Arm. occup. Hyg. VoL 18, pp. 53-62. Pergamon Preu 1973. Printed in Great Britain
ELECTROCHEMICAL CARBON MONOXIDE SENSORS BASED ON THE METALLISED MEMBRANE ELECTRODE* I. BERGMAN
Safety in Mines Research Establishment, Sheffield S3 7HQ
people are exposed to potentially harmful concentrations of carbon monoxide it is often useful to know what the variation in concentration is with time and in space. The primary consideration may be to protect a number of men with individual alarms. A variety of instruments can be useful: recorders, alarms and meters, some preferably compact enough to be carried in a pocket. A number of possible means of detecting and measuring carbon monoxide have been utilised in commercial instruments; these range from chemical detector tubes utilising the reaction between carbon monoxide and periodic acid to sophisticated infrared spectrophotometers. This paper describes the development at SMRE of an electrochemical system, the essential feature of which involves the oxidation of carbon monoxide at a metallised membrane electrode. The "metallised membrane electrode" (BERGMAN, 1970), is made by evaporating or sputtering metal on to a non-porous, but gas-permeable, polymer membrane. In the "amperostat" system (BERGMAN, 1971), an electrochemical cell with two such electrodes and an auxiliary electrode in a control circuit is used to minimise the effect of temperature on the background current. It also allows the electrocatalytic activation of carbon monoxide sensors to be controlled, and the activity to be maintained. WHERE
CURRENT-GENERATING ELECTROCHEMICAL MONITORS Setting the potential of an electrode to a particular value with respect to an electrolyte solution may be thought of as giving the electrons available at the electrode a particular energy. In a current-generating electrochemical monitor, this energy is chosen to be sufficient for electron transfer to take place efficiently to or from the species to be monitored, but not to the electrolyte solution. In a "polarographic" • Crown Copyright 1975. 53
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Abstract—The 'metallised membrane electrode' is made by evaporating or sputtering metal on to a non-porous but gas-permeable polymer membrane. The 'amperostat' system consists of an electrochemical cell with two such electrodes and an auxiliary electrode in a control circuit. The system minimise the effect of temperature on the background current. It also allows the electrocatalytic activation of carbon monoxide sensors to be controlled, and the activity to be maintained. Such sensors, made at SMRE or by collaborators, have given useful results in the field and in the laboratory over a number of years. Preliminary results suggest that, with ancillary equipment, they can make a significant contribution to the measurement of carbon monoxide at concentrations of 10 ppm upwards, and in a wide variety of gases: internal combustion exhaust gases, flue gases, cigarette smoke, exhaled air, polluted air, and air from environments that are subject to spontaneous combustion.
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monitor the rate of arrival of the active species at the electrode, and hence the output current, depends on the permeability of a diffusion-limiting layer.
