membrane 205 'at one end, and in which a buffer solution 213' added is, in which a discharge electrode 211 'is immersed. The reference electrolyte 217 'is accommodated in a chamber formed by a shaft tube 207' connected to the inner tube 203 '. The potential of the reference half-cell is derived by a discharge electrode 219 'immersed in the reference electrolyte. By means of a diaphragm 221 'is a charge carrier exchange between the reference electrolyte 217' and the measuring medium 225 'gewährleitstet. The diverting electrodes are connected to the contact points 215 ', 223' outside the immersion region of the measuring probe 201 '. On the outer wall of the shaft tube 207 'is formed as a ring auxiliary electrode 233', for example, melted.

The additional electrode 233 'is connected by means of an electrically conductive coating 235' to the outside of the Eäntauchbereichs arranged contact point 229 ', via which an electrical characteristic, such as a potential can be tapped or adjusted. The layer 235 'may, for example, consist of a conductive oxide. The layer thickness of the layer 235 'is for example 310 nm. The coating 235' is a large-area coating all around on the Zylindermantetfläche the shaft tube 207 'applied. It is covered with an insulating layer 237 ', which consists, for example, of splintered SiO ₂ , with a layer thickness of, for example, 500 nm.

On this insulating layer 237 ', a further electrically conductive coating 236' which in turn extends all around the cylinder jacket surface of the shaft tube 207 'is applied, which preferably consists of an optically transparent conductive oxide and whose thickness is approximately 300 nm. This additional electrically conductive layer 326 'serves as electromagnetic shielding and, as such, can be acted upon or grounded by way of a (not shown) additional contact point with a defined potential.

Optionally, the additional electrically conductive layer 236 'may be overcoated with at least one further insulating layer 238', which protects it from aggressive media, or is electrically insulated from the surroundings of the measuring probe. The additional insulating layer 238 'may be made of Teflon or of sputtered SiO ₂ , for example.

Furthermore, the additional electrically conductive layer, which is covered with an electrically insulating layer, also serve for the electrically conductive contacting of a further additional electrode, for example a further platinum ring. Thus, it is possible, for example, with the help of the two additional electrodes, which are electrically connected via a contact point to a corresponding higher-level unit, such as a transmitter, connected to determine the electrical conductivity of Messmedäums.

FIG. 5 shows a measuring probe 501, likewise designed as a single-rod measuring chain, in which the supply line to the reference electrode 519 is realized by means of an electrically conductive coating 537.

In this case, the electrically insulating measuring probe housing is of analogous design as in the measuring probes shown in FIGS. 1 and 2, namely with a measuring half cell with a likewise electrically insulating inner tube 503, a Giasmembran 505, a recorded in the inner tube 503

Buffer solution 513 and a discharge electrode 511 for deriving the measuring half-cell potential, which is electrically connected via a contact point 515 in connection with the measuring electronics, and a

Reference half-cell, the liquid-isolated in a second, compared to the measuring half-cell

Housing part in the form of a connected to the inner tube 503, electrically insulating shaft tube

507 is recorded. The reference half-cell comprises a reference electrolyte 517, which has a

Opening 521, for example, a diaphragm for charge carrier exchange with the measuring medium in contact. The discharge electrode 519 of the reference cell is configured as a silver / silver chloride layer on the outer wall of the first housing part, namely the electrically insulating inner tube 509. The silver / silver chloride layer is connected via an electrically conductive coating 539 to a contact point 523 outside the reference half cell, by means of which the discharge electrode 519 can be electrically connected to the measuring electronics. The electrically conductive coating 539 is covered in a region between the contact point 523 and the reference electrode 519 with a further layer 541, for example made of SiO ₂ , and thus electrically and chemically isolated from the reference electrolyte 517 and the environment.

