GB2078962A - Metal hydride reference electrode - Google Patents

Abstract

A metal hydride reference electrode, for example an ( alpha - beta ) palladium hydride electrode, is provided with means 8 for supplying hydrogen to the metal and means for controlling the supply of hydrogen so that the electrode potential remains substantially constant. The control means comprises means for measuring or comparing the electrical resistance of said electrode 1 with that of another component, preferably a second electrode, 2, and means for monitoring the ratio of said resistances and maintaining it within a predetermined range, the resistance of the other component being related to the resistance of the reference electrode when no hydrogen is contained in the latter. The reference electrode may comprise a central elongate core of electrically conductive metal, the core being insulated but having an exposed terminal portion 3, and a layer of hydride forming metal provided over the insulation and the terminal portion. The second electrode may be located adjacent the reference electrode. The hydrogen supply is preferably electrochemically generated. <IMAGE>

Description

SPECIFICATION Reference electrodes This invention relates to reference electrodes, especially palladium hydride reference electrodes, and, more especially, to the use of such electrodes in in vivo applications.

The term "reference electrode" as used herein means an electrode whose potential in a liquid medium, such as a solution, relative to a reference potential, depends on the concentration of an ion, usually an H+ ion, or a particular group of ions, in the liquid medium.

Palladium charged with hydrogen has been used as a hydrogen reference electrode for many years (see, for example, Hills G.J. and Ives G.J.G. "Reference electrodes, theory and practice, III", Academic Press, New York/London (1961)). However, despite careful precharging and rigorous protection from oxidation, such electrodes are frequently stable only for a few days.

The limiting factor in destabilisation is the gradual discharge of hydrogen, which is accelerated in the presence of oxygen. If gaseous hydrogen is fed to the electrode, it is possible to achieve long-term stability. However, there are practical difficulties and drawbacks in using gaseous hydrogen and, in addition, the inherent lack of control can lead to excessive hydrogen occlusion, shifting the rest potential of the (a-ss) palladium hydride electrode away from +50 MV against RHE (reversible hydrogen electrode) to more cathodic values. Repeated phase changes lead to instability and dimensional changes in the electrode.

One proposal for overcoming these disadvantages is the "generation" electrode of Schwing and Rogers ((1956) Anal. Chim. Acta 15, 379) in which part of the palladium electrode is continuously charged cathodically in a separate electrolyte compartment.

The idea of continuously supplying hydrogen in a controlled fashion to a palladium hydride reference electrode in order to keep its potential stable is incorporated in the proposals of Dobson in British Patent Specification No. 1,481,509, the disclosures of which are also incorporated herein by reference.

In that patent there is claimed apparatus for use in measuring the concentration of a particular ion in a liquid, comprising an ion selective electrode which includes a material capable of forming a hydride, the material being a metal or a metal alloy and the hydride being as hereinafter defined, supply means for supplying hydrogen to the metal or metal alloy, and control means for so controlling the supply of hydrogen to the metal that the concentration of hydrogen in the metal is, in operation, maintained within a predetermined range such that the electrode potential remains constant when measured in a solution having a constant concentration of a particular ion.

The control means is preferably actuated by a monitoring means for determining the concentration of hydrogen in the metal or-metal alloy by measuring a hydrogen concentration-dependantWproperty such as electrical resistance and preferab,)" includes means for temporarily applyig a positive potential to the electrode; the positive pulses are said to have the advantages of cleaning the electrode surface and eliminating the damaging effect of dissolved oxygen and reducible ions.

The ion-selective electrodes (or reference electrodes in accordance with the above definition) are preferably palladium hydride electrodes, especially those in which the palladium hydride is in the (o!+ss) phase, the term "hydride" being used to include combinations of hydrogen with a metal or metal alloys in which the hydrogen is absorbed instead of forming a stoichiometric compound. However, metals other than palladium may be employed, including, for example, titanium, vanadium, zirconium and yttrium as well as alloys containing two or more of these metals andior palladium.

As mentioned above, the reference electrodes are generally used to measure H+ concentration but may also be used to measure the concentration of other ions or specific groups of ions by partially coating the metal with a mixture of an acid and a salt thereof which contains the ion(s) concerned.

