WO1996035116A1

WO1996035116A1 - All-solid state reference electrode

Info

Publication number: WO1996035116A1
Application number: PCT/NO1996/000093
Authority: WO
Inventors: Kalman Nagy; Kristin Eine; Tormod FÖRLAND
Original assignee: Sinvent As; FÖRLAND, Katrine, Seip
Priority date: 1995-05-03
Filing date: 1996-04-23
Publication date: 1996-11-07
Also published as: NO951705D0; EP0827589A1; AU5705096A; JPH11508994A

Abstract

All-solid state reference electrode for use in e.g. potentiometric, amperometric and voltametric measurements, comprising: a first and second resistor (102a, 102b) connected to a first and second wire (101a, 101b) respectively; a first and second conductive element (105a, 105b) externally coated with a first and second solid electrically conductive layer (106a, 106b), in which the element (105a, 105b) is connected to the idle connection of said first and second resistor (102a, 102b) or to the free end of said first and second wire (101a, 101b), respectively; and a first and second membrane (107a, 107b) surrounding said first and second solid electrically conductive layer (106a, 106b), respectively, the first membrane (107a) comprising a matrix, an anion-exchange material and optionally a lipophilic salt, and the second membrane (107b) comprising a matrix, a cation-exchange material and optionally a lipophilic salt, thus forming an anion-conducting membrane (107a) and a cation-conducting membrane (107b)

Description

All-solid state reference electrode.

The invention concerns an all-solid state reference electrode as recited in the introductory of claim 1.

Background

Ion-selective electrodes (ISEs) are widely used. An increasing number of the ISEs are all- solid state electrodes. An electrode cell assembly consists of a reference electrode and e.g. an ion-selective electrode. Ideally, the reference electrode gives a known and constant potential, irrespective of changes in ionic concentrations in the sample solution. The ion- selective electrode will be the measuring electrode, sensing changes in the electrode ion concentration.

Extensive work has been carried out on all-solid state electrodes. Nagy and Fjeldly reported on a solid-state differential electrode combining a pH-glass membrane and a LaF₃ membrane (Glass Electrodes with Solid-State membrane contacts and their Application in Differential Potentiometric Sensors, Sensors and Actuators, vol. 8, (1985) 261-269). An all-solid state glass membrane as a reference element appeared to have interesting applications. However, despite all the work carried out, no real all-solid state reference electrode is commercially available today. Sethi et al. wrote the following concerning micro multi-sensors (Transducer aspects of biosensors, Biosensors & Bioelectronics, vol.9, (1994) 243-264): "the incorporation of a 'true' reference electrode still remains one of the major hindrances to the reliable operation of these devices". Lisdat reports on a reference electrode based on a solid state structure, a REFET (reference field-effect transistor) consisting of a fluoride sensitive layer (LaF₃), a porous fluoride salt (CaFj) and a polymer layer (A reference element based on a solid state structure, Sensors and Actuators B, vol.15-16, (1993) 228- 232.). The potential of the interface is stable only if the activity of fluoride ions remains constant. A stable reference electrode requires a constant activity of the reference ion. In conventional liquid based reference electrodes this is accomplished by isolating the reference cell from the sample solution by e.g. a liquid junction. Sinsabugh reports on an integrated reference micro electrode fabricated on a silicon substrate (A batch-processed micro electrode integrated on a silicon substrate, Proc. Electrochem. Soc, col.86 (1986) 66-73). However, they avoid the problem of all-solid state contacts. The integrated reference electrode is a Ag/AgCl-electrode with a liquid junction containing a potassium chloride solution. Constructing integrated micro reference electrodes with the liquid junction/salt- bridge as a contact to the sample solution seems to be quite usual (EP-A2-215614). An all-solid state reference electrode has several advantages over the conventional reference electrode with internal filling solution. An all-solid state electrode can be miniaturized and is therefore well suited for micro as well as macro multi-sensor systems. The all-solid state concept makes it easy to incorporate micro-electronics in the vicinity of the membrane. An all-solid state electrode may also be exposed to higher and lower temperatures and pressures than the liquid based electrode, it may be fabricated in various shapes and sizes and is not restricted to be used vertically. There is no need for refilling of internal reference electrolyte in all-solid state electrodes.

