GB2169709A

GB2169709A - Electrochemical sensor

Info

Publication number: GB2169709A
Application number: GB08529686A
Authority: GB
Inventors: Imanuel Bergman
Original assignee: NAT RES DEV; National Research Development Corp UK
Current assignee: NAT RES DEV; National Research Development Corp UK
Priority date: 1985-01-14
Filing date: 1985-12-02
Publication date: 1986-07-16
Also published as: DE3544462A1; GB8529686D0; JPH0679006B2; JPS61164149A; GB2169709B; GB8500813D0

Abstract

An electrochemical sensor for determining the mean concentration of gas in a space such as a flue. The sensor has an elongated gas-permeable electrolyte container disposed across the required space. In the container, the sensing electrode can be a wire many times longer than the container, spirally wound, to contribute sensing signals throughout the length of the container. An inflatable tube inside the container, may press the wire or spiral against the container so that the gas permeates to it quickly. The sensing electrode may be porous and at least one other electrode must be present in the container.

Description

SPECIFICATION Electrochemical sensor This invention relates to an electrochemical method and sensor which can determine a mean concentration of a desired fluid in a space (such as a chamber or duct) in which this concentration varies from place to place.

It is often undesirable or impractical to move a single sensor around the space, or to instal multiple sensors. One alternative approach is to use a spectrophotometer, in which a beam of radiation traverses the space by an appropriate path, but interfering particles in the fluid can absorb or scatter the radiation. Another approach is to pump a continuous or discontinuous sample of the fluid to a sensor mounted outside the space. One advantage is that the sample can be cooled, or filtered, or both, before reaching the sensor.

However, undesirable changes in the composition of the sample can also take place before it reaches the sensor, for instance by condensation or adsorption on the filter.

According to the present invention, a method of measuring a mean concentration of a fluid in a space comprises (a) providing a sensor comprising: a container permeable to the fluid; an electrolyte within the container; a sensing electrode in contact with the electrolyte; and one or more further electrodes in contact with the electrolyte acting as sensing, reference or auxiliary electrodes, (b) disposing the container of the sensor to traverse that part of the space whose fluid concentration is to be measured and permitting the fluid to permeate the container and migrate to the sensing electrode(s), and (c) making an electrochemical determination (e.g. by the voltage, current or resistance between the electrodes) of the fluid at the sensing electrode(s).

Where electrochemical determination is by voltage, the further electrode will normally be a reference electrode distant from permeating fluid, i.e. not readily subject to migration of the permeating fluid. When it is by current, the further electrodes will normally be a reference electrode and an auxiliary electrode both distant from permeating fluid, or a combined reference/auxiliary electrode, also preferably distant from permeating fluid. Where it is by resistance, the further electrode will normally be a further sensing electrode as nearly equivalent as practical to the first sensing electrode. The sensing electrode(s) is/are preferably positioned such that migration of the fluid thereto is as fast as practicable.

Also according to the present invention, a sensor for measuring a mean concentration of a fluid in a space comprises: an elongated container permeable to the fluid and containing an electrolyte and a sensing electrode; and one or more further electrodes in contact with the electrolyte acting as sensing, reference or auxiliary electrodes, characterised in that the sensing electrode is a wire many times longer than the container. Preferably, the wire is at a substantially constant spacing from the container wall. Preferably, the spacing is at most half of the maximum possible, more preferably at most one-quarter. Preferably the wire is pressed against the container but still allows a film of electrolyte to remain between the container and the electrode.If the electrode is porous to the electrolyte, the electrode can be pressed tightly against the container without an intervening film of electrolyte, or even bonded to the container. Preferably, for measuring an overall mean, the wire is disposed with equal lengths per unit length of the container, such as a regular spiral. However, for measuring a weighted mean, proportionately more wire may be disposed at the more 'important' part(s) of the container. The wire is preferably at least three times the length of the container, more preferably at least six times, more preferably at least twelve times.

Also according to the present invention, a sensor for measuring a mean concentration of a fluid in a space comprises: an elongated container permeable to the fluid and containing an electrolyte and a sensing electrode; and one or more further electrodes in contact with the electrolyte acting as sensing, reference or auxiliary electrodes, characterised in that the sensing electrode is of a porous electronic conductor (usually a metal) on the elongated (preferably tubular) electrolytecontacting surface of the container.