Amongst such ions there may be mentioned, for example, Ca++, Mg++, Sir++, Boa++, Y++ and La++.

The hydrogen may be supplied to the electrode by electrolysis, at intervals between ion concentration measurements, of the solution whose ion concentration is to be measured, or by electrolysis of a different and separate electrolyte whose concentration is not being measured or the supply may be from a gas supply applied to part of the palladium which in operation is not immersed in the solution.

The gas supply itself may include means for electro lysing a solution to provide hydrogen.

The proposals in British Patent Specification No.

1,481,509 provide the facility for automatically maintaining the electrode in a preselected state of charging but the electrode designs and the methods of control which are specifically described have certain disadvantages. For example, the methods of monitoring specifically described, which are based on an AC system, introduce a finite error in control under changing conditions and, in particular, the various electrodes shown and described are unsuitable for in vivo applications.

Attention is drawn in this respect to two papers by A.C. Tseung and A. Goffe in Medical and Biological Engineering and Computing, November 1978, pages 670-680, the disclosures of which are also incorporated herein by reference. As is discussed in those papers, the reference electrodes currently used for in vivo studies fall into two basic categories: (a) glass micropipette electrodes and (b) metal microelectrodes.

In the former, a reference electrode, for example a calomel orAg/AgCl electrode has a micropipette attached to the tip thereof to provide the electrolytic solution. The tip is normally too fragile to penetrate the skin and an incision has to be made beforehand with a scalpel; even then there is the danger of breakage. In addition the tip, whichis filled with electrolyte, e.g. 3M KCI, constitutes an electrical conductor of small cross-sectional area and, as such, may interfere with measurement, for example by giving rise to electrode noise at all frequencies.

Furthermore, the diffusion of the electrolyte into the penetrated body cell may affect the biological environment.

In the latter, electrodes made from stainless steel, tungsten or precious metals are employed which have sharpened tips of about 1Fm in diameter. They are more robust than glass micropipette electrodes but the small area of the metal/electrolyte junction makes it difficult to obtain a high enough input resistance to measure the membrane potential. In addition, if metal microelectrode is implanted for any length of time in an animal it is likely to induce metal/tissue reactions, and the absorption of proteins and other necrotic debris at the electrode/tissue interface causes progressive changes in the rest potential of the electrode.

The need remains, therefore, for an effective electrode for in vivo biomedical investigation.

The present invention provides apparatus for measuring the concentration of a particular ion or a particular group of ions in a liquid, which apparatus comprises (a) a first electrode which includes a metal capable of forming a hydride, (b) means for sup piying hydrogen to the metal, and (c) means for controlling the supply of hydrogen to the metal so that the concentration of hydrogen in the metal during operation is maintained in a predetermined range such that the electrode potential remains substantially constant, characterised in that the control means (c) comprises means for measuring or comparing the electrical resistance R of the first electrode and the electrical resistance of another component, preferably a second electrode, the resistance of which is related to and, preferably, substantially equal to, Ro, i.e. the resistance of the first electrode when no hydrogen is contained therein, and means for monitoring the ratio of said resistances, e.g. preferably R/Ro, and maintaining it within a predetermined range thereby to ensure that the potential of the first electrode remains substantially constant.

For convenience the present invention will from now on be described with reference to an (an(3) palladium hydride electrode for measuring H+ ion concentration but it will be appreciated that it is equally applicable to other electrodes, including, for example, the metal (including metal alloy) hydride electrodes mentioned above, for measuring both the concentration of H+ and other ions in solutions and suspensions.

The basis of the present invention is to measure the resistance of an (a-ss) palladium hydride electrode and use this as a control parameter in a feed-back circuit. In accordance with the present invention this is effected by indirectly, or, preferably directly, monitoring the resistance ratio R/Ro. The R/Ro ratio has an almost linear relationship to the hydrogen content of the palladium hydride and a hydrogen content corresponding to R/Ro lying in the range of from about 1.3 to 1.5 establishes the (a-ss) phase palladium hydride in the middle of the potential plateau. Thus by controlling the R/Ro ratio it is possible to maintain the stable reversible potential of an (a-p) Pd/H electrode at about +50mv vs. RHE.The R/Ro ratio is preferably determined directly by comparing the resistance R of the first working electrode (a) with the resistance Ro of a second electrode which has the same hydrogen-free resistance as electrode (a). For convenience, therefore, the second electrode is preferably a palladium electrode having identical dimensions and structure to those of the working electrode. However, it will be appreciated that the reference electrode (or other reference member) may, if desired, have a different constant resistance, e.g. Ro/2.