EP-A1-193676 discloses an all-solid state reference electrode, comprising a cellulosic membrane which is said to be completely non-selective. Even though the membrane is said to be hydrophobic, a membrane which transmits all ion types will also, to a certain extent, be permeable to water. According to the description of this application, "trace amounts of water are absorbed through the membrane during operation. This produces a saturated solution of the second layer salt; the solution composition will remain fixed as long as some solid from the second layer remains". The disadvantage of such an arrangement is that the salt layer located between the membrane and the conductor gradually will be solved in the water and washed away from the electrode. Accordingly, when the salt layer finally has been dissolved and removed from the electrode, the system will no longer be at equilibrium, or in other words, the electrode has a very limited service life.

The latter patent publication also describes a similarly constructed ion-selective electrode. Experiments carried out in connection with the present invention has shown that also this construction suffer from the same defects as the non-selective electrode described above. After about 2 days of conditioning, the electrode gave a response of about 70 mV/decade, which clearly indicates that the system is out of equilibrium (an electrode at equilibrium should have a response of 59.2 mV/dec). Object

The object of the present invention is to provide an all-solid state reference electrode that alleviates the problems with known partly solid state reference electrodes and enables production of small scale electrodes for use in e.g. in vivo analysis.

The invention

The objects above are achieved by an all-solid state electrode according to the characterizing part of claim 1. Further advantageous features appear from the dependent claims. The present invention concerns an all-solid state reference electrode for use in e.g potentiometric, amperometric and voltametric measurements, the electrode comprising an electrically conductive wire or similar, an electrically conductive metal-containing element, a solid electrically conductive layer embedding the electrically conductive element, and a hydrophobic membrane embedding the solid electrically conductive layer. According to the invention the reference electrode is a two-piece electrode arranged in parallel comprising a first and second wire, a first and second resistor connected to the end of, or in-line with, said first and second wire, respectively, each resistor having an impedance substantially larger than the impedance of said membrane, a first and second conductive metal-containing element externally coated with said first and second solid electrically conductive layer comprising an electrically conductive polymer or a solid salt layer comprising a cation in common with the metal in the electrically conductive element and an anion, in which the electrically conductive element is connected to the idle connection of said first and second resistor, respectively, or to the free end of said first and second wire, respectively, and a first and second membrane embedding said first and second solid electrically conductive layer, respectively, and preventing the latter from contact with a sample solution, the first membrane comprising a matrix, an anion-exchange material and optionally a lipophilic salt, and the second membrane comprising a matrix, a cation-exchange material and optionally a lipophilic salt, thus forming an anion conducting membrane and a cation-conducting membrane respectively.

Alternatively, when said first and second electrically conductive layer comprises a solid salt layer, a first and second secondary solid salt layer is applied onto the respective electrically conductive layer, or primary solid salt layer. The secondary solid salt layer comprises an anion in common with the primary solid electrically conductive layer (primary solid salt layer) and a cation.

Due to the construction recited above, the amount of electrical charge transferred in the two parallel circuits is approximately the same. This provides contributions to the EMF which are equal in size, but of opposite sign. When the concentration of electrolyte is changed, the net change in EMF from the reference electrode is consequently approximately zero. Either shielded wires are used due to high impedance input or impedance transforming electronics are sited in the vicinity of the membrane to ensure low impedance in the system.

The membranes should be thin in order to ensure a rapid establishment of the exchange equilibrium between the membrane and the electrolyte, and a preferred membrane thickness is from 0.01 to 0.5 mm. The membrane should have a thickness sufficient to completely cover the solid electrolyte, to avoid the presence of pinholes and to provide mechanical stability.

The typical field of application of the present all-solid state reference electrode is chemical and biological analyses in-vivo and in vitro, potentiometric and amperometric measurements as well as voltametric measurements etc.

In the following, the invention is described in further details with reference to drawings, which illustrate preferred embodiments of the present invention, wherein

Figure la is a schematic drawing of an all-solid state reference electrode according to the present invention,

Figure lb is a schematic drawing of a liquid based reference electrode prepared for comparison, constructed similarly to the all-solid state electrode of Figure la,

Figure 2 illustrates response curves for the anion conducting electrodes indicated in Table 1 below, recorded with 10^"4 - 1C¹ M KNO₃, in which the electrodes were used against a Radiometer Ag/AgCl-reference electrode,

Figure 3 illustrates calibration curves for the all-solid state anion and cation conducting electrodes according to the invention (Table 1), in which the electrodes were used against a Radiometer Ag/AgCl-reference electrode,