Electrocatalytic activity Since carbon monoxide is not easily oxidised (it is not a good reducing agent at room temperature), an electrode at which it is to be oxidised needs to have some electrocatalytic activity for the process in question. The electrode must have the appropriate degree of affinity for carbon monoxide and its reaction products. The carbon monoxide must normally be adsorbed on to the electrode surface strongly enough to be converted into an activated form, that will give up an electron to the electrode. Once this electron transfer, and perhaps other reactions with the solvent have taken place, the reaction product must be desorbed, to allow further molecules of carbon monoxide to adsorb on to the electrode; otherwise the reaction is selfinhibiting. Further technical details concerning the metallised membrane electrodes and the amperostat system may be found in the Appendix. APPLICATIONS Carbon monoxide sensors incorporating metallised membrane electrodes and the amperostat system have been on trial at SMRE for a number of years. Five examples of applications in which they have been found to be of value are: 1. Spontaneous combustion in coal mines: a pocket "sniffer" Spontaneous combustion occurs in some coal mines. At one time, the only warning in advance of over-heating and "sweating" of the strata was the "gob-stink" that some miners claimed to be able to detect. Carbon monoxide is produced in the spontaneous combustion of coal at a fairly early stage. In recent years the National Coal Board has been installing "tube-bundle" systems in some pits. Air from a number of selected points in the pit is pumped to the surface to a carbon monoxide
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The metallised membrane electrode A non-porous, but gas-permeable, polymer membrane is used to separate the carbon monoxide-containing air from an electrolyte solution, as in the earlier method of CLARK (1956). Consequently only those materials that can diffuse through the membrane (usually of PTFE) can gain access to the electrolyte; particulate materials have no effect. However, in contrast to previous devices such as that of Clark, in which a film of electrolyte separates the PTFE membrane from a solid electrode, the metallised membrane electrode (BERGMAN. 1970; BERGMAN and WINDLE, 1972) consists of a porous or otherwise permeable metal film evaporated or sputtered on to the inner surface of the polymer membrane. This is supported and protected by a "salt bridge", usually a dialysis membrane, which is permeated by the electrolyte solution; it acts virtually as a solid electrolyte. The disposition of the electrode and the geometry of diffusion of carbon monoxide to the electrode are kept stable more easily with the metallised membrane electrode than with earlier devices. The carbon monoxide diffuses from the atmosphere (via high-porosity sintered metal and sintered polythene discs) through the PTFE film which acts as a diffusion barrier, and immediately encounters wet electrode material in intimate contact with the whole of the diffusion barrier.
Electrochemical carbon monoxide sensors
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2. Carbon monoxide recorder with periodic zero checks A carbon monoxide monitor to spot the upward trend in concentration that indicates that spontaneous combustion has flared up in a pit should probably cover the range 0-20 ppm, discriminate to 0-2 ppm or better, and reliably show a rise of about 1 ppm per day. The biggest problems for a metallised membrane carbon monoxide detector working in this range are noise and zero drift. However, the noise can be reduced by capacitative damping, and the zero drift can be compensated for by periodic exposure of the cell to air from which the carbon monoxide has been removed. Indeed the latter function has been carried out in preliminary trials by a pump of power consumption less than 10 mW. This makes even a portable recorder to cover the range feasible. 3. Industrial hygiene: meters and alarms for instantaneous or integrated exposures Another application of carbon monoxide monitors is in industrial hygiene; to prevent workers suffering harmful exposures. The limit currently recommended by the Department of Employment for industrial exposure to carbon monoxide is an average of 50 ppm for a week of five days (seven or eight hours per day). This value is derived from the list issued by the ACGIH (American Conference of Governmental Industrial Hygienists) in which a "threshold limit value" of 50 ppm is associated with a "peak" limit of 75 ppm. This latter value is probably inappropriate for carbon monoxide, which is thought to act almost exclusively on the haemoglobin of the blood in a relatively cumulative manner, so that peak concentrations are less important than with corrosive materials, for instance. The United States National Institute for
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monitor, usually based on a non-dispersive i.r. transducer. If an upward trend in carbon monoxide concentration suggests that spontaneous combustion has started in a district, the problem arises of localising the combustion. What is needed is a carbon monoxide meter that is compact enough to be easily carried, and that responds quickly enough for a source of carbon monoxide in the strata to be located. It was originally suggested that the meter should be calibrated for a range of concentrations of 0-500 ppm in order to avoid large swings of the meter being caused by the carbon monoxide from diesel engine exhaust gases or shot-firing fumes. However, these other sources of carbon monoxide do not occur in all pits, and in some circumstances a more sensitive scale may be useful. A variety of prototype carbon monoxide "combustion sniffers" based on the metallised membrane electrode have been constructed over a period of years, and have received extensive laboratory trials and some trials underground. One form is of dual range: normally 0-100 ppm, but with a push-button to give a range of 0-500 ppm. The metallised membrane electrode transducer does not react to methane, which can be encountered in coal mines in concentrations of 1 % or more. The sensitivity to ethylene is normally about 20 % of that to carbon monoxide. The sensitivity to hydrogen can be reduced to this magnitude by choice of an anodic potential for the electrode. Ethylene and hydrogen will usually be encountered in significant concentrations in coal mines, emanating from spontaneous combustion sites that are already giving off even larger concentrations of carbon monoxide, although some coal-mine methane does contain hydrogen.