In preferred embodiments of the present invention, as will be described with more detail hereinafter, the second palladium electrode is arranged to bq, in substantially the same environment as the working electrode, thus minimising any discrepancies which might otherwise be caused by temperature and/or other localised variations in environment.

in especially preferred embodiments of the present invention the R/Ro ratio is measured and used to govern the control means by the use of a Wheatstone bridge; this provides an especially effective and simple means for controlling the supply of hydrogen to the electrode. Accordingly, in one preferred aspect of the present invention, there is provided an apparatus of the type described above in which the first electrode and the reference component form one arm of a Wheatstone bridge circuit.

The supply means (b) may be selected from a variety of devices which can be electrically controlled, but, as will be apparent from the following description, it is preferably one which is adapted to supply hydrogen electrolytically directly to the electrode, i.e. the electrode is internally charged.

For example, the electrode is preferably a palladium hydride electrode of the type proposed by Schwing and Rogers (vide supra) in which part of the palladium electrode is continuously charged cathodically in a separate compartment. Some of the hydrogen atoms evolved then diffuse into the bulk of the palladium metal to form palladium hydride. In this way there is no need to bubble hydrogen to the tip of the electrode, which is especially advantageous in in vivo applications.

As will be appreciated from the foregoing discussion, the present invention is especially directed to the provision of a reference electrode which is suitable for in vivo investigations. To make such investigations using the apparatus of the present invention it is, of course, necessary to make an effective electrical connection to the tip of the electrode in orderto measure its resistance and it .

will be appreciated that a direct connection is not compatible with the other requirements of the electrode.

In a second aspect of the present invention, therefore, there is provided an electrode which comprises a central elongate core of an electrically conductive metal or other material (i.e. one, the resistance of which can be ignored), said elongate core being insulated but having an exposed terminal portion and being provided over said insulation and said terminal portion with a layer of palladium (or other hydride-forming metal). The length of the terminal portion is preferably very small compared to the length of the insulated portion, in some cases it may be nothing more than the exposed end of the core, because, as will be understood from the following discussion, its function is simply to provide the necessary electrical connection to the tip of the palladium coated electrode.

Such an electrode has many advantages, especially when used in an internally charged electrode of the Schwing and Rogers type. Because the conductive core makes an effective electrical connection at its terminal portion to the tip of the electrode, the resistance of the working part of the electrode can readily be measured without any structural alteration to the tip. In addition the provision of the palladium coating facilitates measurement of the "surface" resistance of the electrode. If solid palladium wire were to be used the resistance measured would be that of the bulk. By keeping the palladium film thickness as low as practicable the surface properties predominate and the control of the surface hydrogen concentration and, therefore, the electrode potential, will be more effective.

A further advantage is that the provision of the palladium coating facilitates the diffusion of hydrogen from the cathodic charging compartment to the working section of the electrode in a Schwing and Rogers type arrangement. Finally, as will be seen from the specific description of the preferred embodiment, the use of an electrode of this type makes it very easy to prepare the second comparative electrode and also to set up the R/Ro portion of the Wheatstone bridge circuit.

Modification of this electrode also make it possible to provide an electrode for deep probe (10 cm) in vivo investigations in, for example, brain surgery.

The central core is advantageously a copper wire, although other conductive metals, e.g. silver and aluminium, may be used, and the insulation may simply be that normally provided on such wire, e.g.

vinyl acetate, enamel etc., although other forms of insulation, e.g. thermoplastics materials such as polyimides, may also be used, especially where additional strength and/or resistance to heat and/or chemical attack are desired.