Figure 4 illustrates response time for the anion conducting electrode by changing the concentration of KNO₃ from 10^-2 to 10^"1 M, Figure 5 illustrates response time for the cation conducting electrode by changing the concentration of KNO₃ from 10^"2 to 10^"1 M,

Figure 6 illustrates calibration curves for an Orion fluoride electrode used with 3 different reference electrodes: 1) a liquid based two-part reference electrode, 2) an all-solid state reference electrode according to the invention and 3) a Radiometer Ag/AgCl-reference electrode, and

Figure 7 illustrates the response curves for an Orion fluoride electrode used in conjunction with an all-solid state reference electrode according to the invention and the reference on an Orion fluoride electrode.

Figure la illustrates schematically an embodiment of the all-solid state reference electrode in accordance with the invention. A shielded electrically conductive wire 101 or similar is split up in a first and second wire 101a and 101b, and a first and second resistor 102a, 102b is connected at the end of the first and second wire 101a, 101b respectively or in-line with the same. Each resistor should have an impedance substantially larger than the impedance of the membrane material, and in general, the impedance should be at least 100 times the impedance of the membrane.

A first and second electrically conductive metal-containing element 105a and 105b is connected to the idle connection of the first and second resistor 102a and 102b, respectively or to the free wire end 101a, 101b connected to the respective resistor. The first and second conductive elements are embedded within a first and second solid electrically conductive layer 106a and 106b, respectively. The solid electrically conductive layer, which serves as a transition between ionic and electronic transmission, can be comprised of an electrically conductive polymer doped with either positive or negative sites, described in further detail below, or of a salt layer comprising a cation in common with the metal in the electrically conductive metal-containing element 105a and 105b, and an anion.

A first and second membrane 107a and 107b embeds the respective solid electrically conductive layer 106a and 106b, and prevent the latter from contact with a sample solution. As indicated with dashed lines in Fig. la, a secondary layer 108a and 108b is arranged between the solid electrically conductive layer 106a/106b and the membrane 107a/107b, respectively. The secondary layer 108a and 108b, which is optional, comprises a solid salt and is applied when the solid electrically conductive layer 106a and 106b also comprises a solid salt. The secondary solid salt layer 108a/108b comprises an anion in common with the salt-containing electrically conductive layer 106a/ 106b, and a cation. This arrangement provides a reservoar of anions and cations which are to be transported across the membrane and promotes the reversibility in the contact. When using e.g. Pb | PbSO₄ the secondary salt layer could be BaSO₄. Another example of the combination of the primary and secondary salt layer 106/108 is Agl and KI, respectively, wherein the electrically conducting element 105 comprises Ag. However, the solubility of the primary salt layer in the secondary salt layer should be low to establish a neglible interdiffusion in the membrane. This sandwhich structure ensures the reversibility in the solid contact. The electrically conductive element is typically formed of silver or a silver alloy, such as Ag₂S, but other electrically conductive and non-reactive metals or alloys capable of forming an insoluble salt and capable of forming a reversible electrode with its cation, can be used. Examples of other materials are mercury, lead and platinum. However, silver is the preferred material both with respect to handling, and toxicity for use in vivo. When using Pt as the conductive element, a layer with conductive polymer is often used between the conductive element (the electronic conductor) and the membrane (the ionic conductor). The term "conductive polymer" means a conjugated organic polymer that can be made electrically conductive by a process called "doping" which is equivalent to oxidation or reduction of the conjugated polymer backbone with simultaneous incorporation of charge compensating ions. Common conductive polymers are polyaniline and poly(3- octylthiophene). However, other polymers can be used, provided that they can be made electrically conductive by oxidation or reduction and that they are insoluble in their doped condition.

When using e.g., silver, lead or mercury, in combination with a salt layer, the salt layer must have a cation in common with the conductive element. The salt layer should form an almost insoluble metal salt, but the salt must be capable of establishing an equilibrium with the metal. In the preferred assembly, the salt layer is composed of silver chloride formed by the electrochemical oxidation with the silver wire in a dilute solution of HCI. Other salts that may be used is Agl (with Ag) and PbS (with Pb).

The first membrane 107a can be comprised of a matrix, an anion-exchange material and optionally a lipophilic salt, and the second membrane 107b can similarly be comprised of a matrix, a cation-exchange material and optionally a lipophilic salt, thus forming an anion conducting membrane and a cation-conducting membrane, respectively.