The conductor may be applied by various techniques such as sputtering or chemical deposition (for example electroless plating).

Also according to the present invention, a sensor for measuring a mean concentration of a fluid in a space comprises: an elongated container permeable to the fluid and containing an electrolyte and a sensing electrode; and one or more further electrodes in contact with the electrolyte acting as sensing, reference or auxiliary electrodes, characterised in that the sensing electrode is restrained laterally (preferably substantially off-centre to the container) by a flexible member inside and running the length of the container. The flexible member can be a wall of the container (typically a wall inaccessible to the fluid) or a tube or rod within the container. The tube (or the face of the wall outside the container) may be connectable to a source of pressure so as to inflate the tube (or displace the wall) whereby to increase the lateral restraint on the electrode.If the electrode is a spiral, it may be wound around the tube or rod.

Any gas can be monitored in principle that can be oxidised or reduced at an electrode, that is acidic or basic, or that will modify the conductivity of a suitable reagent electrolyte.

The acidic or basic gas could be monitored via its ability to change the potential of an electrode, via its hydrolysis of a material contained in the electrolyte to give a substance that was reducible or oxidisable, or via a change in electrolyte conductivity.

The invention will be described by way of example. Consider a boiler flue in which it is desired to monitor oxygen, carbon monoxide, sulphur dioxide or nitric oxide. Each of these gases may be monitored separately or in turn with one or more amperometric sensors. Although the latter three gases chosen as examples could, in principle, be reduced electrochemically, they would normally be monitored via their electrochemical oxidation. Oxygen would be monitored via its reduction. The electrochemical sensor comprises a sensing electrode bathed in electrolyte in a gaspermeable container.

The sensing electrode consists in this example of an electronically conductive member such as wire in an electrolyte contained in a thin-walled tube of a gas-permeable material such as polytetrafluorethylene (PTFE). The wire extends along the bore of the tube, and is of gold, or of other metal or a fibre of carbon or of another electronic conductor such as a semiconductor or an organic conductor stable in the electrolyte at the operating temperature and effective in sensing the fluid of interest. The wire however is not necessarily straight, as will be explained later. The electrolyte also needs to be stable under the operating conditions; for operation in a flue at up to 250or, pyrophosphoric or metaphosphoric acid or concentrated sulphuric acid will be suitable acidic electrolytes.Molten salts such as sodium or potassium thiocyanates are likely to be useful.

The fluid or fluids of interest (such as the above-named gases) need to traverse the wall of the gas-permeable tube sufficiently rapidly to give an easily measured signal with a sufficiently rapid response time. In principle, a porous tube could be used, if the electrolyte were retained despite the porosity. In practice, it is normally better to use a material such as PTPE, through which gases and vapours diffuse by activated diffusion. When the temperature of operation and the reactivity of the electrolyte and the fluids being sampled are not excessive, other gas-permeable materials such as silicone rubber and polyethylene may be used.

In general, the output signal of an amperometric sensor will be inverseiy proportional to the wall thickness, and the response time will be proportional to the square of the thickness.

The thinnest membranes of PTFE available are about 3 microns thick. The thinnest tubes of PTFE available have a wall thickness of about 40 microns. The thinner the wire or fibre electrode, the less will movement of fluid affect the output signal. Pure gold wire is commonly available with a diameter of 25 microns.

The wire need not be of circular cross-section. Indeed it might be of advantage to have it in the form of a thin strip. For a given gold wire, the total surface area would be increased if the wire were flattened into a strip.

The catalytic activity of the surface might be modified by such treatment. The background current would normally increase with the surface area. The interaction of gases diffusing through the outer PTFE membrance with the surface of the gold strip might be better than with a circular wire, in that the strip might lie against the PTFE with a thinner layer of electrolyte in between them. On the other hand, a circular-section wire might fit better into a circular bore tube. In any case, the back of the flat strip would not be as easily accessible to gas diffusing through the PTFE tube, as the back of a circular section wire. It might be easier or more difficult to thread a thin strip through a tube, compared with a wire of the same cross-sectional area.In either case, the best way of threading the wire or strip through is probably first to thread the narrower tube through the wider tube, until the narrower tube protrudes by an amount at least equal to the length of the wider tube.

