GB2245707A

GB2245707A - Screened electrochemical electrode

Info

Publication number: GB2245707A
Application number: GB9014027A
Authority: GB
Inventors: James Hambleton
Original assignee: SYCOPEL SCIENT Ltd
Current assignee: SYCOPEL SCIENT Ltd
Priority date: 1990-06-23
Filing date: 1990-06-23
Publication date: 1992-01-08
Also published as: GB9014027D0

Abstract

An electrochemical electrode has a screened cable 11 electrically connected to it and a further screen 17 enclosing at least a major part, but not all, of the electrode itself, the latter screen in turn being electrically connected to the cable screen. The electrode screen may be spaced by a small axial distance from the cable screen and connected to it by a separate axial link. The screen is preferably of steel, aluminium or mumetal coated on one or both of its faces with a non-conductive coating. The electrode may be a pH electrode, an ion-specific electrode, a glass electrode or a micro referecence electrode. <IMAGE>

Description

Improved Electrochemical Electrode Assembly The invention relates to electrodes for use in electrochemical analyses and is an improved electrode assembly for such purposes.

In electrochemical analytical methods, a test cell is formed which cccprises three electrodes immersed in an electrolyte. The material under test forms the working electrode and a current is passed through the electrolyte from the so-called secondary or auxiliary electrode to the working electrode. A third electrode, the reference electrode, is placed close to but not in contact with the working electrode and the potential at the surface of the working electrode is measured between the working electrode and the reference electrode. The resulting figure is an indication of the electrochemical relationship between the material under test and the electrolyte and may be used to determine such characteristics as the rate of corrosion of the test material in the electrolyte.

In order to obtain accurate results, it is essential that any current flow between the working electrode and the reference electrode be minimised, since otherwise the presence of the reference electrode distorts the system under test. To this end, the reference electrode must have a high impendance, typically of the order of 1014 ohms.

As a result, especially where a protective film has been applied to the surface of the working electrode, the currents and voltages to be measured are very small, for exanzole in the region of nanoaperes and microvolts. The measurment techniques developed in recent years are able to monitor such low values without undue difficulty but at the same time, these high-impedance reference electrodes are seriously vulnerable to picking up extraneous signals, which of course give rise to spurious measurements. One method of countering this problem which is sometimes adopted is to place the whole cell within a Faraday screen, but this is an extremely cumbersome and costly procedure.There therefore remains a need for a way of overcoming or lessening this problem which is mare practicable and less expensive than the use of a Faraday screen.

It is an object of the present invention to provide an improved electrode assembly which meets that need.

The improved electrode assembly according to the present invention is characterised by having a screened cable electrically connected to it and a further screen enclosing at least a major part, but not all, of the electrode itself, the latter screen in turn being electrically connected to the cable screen. The effect of screening the electrode in this way and of connecting it to the cable screen is to greatly reduce or eliminate the adverse effects of extraneous electrical noise or signals on the accuracy of the analysis.

The use of a screened cable to form the electrical connection to the electrode is conventional but in the case of the present invention, the electrode itself is largely enclosed in a screen. In one form of the invention, the electrode screen is spaced by a small axial distance from the cable screen and is electrically connected to it by a separate electrical link. However in an alternative preferred form, the electrode screen and the cable screen together form a continuous cover for the cable and for a major part of the electrode. The electrode must not be wholly screened or it could no longer perform its intended function but it is only necessary to leave the tip of the electrode exposed, that is, the part of the electrode which provides the electrical interface with the electrolyte.

The screen covering the electrode, which is preferably of the same material as the cable screen, may be of any suitable metal, usually steel or aluminium, but preferably metal, which gives both electrostatic and magnetic screening; the screen is preferably in the form of a tube. Preferably the screen is coated on one or both of its faces with a nonconductive protective coating, for example of glass or a plastics material, in order to minimise the possibility of the screen intruding upon the electrochemical system.

The invention is of particular benefit when applied to reference electrodes, wherein the adverse effects of extraneous signals are particularly felt, but may also be applied to working electrodes, especially to the "micro" working electrodes which carry currents in the picoepere range. The reference electrodes to which the invention is applicable include those in which a metallic or soluble phase is in equilibrium with its ion, those in which a metallic phase is in equilibrium with its soluble metal salt, pH electrodes, ion specific electrodes, glass electrodes and "micro" reference electrodes.

A problem which can affect the accuracy of electrochemical analyses arises where there is a significant drop in potential (an "IR" drop) between the working electrode and the reference electrode. One method of reducing this drop is to extend the reference electrode by incorporating it in a so-called Luggin capillary probe, the end of which can be positioned very close to the working electrode, the extension being filled with a second electrolyte separated from the first electrolyte by a capillary tube or a porous "frit" of sintered glass. The present invention may advantageously be applied to such a reference electrode by extending the electrode screen to enclose the electrode and the secondary electrolyte down to the region of the frit.

As already stated, the electrode screen is electrically connected to the cable screen. Further improvements in performance of the cell may be obtained by operating the two screens at the same potential as the electrode and as the signal lead within the cable. One method of achieving this is to interpose a non-inverting unity gain amplifier, driven by the signal lead, between the signal lead and the cable screen. The screen may in this way be placed at a potential which is effectively earth potential.