In general, the membrane should be hydrophobic, non-selective with regard to specific ions and permeable to either anions or cations. The main purpose of the membrane matrix is to immobilize the ion-exchange material and to obtain a durable and permeable membrane. A polymer based matrix should be relatively high molecular.

The membrane can be prepared by dissolving a polymer in a solvent, and supplying a plasticizer (if needed), particulate and/or polymeric ion-exchange material, and optionally a lipophilic salt, whereupon the matrix is set by evaporation of the solvent. A lipophilic salt is used to prevent anions from entering the cationic membrane and/or to prevent cations from entering the anionic membrane, and in both cases, the lipophilic salt serves to increase the electrical conductivity of the membrane. In addition to serving as a softener, the plasticizer can increase the membrane conductivity.

However, other polymer based matrix materials are also conceivable: the membrane can be prepared by in situ polymerisation or co-polymerisation of suitable monomer materials in conjunction with the remaining membrane components directly onto the electrically conductive element and solid electrically conductive layer.

Examples of the polymer matrix are: PVC, silicone polymers, polystyrene, polyhydroxy ethylmethacrylate, polyvinylidenechloride and polyurethane. Examples of suitable solvents are THF (tetrahydrofuran) and cyclohexanone, where THF is preferred due to its high volatility.

Examples of plasticizers are phtalic acid and its derivatives, citric acid and its derivatives, adipic acid and its derivatives, sebacic acid and its derivatives.

An example of a lipophilic salt for the anion-conducting electrode is tridodecylmethyl- ammonium chloride, and potassium tetrakis(p-chlorophenyl)borate or sodium tetraphenyl borate (NaTPhB) for the cation-conducting electrode. The ion-exchange material can be provided as a commercially available insoluble ion- exchange material, or a soluble polymer made from monomers capable of exchanging anions or cations due to ion-exchange functional groups. The term "insoluble" and "soluble" means, in this connection, that the ion-exchange material is insoluble or soluble in an organic solvent, respectively. In order to provide a sufficient ion-exchange activity, the ion-exchange material should constitute 1-15% of the membrane, depending on the activity of the ion- exchange material used and on the porosity of the resulting membrane. However, a membrane consisting essentially of polymerized ion-exchange material only is also conceivable, and the limits above should for that reason not be interpreted as absolute. As commercially avilable insoluble anion-exchange material for the anion electrode, Dowex 2x8 can be used, and for the cation electrode, Dowex 50Wx8 is suitable. However, other commercially available ion-exchangers can be used, e.g. from Dowex or Amberlite. The commercially available insoluble ion-exchange materials are usually purchased in particulate form, and in order to obtain an acceptable dispersion of the ion-exchange material in the membrane matrix, the ion-exchange material should be milled and mixed with the remaining membrane components as a powder. It is preferred that such a solid insoluble ion- exchange material is milled and sieved to a particle size of, e.g., less than 40 μm, and the resulting membrane mixture should be homogenized, e.g. by means of an ultrasound treatment, prior to application on the electrically conductive element and solid electrically conductive layer.

The soluble polymeric ion-exchange material can be mixed directly with the membrane matrix and the remaining optional components or combined with a commercial solid ion- exchange material described above. Examples of monomers for preparation of soluble cation-exchange polymers are 4-vinylbenzene sodium sulphonate, methacrylic acid, methacrylic acid/methyl methacrylate (50:50 co-polymer), 2-acrylamide-2-methyl-propane sulphonic acid, and 2-acrylamide-2-methylpropane sulphonic acid/methyl-methacrylate (50:50 co-polymer), in which the copolymer of methacrylic acid/methyl methacrylate (50:50) is preferred. An example of a suitable soluble anion-exchange polymer is N-(4-vinylbenzyl)- N,N-dimethyl amine. The polymers are preferably pre-polymerized and supplied to the remaining membrane components in an organic solution. The polymerisation degree is not critical, but the molecular weight should be sufficient to avoid scouring or similar of the ion- exchange material.

The soluble ion-exchange polymer is preferred since the material is soluble in organic solvents and therefore the active ion-exchange sites will be more uniformly dispersed in the membrane material, thus establishing a more ideal and reliable electrode. In order to achieve the desired electrode quality in terms of sensitivity, response time and stability, it is essential that all solid contacts are reversible.