The wire or strip is then fastened to the part of the narrower tube just protruding from the wider tube, and then pulled back through the wider tube with the protruding portion of the narrower tube.

A further option at this stage is that the wire or strip can be made porous. This could be achieved by making the wire or strip of a suitable alloy, then assembling it as described, then etching out one component or phase in situ; in this way, the full strength of the wire or strip is available during its manipulation, its relative weakness after being made porous being relatively unimportant.

The PTFE tube containing electrolyte would then be allowed to dip into the electrolyte in a part of the detector cell containing another electrode or other electrodes. If, as suggested above, the thin-wire electrode was used in an amperometric sensor in which, for instance, oxygen could be reduced and the other gases mentioned above could be oxidised, currents would be generated proportional to the partial pressures of the gases being monitored.

These currents would be generated via an auxiliary electrode, when the thin-wire electrode was held with a potentiostat at appropriate potentials with respect to a reference electrode. A separate cell could be used for monitoring each fluid, or the potential could be pulsed to a succession of potentials appropriate for each fluid in turn, or several sensing electrodes could be used (for simultaneous measurement of several gases).

As an alternative to an amperometric cell, a potentiometric sensor could be constructed to give a signal that represented a mean concen tration over the space covered by the sensor.

To give yet another alternative, two wires could be mounted side by side on an insulating substrate, which was then threaded through the PTFE or silicone tube. This assembly could then be used as a conductimetric sensor or amperometric where two electrodes are at different potentials with respect to the reference electrode.

One of the problems likely to arise with the operation in a boiler flue of a gas-permeable sensor cell incorporating hygroscopic materials such as pyrophosphoric, metaphosphoric or sulphuric acids, is that water diffusing into the acid during wet and cold periods will tend to vaporise at high temperatures. One solution would be a heater to keep the cell permanently hot. Another solution is to try to keep a portion of the acid dry by electrolysis during the period in which the boiler is cold. The gases released by the electrolysis might generate the pressure needed to force the dry acid into the polarographic-electrode tube ready for the boiler to be lit again.

So far, the invention has been described in terms of one or more electrode wires threaded through a gas-permeable tube, but care must be taken over likely problems.

Thus, for example, if the bore of the tube is too small, it may be difficult to thread the electrode down the tube. If on the other hand the bore is too large, the wire and the electrolyte can move with respect to each other, and hence the background current will be larger and less stable than necessary. Any movement to the electrode of electrolyte with which the electrode has not been equilibrated is likely to lead to variable currents resulting from faradaic and adsorption/desorption processes. This undesirable effect will be less for a thinner electrode, because a bigger proportion of the transport of the electroactive species takes place through the layer of electrolyte which is tightly bound to the electrode surface.Also if the bore is too large, unless all the wire is pressed against the wall of the tube, some of the electroactive gas will need to diffuse through an unnecessarily thick and mobile layer of electrolyte to the electrode.

The magnitude of the gas-sensing current will thus be limited not mainly by the permeability of the tube but by diffusion of the gas through the electrolyte and by bulk movement of the electrolyte. However, even if the wire is pressed against the wall of the tube, the response of the sensor to a disappearance of the electroactive gas will be slower than its response to an exposure to the gas. This arises because, with a large-bore tube, some of the electroactive gas will permeate through it at points distant from the wire. Thus, even after permeation ceases, this gas will continue to be transported slowly to the electrode by diffusion, and more quickly by movement of the electrolyte.