FIG. 6 shows in more detail the layer structure of the discharge electrode 519 and the feed line formed by the electrically conductive coating 539 for the measuring electronics. Fig. 6 a) shows a plan view of the layer system, while Fig. 6 b) shows a cross section through the layer system and the wall of the inner tube 503 along the line A, and Fig. 6 c) shows a cross section through the layer system and the wall of the Inπenrohres 503 along lines B and C shows.

On the electrically insulating inner tube 503, which is preferably formed of glass, the electrically conductive coating 539, for example of ITO, applied with a layer thickness of, for example, 310 nm. As described above with reference to FIG. 3, an intermediate layer of SiO ₂ may be provided between the ITO layer and the glass substrate. The electrically conductive coating 539 is covered in a first region with a silver / silver chloride layer. This silver / Siiberchlorid layer forms the Ableitelektrode 519. It can be prepared, for example, by first electrochemically, by sputtering or by adhering a silver foil, a silver layer is applied to the ITO layer, which is then chlorided. In a second region, the electrically conductive coating 539 is covered with at least one further layer 541, which is directly laterally adjacent to the silver / silver chloride layer, of electrically insulating and preferably chemically inert material, for example sputtered SiO ₂ , with a layer thickness of, for example, approximately 500 nm. Only in one end region of the electrically conductive coating remains uncovered. This area serves as a contact point 523, to which a connection wire can be attached, which connects the contact point 523 in an electrically conductive manner to a measuring electronics. Alternatively, the electrically conductive coating 539 in the region of the contact point 523 may also be covered with a metal layer, for example of gold, in order to provide a better material transition to the connection wire.

In an alternative embodiment, the electrically conductive coating itself may be formed as a silver layer, which is chlorided in an end region, and so in this area

Reference electrode is used. In as an electrically conductive connection of the contact point to Reference area serving further area, the silver layer is covered in this embodiment with an electrically insulating layer.

An advantage of this embodiment of the measuring probe with an electrically conductive coating 539, which is covered with a further electrically insulating layer 541, on the one hand is the simplified sealing of the reference half cell. This results from the fact that on a difficult to tight implementation of a metal wire or pin as Ableitelektro, as in the measuring probes gem. Fig. 1 or 2 necessary, can be omitted.

As in the example of FIG. 4, a further electrically conductive coating for electromagnetic shielding can be provided. This can in turn be optionally covered with a further insulating layer.

An additional electrode is not shown in the example of FIG. 5. Of course, however, the measuring probe 501 can also be equipped with such an additional electrode, for example in the embodiment according to FIG. 2.

In a modification, in the case of a measuring probe of the type shown in FIG. 5, the lead-in electrode of the measuring half-cell can also be electrically conductively connected to the pad 515 by means of an electrically lettable coating, which is covered with an electrically insulating further layer. The discharge electrode and the electrically conductive coating can be configured analogously, as described for the reference electrode 519 of the reference half-cell, and applied to the inside of the inner tube 503,

FIG. 7 schematically shows a device for carrying out potentiometric measurements. In the example of FIG. 7, this is a device for carrying out a pH measurement mitteis a measuring probe, which is designed as described with reference to FIG. 2, and a connected to the probe via a connector SK transmitter MU, the For example, it may be connected to a process control center L. The measuring electronics (not shown) may be completely housed in the transmitter MU, but it may also be housed, at least in part, in the plug head of the probe, which forms the sensor-side part of the connector. It comprises, inter alia, means for setting a defined potential at the additional electrode 233 connected to the contact point 229 by means of the electrically conductive coating 235, so that the additional electrode 233 can be used, for example, to lay the measuring medium 225 at a defined potential. Alternatively or additionally, the measuring electronics may further comprise means to connect one between the additional electrode 233 and another part of the device, for example one of the discharge electrodes 211, 219 to determine prevailing electrical or electrochemical characteristic. As such characteristic, for example, the impedance comes into question. This can be determined for the purpose of sensor diagnosis. Another such characteristic may be a potential, for example, between the additional electrode and the Referenzhaibzeile. In this case, the additional electrode serves as a redox electrode. By means of two additional electrodes next to it, for example, the electrical conductivity of the medium can be determined.