Example

The present examples are provided in order to verify the viability of the all-solid state reference electrode according to the invention. First, the anion- and cation conducting parts of the reference electrode were examined separately. In order to further support the principles of the invention, a liquid based reference electrode was prepared with a construction similar to the all-solid state reference electrode in accordance with the invention. All chemicals used were of p. a. grade. Only doubly distilled water was used.

Electrode manufacture

Figure la shows a schematic drawing of the all-solid state reference electrode according to the invention. The composition of the membranes in this example was:

Anion-conducting membrane

Membrane matrix 23-27% PVC Plasticizer 65-69% bis(2-ethylhexyl)sebacate (DOS) Anion-conducting material 2-10% Dowex 2x8, or

2-10% N-(4-vinylbenzyl)-N,N-dimethyl amine

Lipophilic salt 2% tridodecylmethyl-ammonium chloride (TDMACl) Cation-conducting membrane Membrane matrix 23-27% PVC

Plasticizer 65-69% bis(2-ethylhexyl)sebacate (DOS)

Cation-conducting material 2-10% Dowex 50Wx8, or

2-10% metacrylic acid/methyl methacrylate (50:50) Lipophilic salt 2% potassium tetrakis(p-chlorophenyl)borate

(KTpClPB)

The commercially available Dowex ion-exchange material, which was obtained in the form of small spheres, was milled and sieved to a homogenous particle size of <40 μm. The polymeric membrane constituents were dissolved in tetrahydrofuran (THF) and mixed with the ion-exchange powder. The preparative membrane mixture was then homogenized in an ultrasonic bath.

The polymer based ion-exchange materials were simply mixed with the remaining membrane mixture components.

Preparation of all solid state electrodes.

A shielded copper wire 101 was separated in two parts 101a and 101b, and a resistor 102a and 102b having an impedance of about 100 times the impedance of the membrane was soldered onto each of them. Then, Ag-wires 105a and 105b were soldered onto the free end of the respective wires 101a and 101b (at 104a and 104b in Fig. la) to provide the electrode. In addition, a Ag-wire (not illustrated) was soldered onto each of the electrode wires 105a and 105b, respectively, to provide an electrical circuit to enable the electrolysis of a corresponding metal salt: AgCl. The metal salt was electrolyzed onto the electrical conducting elements. An insulator 103 a and 103b comprising an epoxy resin was applied to embed the free ends of the wires 101a and 101b, the soldering 104a and 104b and a short section of the Ag-wires 105a and 105b, and allowed to set. The electrodes were then dipped several times in the respective membrane mixtures, one in the anion and one in the cation conducting membrane mixture. The organic solvent was allowed to vaporize before use. Separate anion and cation conducting electrodes were prepared by the same procedure. However, the copper wire was not separated and no resistors were required.

Preparation of liquid based electrodes.

The liquid based electrode is illustrated schematically in Figure lb, in which only one part of the two-part electrode is described for simplicity. The procedure was almost the same as for the preparation of the all-solid state electrodes according to the invention. The membrane solution was cast onto a mould, and the organic solvent was allowed to vaporize before use. The membrane 1 was attached to a plastic tube 2 with an epoxy-based adhesive. The tube 2 was filled with an inner reference electrolyte 3 of 1 M KC1 (1) saturated with AgCl, and the electrical conductor 4 covered with metal salt was incorporated into the tube. Resistors 5 were used in the reference electrodes but not in the anion and cation conducting electrodes.

EMF measurements

EMF-measurements were carried out in the same way for the all-solid state electrode according to the invention and the liquid based electrodes. The voltmeter used was a Keithly Electrometer.

The anion and cation conducting electrodes were tested in conjunction with a Radiometer Ag/AgCl-single junction reference electrode (K-401). The cell can be schematically described as: Ag/AgCl | Anion or cation conducting membrane | Sample solution || Radiometer Ag/AgCl-reference electrode.

Response curves (potential vs time) were recorded using lO^' O^"1 M KNO₃ (aq) solution. From the response curves the calibration curves of the electrodes (potential vs log concentration) were plotted. No supporting electrolyte was used in these experiments. The slopes of the calibration curves (mV/dec) were used as a measure of ideality (ideal Nernst response is 59.2 mV/dec). Liquid junction potentials were corrected according to the Henderson formalism, and the activity coefficients were calculated with the extended Debye- Hϋckel equation.