The improved electrode according to the invention may be used in a wide range of electrochemical analyses, among which may be mentioned corrosion analysis, stripping voltaumetry, cyclic voltammetry, polarography and AC impedance and harmonic measurements.

The invention will now be further described and illustrated by reference to the acoompanying drawings, which show, by way of example only, two alternative preferred embodiments of the present invention and wherein: Fig. 1 is an elevation, partly in section of a first embodiment of reference electrode; Fig. 2 is a corresponding view of a second embodiment of reference electrode, incorporating a Luggin probe; and Fig. 3 shows diagrammatically one arrangement for electrically inter connecting the signal lead and screens.

Referring firstly to Fig. 1, the simple reference electrode 10 there shown is connected to a screened electrical cable 11 via a cap 12. The electrode may, for example, comprise a piece of silver 13 immersed in a solution 14 containing silver ions, for example a silver nitrate solution. The electrode, when in use for electrochemical test purposes, is imnersed in an electrolyte, electrical contact with the latter being via a short length of conductive wire 15 at the tip of the electrode. The electrode is formed as a glass tube 16 and a tubular steel screen 17 is incorporated in the thickness of the glass, giving thereby a glass/steel/glass laminate.Within the cap 12, the screen 17 is electrically connected to the screen of cable 11, thereby effectively screening almost the whole of the electrode from the cap to the tip, apart from the tip itself.

Fig. 2 illustrates a reference electrode 20 incorporated in a Luggin probe 21. The probe comprises a generally tubular glass body terminating at its lower end in a capillary section 22, which section curves away from the axis of the main body and is closed off by a frit 23. The whole probe is immersed in the electrolyte of an electro chemical cell when it is used. Within the probe 21, the reference electrode 20 is partially immersed in a bridging electrolyte 24.

A generally tubular metal screen 25 is incorporated in the thickness of the glass probe 21 and extends from the upper end of the probe surrounding the reference electrode 20 down to close to the frit 23 at the lower end of the probe. The screen 25 is electrically connected to the screen of the cable 26 via a line 27.

As explained above, it is advantageous to operate the improved electrode of the present invention with the electrode screen and cable screen at the same potential as the signal lead and the electrode itself. Fig. 3 is a diagrammatic illustration of one arrangement for achieving this. In that figure, a reference electrode 30 is shown connected via a cap 31 to a cable 32. The cable is a coaxial cable in which the signal lead 33 is axially disposed within a tubular metal screen 34, which in turn is surrounded by a protective sheath 35. The screen 34 is electrically connected to a screen surrounding the electrode (for example, the screen 17 or 25 of Fig. 1 or 2), and the signal lead 33 is connected to the reference electrode to receive the signal therefrom.

As shown, in the illustrated circuit the signal from the lead 33 is led to a non-inverting unity gain amplifier 36.

This input to the amplifier has a very high impedance close to that of the reference electrode. However, the output voltage of the amplifier 36 follows that of the input voltage, with the advantage that the output impedance is very low and is effectively at earth potential. The output is connected via a line 37 to the screen 34, thereby effectively earthing the electrode screen.

Claims

1. An improved electrode assembly for use in electrochemical analyses, which has a screened cable electrically connected to it and a further screen enclosing at least a major part, but not all, of the electrode itself, the latter screen being electrically connected to the cable screen.

2. An improved electrode assembly as claimed in Claim 1, wherein the electrode screen is spaced by a small axial distance from the cable screen and is electrically connected to it by a separate electrical link.

3. An improved electrode assembly as claimed in Claim 1, wherein the electrode screen and the cable screen together form a continuous cover for the cable and for a major part of the electrode.

4. An improved electrode assembly as claimed in any of the preceding claims, wherein the electrode screen is of steel, aluminium or mumetal.

5. An improved electrode assembly as claimed in any of the preceding claims, wherein the electrode screen is coated on one or both of its faces with a non-conductive coating.

6. An improved electrode assembly as claimed in Claim 5, wherein the electrode screen is retained between two layers of glass.

7. An improved electrode assembly as claimed in any of the preceding claims, wherein the electrode is one in which a metallic or soluble phase is in equilibrium with its ion, or one in which a metallic phase is in equilibrium with its soluble metal salt.

8. An improved electrode assembly as claimed in any of the preceding claims, wherein the electrode is a pH electrode, an ion specific electrode, a glass electrode or a micro reference electrode.

9. An improved electrode assembly as claimed in any of the preceding claims, wherein the electrode is a reference electrode incorporated in a Luggin capillary probe.

10. An improved electrode assembly as claimed in any of the preceding claims, comprising a non-inverting unity gain amplifier, driven by the signal lead and interposed between the signal lead and the cable screen.

11. An improved electrode assembly for use in electrochemical analyses, which is substantially as hereinbefore described with reference to, and as illustrated in, Fig. 1 or Fig. 2 of the accompanying drawings.

12. A corrosion analysis, carried out using an improved electrode assembly as claimed in any of the preceding claims.