Insulated copper wire is at present preferred because it is commercially available in a variety of sizes and with various forms of insulation, most of which are amenable to the coating of palladium by conventional methods and which are durable and resistant to acids and alkalis. In addition, besides being a very good conductor, copper is not subject to attack by the occluded hydrogen (for example, iron and nickel become embrittled under the operational conditions); in fact copper does not occlude hydrogen efficiently under the conditions.

The palladium layer may be provided as a preformed integral component but is preferably formed by coating by conventional methods such as vacuum deposition or electroless or electrolytic deposition or combinations of such methods.

In one especially preferred embodiment of the present invention the first electrode and the reference component, e.g. the second electrode, are formed as a single unit. This may be achieved by forming both electrodes from a single length of coated copper wire which is coated with palladium, as described above, and further coating the reference electrode with a layer of a material, e.g. an epoxy resin, which protects it against attack by the electrolyte and which, in particular, ensures that no hydrogen is occluded within it so as to alter its resistance.

Various embodiments according to the present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which; Figure 1 is a schematic representation of apparatus according to the present invention comprising a working electrode and a comparison electrode; Figure 2 shows details of the design of the electrode combination of the apparatus of Figure 1; Figure 3 shows an electric circuit suitable for monitoring the apparatus of Figures 1 and 2; and Figure 4 illustrates a modified form of electrode for use in deep implantation in vivo studies.

Referring now to the drawings, Figures 1 and 2 illustrate one form of apparatus according to the present invention comprising a working electrode, indicated generally by the reference numeral 1, and a comparison electrode, indicated generally by the reference numeral 2.

The working electrode 1 and the comparison electrode 2 are formed from a single length of copper wire 3 which is insulated by vinyl acetate layer 4. A layer of palladium 5 is coated over the layer 4.

The working electrode 1 has a resistance R (as shown) and is supplied with hydrogen by the electrolytic cell 6 which comprises electrolyte 7, e.g.

3m KCI solution, and platinum screen electrode 8.

The comparison electrode 2 is protected from the electrolyte 7 by encapsulation within a layer 9 of an epoxy resin and has a resistance Ro, the effective dimensions of the working electrode 1 and the comparison electrode 2 being identical.

The electrodes were made by initially depositing a thin (less than 0.0025 mm) layer of palladium onto 27 swg vinyl acetate-coated copper wire, supplied by Temple Electrical Company Limited, by vacuum evaporation with simultaneous rotation of the wire target. The required thickness of palladium was then obtained by electroplating with a Bright Palladium Plating solution, supplied by Johnson Matthey Chemicals Limited. The preferred thickness of palladium after this treatment is between 0.02 and 0.04 mm, especially between 0.03 and 0.035 mm. The whole electrode assembly was subsequently annealed at 200"C for 24 hours in an argon atmosphere. This treatment has been found greatly to improve the hydrogen diffusion rate and also helps to remove hydrogen from the comparison electrode 2 before it is encapsulated in an epoxy resin.The combined working and comparison electrode was then formed into the shape shown more particularly in Figure 2 leaving various parts of the copper wire exposed (as shown).

Electrical connections were made as indicated by the reference letters O/P, C, C-, B and A, respectively.

With the exception of B, the connections made to the combined electrode were solderless and were made by wrapping copper wire around the initially deposited palladium layer at the appropriate position before electroplating.

If desired, the vinyl acetate insulation may be degreased and roughened by treatment with concentrated acids prior to vacuum deposition of the palladium.

Figure 3 shows the electrical circuit which is used to monitor the electrode of Figures 1 and 2, the electrical contacts A, B, C, C' and O/P being indicated together with the working electrode and comparison electrode resistances R and Ro respectively.

The circuit is composed of three sections: 1. The Wheatstone bridge and bridge detector amplifier.

This consists of R1, R2, R3, R, Ro and the 725 operational amplifier and its assocated input and feedback resistors (R4-R8).

As Rand Ro are of the order of 100 m. ohm then the current through these is approximately 100 mA and is maintained constant by R1, R2 and R3 from the other half of a Wheatstone bridge circuit with R, Ro and any imbalance in this is applied to the inputs of the detector amplifier via R4 and R5 and amplified by 100. The amplification may be increased to, say, 200 by increasing the value of Rg to 200 kQ. The 725 is used because of its low drift characteristics and high common mode rejection. High stability close tolerance resistors should be usedthroughoutthis section and it is necessary to compensate for the input offset voltage ( 2mV) using Re.