The response time (the time it takes to achieve 95% of the total change in EMF following a concentration step in the sample solution) of the anion and cation conducting electrodes were measured using the same experimental set-up as described above. Only the practical response time were measured, in order to compare the constructed electrodes with commercial electrodes. The concentration in the sample solution was changed from 10"² to 10^"1 M KNO₃. No supporting electrolyte was used in these experiments.

The two-part reference electrode (both liquid based and all-solid state) was used with an

Orion fluoride combination electrode (No. 96-09; in this case the reference in the fluoride electrode was not used) in order to examine how it performed as an external reference electrode for an ISE. The following cell was used: two-part ref. electrode (both liquid based and all-solid state) | Sample solution || Orion fluoride electrode.

Response curves were recorded using lC O^"1 M of a NaF solution; 0.1 M NaCl solution was used as the supporting electrolyte. The calibration curves of the electrodes were plotted from the response curves.

Results

The results obtained from experiments with the anion and cation conducting electrodes are summarized in Table 1 below, which illustrates the slopes of the linear range obtained from the calibration curves for cation and anion conducting electrodes. The electrodes were used in conjunction with a Radiometer Ag/AgCl-reference electrode. Response curves were recorded using 10^"4 - 10^'1 M KNO₃. No supporting electrolyte were used. The linear range was 10^"3 - 10^"1 M .

Figure 2 shows the response curves of the all-solid state anion conducting electrode and the liquid based anion conducting electrode. Figure 3 shows the calibration curves for the all-solid state anion and cation conducting electrodes.

Table 1

Response of electrode according to the invention

Slopes (mV/dec)

Membrane Ion exchange All-solid Liquid composition material

Cation conducting Dowex 50Wx8 55.3 56.1 electrode

Anion conducting Dowex 2x8 -60.7 -56.5 electrode

Cation conducting Poly(metacrylic acid/methyl 55.2 electrode methacrylate [50:50])

Anion conducting Poly(N-(4-vinylbenzyl)-N,N-dimethyl -58.9 electrode amine)

Figure 4 and 5 show that the response time is within 5 seconds for both the anion and cation conducting electrodes. The results summarized in Table 1 and Figure 2 to 5 show that it is possible to make all- solid state electrodes that are almost completely permselective, i.e. selective to either cations or anions. They respond within few seconds (2-5s) and show nearly Nernstian response. There is no severe differences between the liquid based and the all-solid state electrode. The obtained result from the use of a reference electrode according to the invention and an Orion F-selective electrode are given in Table 2. The table illustrates the slopes obtained from the linear ranges of the calibration curves using an all-solid state reference electrode according to the invention, a two-part liquid based reference electrode and an Orion Fluoride combination electrode. The reference electrodes were used against a F-selective electrode. Response curves were recorded using 10^'5 - 10¹ M NaF; 0.1 M NaCl was used as the supporting electrolyte. The linear range was 10^"5 - 10^"1 M NaF. Table 2

Slopes

Electrode assembly (mV/dec)

All-solid state reference el. and a F^"-selective el. - 57.8 Two-part liquid based reference el. and a F^"-selective el. - 55.9

Fluoride-combination electrode - 59.7 (i.e. Ag/AgCl-reference and a F^"-selective el.)

Figure 6 shows the calibration curves for an Orion F^"-selective electrode used with three different reference electrodes: 1) an all-solid state reference electrode according to the present invention, 2) a liquid based reference electrode and 3) Radiometer's reference electrode.

Figure 7 shows the response curves for the Orion F^'-selective electrode used in conjunction with the all-solid state reference electrode according to the invention and Orion's reference electrode. Figure 6 and 7 and the slopes given in Table 2 show that the all-solid state reference electrode according to the invention is almost as good as the commercial liquid based reference electrodes available. That means, the principle behind the described invention is viable.

The present all-solid state electrode as disclosed above provides a durable and reliable reference electrode which in addition enables the use of small-scale measurement of ionic activity, e.g. in vivo.