It would be desirable to have the fluid being sampled permeating through the wall of the gas-permeable tube, and all of it immediately coming into contact with the sensing electrod. One way to make such an electrode would be to deposit a porous electronic conductor onto the inner wall of the gas-permeable tube, e.g. by sputtering (with or without a mask), or by chemical deposition (electroless plating). Another way, when the electrode is a wire or fibre threaded through the tube, is to form it as a spiral pressed against the wall of the tube. (Where it is desired to take a weighted-average reading, the spiral coils may be wound closer together in the region in whose favour the average is to be weighted.) Another way is to thread two components through the gaspermeable tube: in addition to the wire electrode, an inner tube or rod is inserted, i.e. threaded through.This inner tube can be used to pass a temperature-control fluid. Its main function, however, is to press the wire against the wall of the outer tube and, at the same time, limit the access to the electrode of electrolyte with which it has not become equilibrated. The inner tube is preferably flexible and elastic such that it can be folded into a smaller diameter until it has been threaded through the gas-permeable outer tube. The inner tube can then be expanded by a gas or liquid under some pressure. One or more electrodes could be mounted ready on the inner tube, for threading (deflated) into the outer tube and then being expanded; the outer surface of the inner tube could carry an electrolessly plated porous metal electrode. This electrode could alternatively be made by evaporation or sputtering.If stripes of applied metal were later removed, the remaining electronic conductor could be used as multiple electrodes, such as (as already suggested) conductimetric electrodes or for sintultane- ously measuring several gases.

An alternative to threading an inner tube through an outer tube is to use a double-bore (double-lumen) tube with suitable thicknesses for the wall sections and the septum, with the electrode threaded through one of the lumens.

These ways are inventive in their own right.

If bubbles are allowed to form on the wire electrode, for instance by electrochemical gas evolution, they may impair the performance of the electrode. If the tube electrode is held vertically, alternate inflation and deflation of the inner tube (or of the other bore of a double-lumen tube) can help electrolyte along the electrode-containing tube or bore so as to remove the bubbles.

There are a number of applications for a tube electrode that do not require it to operate at such high temperatures as a flue-gas sensor. For instance a tube sensor could be used to monitor the partial pressure of oxygen and other gases dissolved in chemical or biological reactors. The tubes could easily be sterilised by heat without damaging them. Indeed the wire of the electrode or even the electrolyte itself could be used as a conductor for an AC current to heat the electrode, either to sterilise it, or to maintain it at a fixed temperature.

The simple smooth shape of the tube-electrode sensor is of advantage in a chemical or biological reactor, in that it is easily cleaned.

The fact that the tube can consist of chemically robust materials such as PTFE means that corrosive cleaning chemicals can be used to clean it.

The smooth shape is also of benefit in an application that requires it to be inserted into a biological tissue, because it slides in easily, and does not encourage clotting.

Materials such as PTFE do not favour fouling by biological organisms. A tube sensor will thus be suitable also for monitoring gases dissolved in sewage or natural water.

When the temperature or other conditions allow the very gas-permeable material silicone rubber to be used, the fabrication of electrodes may be facilitated. One caveat is that thin membranes of silicone rubber may be sensitive to attack by alkali.

Because silicone rubber is so elastic, the wire of the electrode may be threaded through the silicone rubber tube by inflating the latter to a suitable internal bore. The silicone tube is threaded over and sealed to the lower end of an upper non-magnetic pressurised tube and the upper end of a lower pressurised tube.

The wire is allowed to hang inside the upper pressurised tube from an upper needle held by a magnet outside the tube, and is kept taut by a lower needle suspended from the wire.

The wire is then threaded through the silicone tube by lowering the magnet.

An alternative way of fabricating an electrode assembly with silicone rubber is to coat the wire with a layer of material that can later be dissolved away without attacking the wire or the silicone rubber. The silicone rubber tube is then deposited from solution onto the layer and cured, before the layer is dissolved away.

For instance a layer of aluminium on top of a gold wire could be dissolved in dilute hydrochloric acid.

It may be desirable to monitor the partial pressure of an electroactive material such as oxygen, not averaged out over a region, but in a particular place, e.g. part of the body. It is possible to do this with an adapted version o the sensor herein described, comprising a thin wire electrode and the coaxial pair of PTFE tubes.

The thin wire is threaded through the inner tube to near its other end, then out through a hole in the inner tube. The wire is then wound once or more times round the inner tube, threaded in through a hole in the inner tube, and back to the original end of the wire. This assembly is then threaded through the outer PTFE tube, such that the coil of wire around the inner tube is surrounded by the outer tube.

The inner and outer tubes are filled with isotonic saline, which has been freed of oxygen, for instance by evacuation, or with a stream of nitrogen. A slow flow of such saline is then pumped through both tubes, such that the saline stream contains reference and auxiliary anperometric electrodes. The tube system can then be used as a catheter, for instance into a vein or artery, and the loop of wire wound round the inner tube can be used as an oxygen electrode.