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4. Carbon monoxide in alveolar air Wald and his co-workers (WALD et al. 1973; WALD and HOWARD, this issue pp. l-14)have shown that smokers whose blood haemoglobin is combined with carbon monoxide to the extent of 5 % (5 % COHb) or more, are 21 times as likely to be affected by atherosclerotic diseases, including ischaemic heart disease, as similar smokers with a COHb level of less than 3 %. GRUT et al. (1970) in their support for a threshold limit value of 50 ppm for carbon monoxide, relate this value to a COHb level of 8-10 % after several hours of exposure. PETERSON (1970) found that alveolar air analysis could be used to estimate the post-exposure COHb saturation accurately. He found that 3 % COHb corresponded to about 15 ppm carbon monoxide in alveolar air, 5 % to about 25 ppm, 8 % to about 35 ppm, and 10% to about 50 ppm. During some early attempts to determine carbon monoxide in breath, we discovered that humans exhale some hydrogen. This is a phenomenon that appears to be familiar only to gastroenterologists, and has some clinical significance in malabsorption in the gut. The experience in gas chromatography of Mr G. Vizard and his colleagues at the National Coal Board Yorkshire Regional Laboratory was of great assistance in the development of an apparatus for the determination of both hydrogen and carbon monoxide in breath. A column of an activated molecular sieve was used at ambient temperature and with air as carrier gas to separate carbon monoxide and hydrogen; a metallised membrane electrode was the detector. Figures l(a) and l(b) show the apparatus for sampling alveolar air, suggested by Wald. Figures l(b) and l(c) show schematic diagrams of the chromatographic system. A small pump provides the air flow through the column. A ganged pair of pneumatic valves serve to inject the sample into the gas flow. The signal peak height, accurately linear with concentration for both hydrogen and carbon monoxide, may be measured with a meter or with a recorder. Figure l(d) is a photograph of the chromatographic system; the commercially-available components are extremely inexpensive.
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Occupational Safety and Health (NIOSH) has recently promulgated limits of 35 ppm mean and 200 ppm peak exposure concentrations for carbon monoxide. Audible and flashing light alarms have beenfittedto a variety of oxygen deficiency monitors based on the metallised membrane electrode. Preliminary studies have also been carried out on peak concentration alarms set at 200 ppm or even 100 ppm carbon monoxide. Their accuracy and reproducibility depend on how extreme are the variations in temperature, and how frequently the zero reading can be checked, e.g. by sucking air through a small carbon monoxide-scrubber tube. Silver microcoulometers are now available for long-term integration of currents. The output of the metallised membrane electrode sensor is linear with partial pressure over a wide range, and can be integrated in such a device. Thus the integrated exposure of a man to carbon monoxide over a shift could be measured. An integratedexposure alarm has been suggested by Mr R. Wilson of SMRE. An amount of silver equivalent to the desired maximum exposure is plated on to the coulometer electrode; the output of the carbon monoxide cell is then used to dissolve the silver again. When all the silver has been dissolved, the alarm is activated.