2. The low pass filter circuit and second stage amplifier.

This consists of Rg - R3, C1 - C3 and the 741 operational amplifier A2.

This amplifier serves two purposes. Firstly it is a low pass filter having a cutoff frequency of approximately 1 Hz thus eliminating any noise generated in the circuit. The capacitance of the filter is provided by C2 which may be reduced to 1 FtF. The reduction in circuit response time is unimportant because of the diffusion time of H+ ions through the Pd film. This circuit also serves as a variable gain D.C. amplifier.

As Rlo is varied from 0 to 50 K the stage gain varies from ca. to 6. Thus if R/Ro = 1.4 and Ro = ca. 100 m the output of the bridge detector stage will be ca 400 mV. Then by adjusting the gain of the second stage the circuit output can be adjusted to the O.C.V. of the charing cell. Thus if R/Ro is at the desired level the charging current will be zero. Rg - R12 should also be high stability components.

3. The output buffer amplifier.

This consists of R14, R15 and the 741 operational amplifier, A3. This is a unity gain amplifier and merely serves to bufferthe amplification stages from overload by the charging cell.

The ammeter and its associated shunt resistors and switches are included in the feedback loop thus preventing output current limiting by the meter circuitry itself.

As discussed above, this circuit enables the resistance ratio of the two similar palladium "resistors" R and Ro to be monitored using a Wheatstone bridge in which the two resistors form one half of the bridge. The set pointforthe control system was selected (for example by selection of R2) so that the (an(3) reversible potential of +50 mV vs. RHS was obtained at the external tip of the working electrode.

During various tests on this electrode it has been found possible to maintain the tip potential to within + 5 mV during tests lasting over 100 hours and the integrity of the palladium film has remained intact during such tests even over periods of 1000 hours or more. In addition, it has been found that the selected potential may be maintained in the presence of oxygen (because the cell was exposed to the atmosphere in the long term tests) and, in addition, various experiments involving purging the cell with an air pump at very fast flow rates have shown that, although there is a shift in potential during purging and vigorous stirring, the original potential is rapidly re-established whereas, without the control circuit, the palladium hydride electrode would have been substantially discharged and would have required a considerable amount of time in order to regain its working potential.

Figure 4 shows a modified form of electrode which is suitable for deep probe implantation. In this modified electrode the basic copper wire core 11 has an insulating layer 12 over which there is provided an additional copper layer 13 which is itself provided with an insulating layer 14. A palladium layer 15 is subsequently deposited on insulation layer 14. The auxiliary copper layer 13 may be connected with the palladium layer 15 at any distance from the extreme tip, thereby effectively determining R, the length of palladium film included within R being varied as required. In this arrangement R is measured between the copper core 11 and the copper layer 13 and the palladium layer 15 covered by the outer insulation 16 acts purely as a diffusion pathway and a reservoir for hydrogen passing through the internal charging cell to the active tip.The outer insulation layer 16 further defines the area of the external tip which is involved in the palladium hydride electrode reaction. The break lines indicate an undefined distance d to illustrate that the length of the probe may be chosen as required. It will be seen that the effect of this arrangement is to remove the working electrode part of the probe (indicated by R) to any desired distance from the glass cell 17.

Various modifications and variations in accordance with the present invention will be apparent to those skilled in the art. For example, although the invention has been particularly described with reference to electrodes suitable for in vivo applications, because the Wheatstone bridge arrangement makes it possible to obtain the required degree of miniaturization which is essential for such work, the apparatus in accordance with the present invention will have many other uses in areas where electrical potential and pH measurements have to be made, including industrial plants where measurements are taken in a flowing system containing dissolved oxygen, and in the long term monitoring for corrosion of metals in pipe lines. The choice of dimensions and specific materials may, of course, vary depending on the application concerned.