Claims

1. A sensor for measuring a mean concentration of a fluid in a space comprising: an elongated container permeable to the fluid and containing an electrolyte and a sensing electrode; and one or more further electrodes in contact with the electrolyte acting as sensing, reference or auxiliary electrodes, characterised in that the sensing electrode is a wire many times longer than the container.

2. A sensor according to Claim 1, wherein the spacing between the wire and the container wall is at most half of the maximum possible.

3. A sensor according to Claim 2, wherein said spacing is at most one quarter of the maximum possible.

4. A sensor according to any preceding claim, wherein the wire is pressed against the container but still allows a film of electrolyte to remain between the container and the electrode.

5. A sensor according to any of Claims 1 to 3, wherein the sensing electrode is porous to the electrolyte.

6. A sensor according to Claim 5, wherein the sensing electrode is pressed tightly against the container without an intervening film of electrolyte.

7. A sensor according to Claim 5, wherein the sensing electrode is bonded to the container.

8. A sensor according to any preceding claim, wherein the wire is at a substantially constant spacing from the container wall.

9. A sensor according to any preceding claim, wherein, for measuring an overall mean, the wire is disposed with equal lengths per unit length of the container.

10. A sensor according to any of Claim 1 to 8, wherein, for measuring a weighted mean, proportionately more wire is disposed at the more 'important' part(s) of the container'.

11. A sensor according to Claim 9, wherein the wire is a regular spiral.

12. A sensor according to any preceding claim, wherein the wire is at least three times the length of the container.

13. A sensor according to Claim 12, wherein the wire is at least six times the length of the container.

14. A sensor according to Claim 12, wherein the wire is at least twelve times the length of the container.

15. A sensor according to Claim 1, substantially as hereinbefore described.

16. A sensor for measuring a mean concentration of a fluid in a space comprising: an elongated container permeable to the fluid and containing an electrolyte and a sensing electrode; and one or more further electrodes in contact with the electrolyte acting as sensing, reference or auxiliary electrodes, characterised in that the sensing electrode is restrained laterally by a flexible member inside and running the length of the container.

17. A sensor according to Claim 16, wherein the or each sensing electrode is a wire nany times longer than the container.

18. A sensor according to Claim 16 or 17, wherein the flexible member is a wall of the container or a tube within the container.

19. A sensor according to Claim 18, wherein the face of the wall outside the container, or the tube, is connectable to a source of pressure so as to displace the wall or inflate the tube, whereby to increase the lateral restraint on the sensing electrode(s).

20. A sensor according to any of Claims 16 to 19, wherein the or each sensing electrode is laterally restrained substantially off-centre to the container.

21. A sensor according to Claim 20, wherein the sensing electrode is pressed against the container.

22. A method of measuring a mean concentration of a fluid in a space comprising (a) providing a sensor comprising: a container permeable to the fluid; an electrolyte within the container; a sensing electrode in contact with the electrolyte; and one or more further electrodes in contact with the electrolyte acting as sensing, reference or auxiliary electrodes, (b) disposing the container of the sensor to traverse that part of the space whose fluid concentration is to be measured and permitting the fluid to permeate the container and migrate to the sensing electrode(s), and (c) making an electrochemical determination of the fluid at the sensing electrode(s).

23. A method according to Claim 22, wherein the reference electrode is in a region of the electrolyte not readily subject to migration of the permeating fluid.

24. A method according to Claim 22, wherein the further electrodes are a reference electrode and an auxiliary electrode (which may be combined) both distant from permeating fluid.

25. A method according to Claim 22, wherein the further electrode is a sensing electrode equivalent to the first sensing electrode.

26. A method according to any of Claims 22 to 25, wherein the sensing electrode(s) is (are) positioned such that migration of the fluid thereto is fast.

27. A sensor for measuring a mean concentration of a fluid in a space comprising: an elongated container permeable to the fluid and containing an electrolyte and a sensing electrode; and one or more further electrodes in contact with the electrolyte acting as sensing, reference or auxiliary electrodes, characterised in that the sensing electrode is of a porous electronic conductor on the elongated electrolyte-contacting surface of the container.