The invention is not limited to the exemplary embodiments shown and encompasses any further technically possible implementation, which falls within the scope of the following claims. Thus, for example, electrodes of a measuring probe of a conductivity sensor can be contacted by means of an electrically conductive coating or a layer stack with a plurality of electrically conductive shafts according to one of the embodiments described above. Likewise, electrodes of a measuring probe of an amperometric sensor can also be contacted by means of such a coating or a layer stack.

Claims

claims

1. Measuring probe (201, 201 ', 501) for electrochemical, in particular potentiometric, measurements comprising at least on a surface of electrically insulating material, in particular glass or

Plastic, formed component, wherein the measuring probe (201, 201 ', 501) has at least one electrode (233, 233', 519), which is provided with an electrically conductive coating (235, 235 ') applied to the component formed from electrically insulating material 539) is electrically conductively connected, characterized in that at least one part of the electrically conductive coating (235, 235 ', 539) is covered with at least one electrically insulating layer (237, 237', 541).

2. Measuring probe (201, 201 ', 501) according to claim 1, wherein the measuring probe (201, 201', 501) has a housing and the at least formed on a surface of electrically insulating material component forms at least part of the housing.

3. measuring probe (201, 201 ', 501) according to claim 2, wherein the electrically conductive coating (235, 235', 539) is at least partially applied to an outer wall of the housing.

4. Measuring probe (201, 201 ', 501) according to claim 2 or 3, wherein the housing of the measuring probe (201, 201', 501) has an immersion area for immersion in a measuring medium (225, 225 ', 525) for carrying out electrochemical, in particular potentiometric, measurements, wherein the at least one electrically conductive coating (235, 235 '539) with an electrical contact point (229, 229', 523) is electrically conductively connected, which is arranged outside the immersion region.

5. measuring probe (201, 201 ', 501) according to one of claims 1 to 4, wherein the electrically conductive coating (235, 235', 539) a Metail, in particular platinum, silver, copper or gold, a composite material with metal pigments, in particular Platinum, silver, copper or gold pigments, for example silver conductive ink, an electrically conductive polymer, in particular from the group comprising polyacetylenes, polyphenylenes, polyphenylenevinylenes, polyfurans, polyanilines, polypyrroles, polythiophenes, poly (3,4-) ethylenedioxythiophene) (PEDOT) and their Derivatives, doped carbon, in particular carbon nanotubes (nanotubes), an electrically conductive ceramic, or an electrically conductive oxide, in particular a transparent electrically conductive oxide such as antimony tin oxide (SnO ₂ : Sb), indium zinc oxide (ZnO: In), aluminum zinc oxide (ZnO: Al), fluorine tin oxide (SnO ₂ : F) or indium tin oxide (ITO).

6. measuring probe (201, 201 ', 501) according to any one of claims 1 to 5, wherein the insulating layer (237 _r 237', 541) applied in particular by means of sputtering, oxide or a plastic, in particular from the group comprising perfluoropolymers, polytetrafluoroethylene , Polyether ether ketones (PEEK), polysulfones (PSU), polyvinyl chloride (PVC), polyethersulfones (PES), or an electrically insulating ceramic.

The measuring probe (201, 201 ', 501) according to claim 6, wherein the oxide is selected from the group consisting of SiO ₂ , silicates, silicate glasses, oxide glasses and Al 2 O 3.

8. Measuring probe (201 ') according to one of claims 1 to 7, wherein the electrically insulating layer (237 ¹ ) at least partially overcoated with at least one further electrically wettable layer (236'), which for electromagnetic shielding or for electrically contacting another Electrode serves.

9. measuring probe (201, 201 ', 501) according to any one of claims 1 to 8, wherein the electrode (233, 233', 519) at least by an electrically conductive layer comprising a metal, in particular platinum or coated with silver chloride Süber, which a further part of the applied on the module or component applied electrically conductive coating is formed.