Claims

1. All-solid state reference electrode for use in e.g potentiometric, amperometric and voltametric measurements, the electrode comprising an electrically conductive wire (101) or similar, an electrically conductive metal containing element (105 a), a solid electrically conductive layer (106a) embedding the electrically conductive element (105a), and a hydrophobic membrane (107a) embedding the solid electrically conductive layer (106a), characterized in that said reference electrode is a two-piece electrode arranged in parallel comprising a first and second wire (10 la, 101b), a first and second resistor (102a, 102b) connected to the end of, or in-line with, said first and second wire (101 a, 101b) respectively, each resistor having an impedance substantially larger than the impedance of said membrane to provide an equal transfer of electrical charge in the respective electrode parts, a first and second conductive element (105a, 105b) externally coated with said first and second solid electrically conductive layer (106a, 106b) comprising an electrically conductive polymer or a salt layer comprising a cation in common with a metal in the electrically conductive element (105a,105b) and an anion, in which the element (105a,105b) is connected to the idle connection of said first and second resistor (102a, 102b) or to the free end of said first and second wire (101a, 101b), respectively, and a first and second membrane (107a, 107b) surrounding said first and second solid electrically conductive layer (106a, 106b), respectively, and preventing the latter from contact with a sample solution, the first membrane (107a) comprising a matrix, an anion-exchange material and optionally a lipophilic salt, and the second membrane (107b) comprising a matrix, a cation-exchange material and optionally a lipophilic salt, thus forming an anion- conducting membrane (107a) and a cation-conducting membrane (107b).

2. The electrode of claim 1, characterized in that said first and/or second membrane (107a, 107b) comprises a soluble ion-exchange material or an insoluble ion-exchange material.

3. The electrode of claim 1, characterized in that said first and/or second membrane (107a, 107b) comprises both soluble and insuloble ion-exchange materials.

4. The electrode of any one of claim 1 to 3, characterized in that said first and/or second membrane (107a,107b) comprises a matrix material selected from the group consisting of silicon rubber, polyvinylchloride, polystyrene, polyhydroxyethyl-methacrylate, polyvinylidene chloride and polyurethane; a plasticizer selected from the group consisting of phthalic acid derivatives, citric acid derivatives, adipic acid derivatives and sebacic acid derivatives, and an anion-exchange material and a cation- exchange material, respectively.

5. The electrode of claim 1 or 2, characterized in that said anion-conducting membrane (107a) comprises a mixture of polyvinyl chloride, bis(2-ethylhexyl)sebacate, tridodecylammonium chloride and Dowex 2x8 anion-exchange material.

6. The electrode of claim 1 or 2, characterized in that said cation-conducting membrane (107b) comprises a mixture of polyvinylchloride, bis(2-ethylhexyl)sebacate, potassium tetrakis(p-chlorophenyl)borate and Dowex 50Wx8 cation-exchange material.

7. The electrode of claim 1 or 2, characterized in that said cation-exchange material comprises a polymeric soluble ion- exchange material selected from the monomer group consisting of 4-vinylbenzene sodium sulphonate, methacrylic acid, methacrylic acid/methyl methacrylate (50:50 co-polymer), 2- acrylamide-2-methyl-propane sulphonic acid, and 2-acrylamide-2-methylpropane sulphonic acid/methyl-methacrylate (50:50 co-polymer), and said soluble anion-exchange material comprises poly-N-(4-vinylbenzyl)-N,N-dimethyl amine.

8. The electrode of claim 1, characterized in that said first and/or second electrically conducting element (105a,105b) consists of a metal or an alloy of Ag, Pb, Hg or Pt.

9. The electrode of claim 1, characterized in that said first and second electrically conducting layer (106a, 106b) comprises a solid salt layer, and a secondary solid salt layer (108a, 108b) is arranged between said electrically conductive layer (106a, 106b) and said membrane (107a, 107b), wherein the secondary solid salt layer (108a, 108b) comprises an anion in common with the salt- containing electrically conductive layer (106a, 106b), and a cation.

10. The electrode of claim 1, characterized in that said first and/or second electrically conducting element (105a, 105b) at least partially comprises Ag, and said salt layer comprises substantially AgCl or Agl

11. The electrode of claim 10, characterized in that each resistor having an impedance of 0.1 - 20 MΩ.

12. The electrode of claim 1, characterized in that said first membrane (107a) consists essentially of polymerized monomer selected from the group consisting of 4-vinylbenzene sodium sulphonate, methacrylic acid, methacrylic acid/methyl methacrylate (50:50 co-polymer), 2-acrylamide-2- methyl-propane sulphonic acid, and 2-acrylamide-2-methylpropane sulphonic acid/methyl- methacrylate (50:50 co-polymer), and/or said second membrane (107b) consists essentially of polymerized N-(4-vinylbenzyl)-N,N-dimethyl amine.