Claims (24)

1. Apparatus for measuring the concentration of a particular ion or a particular group of ions in a liquid, which apparatus comprises (a) a first electrode which includes a metal capable of forming a hydride, (b) means for supplying hydrogen to the metal, and (c) means for controlling the supply of hydrogen to the metal so that the concentration of hydrogen in the metal during operation is maintained in a predetermined range such that the electrode potential remains substantially constant, characterised in that the control means (c) comprises means for measuring or comparing the electrical resistance R of the first electrode and the electrical resistance R' of another component, preferably a second electrode, the resistance of which is related to the resistance (Ro) of the first electrode when no hydrogen is contained therein, and means for monitoring the ratio of said resistances (R/R') and maintaining it within a predetermined range thereby to ensure that the potential of the first electrode remains substantially constant.

2. Apparatus as claimed in claim 1, wherein the resistance R' of the other component is substantially equal to the resistance (Ro) of the first electrode when no hydrogen is contained therein.

3. Apparatus as claimed in claim 1 or claim 2, wherein the first working electrode is an (a-ss) palladium hydride electrode.

4. Apparatus as claimed in claim 3, wherein the R/Ro ratio is maintained within the range of from 1.3 to 1.5.

5. Apparatus as claimed in any one of claims 1 to 4, wherein the R/Ro ratio is determined directly by comparing the resistance R of the first working electrode with the resistance Ro of a second electrode which has the same hydrogen free resistance.

6. Apparatus as claimed in claim 5, wherein the second electrode is substantially identical to the working electrode when each is in the hydrogen-free state.

7. Apparatus as claimed in claim 5 or claim 6, wherein the second electrode is arranged to be in substantially the same environment as the working electrode.

8. Apparatus as claimed in any one of claims 1 to 7, wherein the working electrode and the reference component form one arm of a Wheatstone bridge circuit.

9. Apparatus as claimed in any one of claims 1 to 8, wherein the working electrode and the reference component form a single unit.

10. Apparatus as claimed in any one of claims 1 to 9, wherein the supply means (b) is capable of supplying hydrogen electrolytically directly to the working electrode.

11. Apparatus as claimed in any one of claims 1 to 10, wherein part of the working electrode is capable of being charged cathodically in a compartmenu which is separated from the working compartment.

12. Apparatus as claimed in any one of claims 1 to 11, wherein the working electrode comprises a central elongate core of an electrically conductive metal or other material, said elongate core being insulated but having an exposed terminal portion and being provided over said insulation and said terminal portion with a layer of hydride-forming metal.

13. Apparatus as claimed in claim 12, wherein the central elongate core comprises an insulated metal wire.

14. Apparatus as claimed in claim 13, wherein said metal is copper.

15. Apparatus as claimed in any one of claims 12 to 14, wherein the metal layer is one formed by coating.

16. Apparatus as claimed in any one of claims 12 to 15, wherein the working electrode and the refer ence component are formed from a single length of insulated conductive metal wire which is coated with the hydride-forming metal, the reference component portion being further coated with a layer of material which protects it against the electrolyte and ensures that no hydrogn is occluded with it so as to alter its resistance.

17. Apparatus as claimed in claim 1, substantial ly as described herein with reference to, and as illustrated in, the drawings.

18. A reference electrode which comprises a central elongate core of an electrically conductive metal or other material, said elongate core being insulated but having an exposed terminal portion and being provided over said insulation and said terminal portion with a layer of hydride-forming metal.

19. A reference electrode as claimed in claim 18, wherein the central elongate core comprises an insulated metal wire.

20. A reference electrode as claimed in claim 19, wherein said metal is copper.

21. A reference electrode as claimed in any one of claims 18 to 20, wherein the metal layer is one formed by coating.

22. A reference electrode as claimed in any one of claims 18 to 21, wherein the working electrode and the reference component are formed from a single length of insulated conductive metal wire which is coated with the hydride-forming metal, the reference component portion being further coated with a layer of material which protects it against the electrolyte and ensures that no hydrogen is occluded with it so as to alter its resistance.

23. A reference electrode as claimed in claim 18, substantially as described herein with reference to, and as illustrated in, Figure 1 or Figure 4 of the drawings.

24. A method of measuring the concentration of a particular ion or group of ions in a solution, which comprises employing apparatus as claimed in any one of claims 1 to 17.