10. Measuring probe (201, 201 ') according to one of claims 1 to 8, wherein the electrode (233, 233') by a shaped body, in particular a ring of a material which comprises a metal, an ion conductor or a semiconductor is formed ,

11. Measuring probe (201, 20V, 501) according to one of claims 2 to 10, wherein the housing has a first housing part (203, 203 ', 503), which delimits a first housing interior, and a second housing part (207, 207', 507 ), which defines a second housing interior, wherein the first and the second housing interior are liquid-tightly separated from each other, and wherein the first housing part (203, 203 ', 503) part of a measuring half-cell and the second housing part (207, 207', 507) Component of a reference half cell is.

12. Measuring probe (201, 201 ', 501) according to claim 11, wherein the first (203, 203 \ 503) and / or the second housing part (207, 207', 507) and / or at least one further third housing part with its housing outside in contact with the measuring medium (225, 225 ', 525).

13. Measuring probe (201, 201 ', 501) according to claim 11 or 12, wherein the measuring half-cell comprises: the first, in particular tubular, housing part (203, 203', 503), a measuring diaphragm (205, 205 ', 505), which in particular at a first end portion of the first housing part (203, 203 ', 503) is arranged and the first housing part (203, 203', 503) liquid-tightly seals, a first electrolyte (213, 213 ', 513), in particular comprising a buffer solution, a first diverter electrode (211, 211 ', 511) in contact with the first electrolyte (213, 213', 513) for diverting a first electric potential; and the reference half-cell comprises: the second, in particular tubular, housing part (207, 207 ', 507), at least one second electrolyte (217, 217', 517), in particular a potassium chloride solution or a gel comprising a potassium chloride solution, which in Contact with a, the second housing part (207, 207 ', 507) at least in a partial area surrounding medium

(225, 225 ', 525), a second ablating electrode (219, 219', 519) in contact with the at least one second electrolyte (217, 217 ', 517) for deriving a second electrical potential.

14. Measuring probe according to one of claims 11 or 12, wherein the measuring half-cell comprises: the first, in particular tubular, housing part; a measuring diaphragm, which at a first end portion of the first tubular

Housing part is arranged and closes the first housing part fluid-tight; a solid, electrically conductive subassembly electrically connected to the sensing diaphragm to dissipate a first electrical potential; wherein the reference half-cell comprises: the second, in particular tubular, housing part; at least one electrolyte, in particular a potassium chloride solution or a gel comprising a potassium chloride solution, which is in contact with a medium surrounding the second housing part at least in a partial region, an electrode in contact with the at least one second electrolyte to dissipate a second electrical potential.

15. measuring probe (201, 201 ', 501) according to any one of claims 10 to 14, wherein the at least one at least partially overcoated with another, in particular electrically insulating layer (237, 237', 541), electrically conductive coating (235, 235 ', 539) and the electrode (233, 233', 519) electrically conductively connected thereto are applied to an outer wall of the first (503) and / or the second (207, 207 ') and / or a further housing part.

16. A device for carrying out potentiometric measurements comprising: a measuring probe (201) according to one of claims 1 to 15, and a higher-order unit (MU, L), in particular a measuring transducer (MU) ₁ wherein the superordinate unit (MU, L) with the on the formed of electrically insulating material component applied electrically conductive coating over

Connecting elements, in particular for signal conversion and / or signal transmission and / or energy transmission, is connected.

17. Device according to claim 16, wherein the device for carrying out potentiometric measurements comprises means for setting a defined potential at the at least one electrode electrically conductively connected to the electrically conductive coating and / or means for determining an intermediate between the and the electrically conductive coating (235). 439) electrically conductively connected electrode (233, 419) and another part of the device prevailing electrical or electrochemical characteristic, in particular an impedance, a resistance or a potential difference comprises.