Electrochemical carbon monoxide sensors
(c)
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Air pump-
Ganged pneumatic l ^
Sample loop Chromatographic ~ j l column
-Flowmeter
5. High range meters Metallised membrane electrode sensors may prove useful for monitoring carbon monoxide in the concentration range 0-1-10% in, for instance, cigarette smoke or petrol engine exhaust gases. If a metallised membrane electrode carbon monoxide sensor is exposed to pure carbon monoxide for more than a few seconds, the cell current rises to a peak and then falls ; presumably carbon monoxide or an oxidation product tends to inhibit oxidation. There is also sometimes an effect on the background current of the cell. Both these effects disappear some time after the exposure of the cell to the high concentration of carbon monoxide has ceased. The linearity of cell current with continuous exposures to carbon monoxide has an upper limit dependent on the thickness of the PTFE membrane. With a 3 (xm thick membrane, the limit is somewhere between 0 05 % and 0 1 % ; with 25 (xm thick membranes it is between 0-4% and 1 per cent. However, linearity can be achieved, even up to one atmosphere of carbon monoxide, by means of sample injection techniques such as are used in gas chromatography. The equipment may consist of simple and cheap pneumatic valves such as those shown in Fig. 1. Indeed the gas chromatograph shown in Fig. 1 may be used for high concentrations merely by using a small injection loop. If carbon monoxide is the only component of the gas to be monitored that is oxidisable or reducible at the metallised membrane electrode and is present in significant concentrations, the chromatographic column may be unnecessary. Alternatively, a chemical scrubbing layer may be used to remove unwanted components from the gas mixture, probably before it is injected into the air stream that is fed to the monitor.
ADVANTAGES AND LIMITATIONS OF METALLISED MEMBRANE ELECTRODE CELLS Electrochemical transducers such as cells using the metallised membrane electrode have a number of advantages over other devices; the predominant one is low power
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FIG. 1. Simple chromatographic system for carbon monoxide and hydrogen in alveolar air using the metallised membrane electrode detector, (a) Carbon monoxide sample collection; (b) Flushing-out sample loop (c) Injecting sample into chromatographic flow.
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CONCLUSIONS
There are a variety of situations in which individuals may be exposed to potentially harmful concentrations of carbon monoxide. In some circumstances, protection (or surveillance) may best be provided by continuous monitoring of their environment by a fixed recording instrument; in others individual alarms and/or meters, compact enough to be carried in the pocket may be desirable. As a general method of detecting and measuring carbon monoxide, electrochemical sensors have a number of advantages, predominant among which is low power consumption. Such sensors, made at SMRE or by collaborators, have given useful results in the field and in the laboratory over a number of years. Preliminary results suggest that, with ancillary equipment, they can make a significant contribution to the measurement of carbon monoxide at concentrations of 10 ppm upwards, and in a wide variety of gases: internal combustion exhaust gases, flue gases, cigarette smoke, exhaled air, polluted air, and air from environments that are subject to spontaneous combustion.
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consumption and, consequently, that instruments should need infrequent servicing. For example, a simple pocket carbon monoxide meter with an amperostat control system and a current-voltage converter will run continuously for six to twelve months on four hearing-aid batteries. Moreover, because of the low power requirements, "intrinsic safety" is easily achieved for instruments to be used in coal mines and in other places where explosive gases such as methane may be encountered. Furthermore, the cells are robust, yet can be made very compact and lightweight. They are current generators; the output of a cell will normally bear a linear relationship with the partial pressure of carbon monoxide over several orders of magnitude. The only part of the cell that is consumed during its life is the electrolyte solvent; normally water. If the electrocatalytic activity of the cell falls significantly after its specified life, this activity can be regenerated by means of the cyclic activation procedure used when a cell is brought into operation (see Appendix). For a single design of cell, variations may be made in the thickness and material of the non-porous but gas-permeable polymer membrane, the composition of the metal layer, the solvent, the acidity and ionic strength of the electrolyte, electrocatalytic poisons or activators, activation or deactivation procedures, and the eventual potential of the working electrode with respect to the solution. Because the transducer has so many variables, it is veryflexible,and can be designed to meet potential problems. However this flexibility means that much work is necessary to ensure that a particular device is the optimum for a particular application. So far, metallised membrane electrode transducers are available commercially only for oxygen. Quite apart from commercial considerations, the problems that arise in the conversion of a prototype device into one that can be mass produced are very difficult to predict. Unless the manufacture of an instrument is commissioned by a potential user, the format of the device will depend on the manufacturer. The cost of the instrument is entirely in the hands of the manufacturer. The gas-monitoring instruments currently on the market suggest that electrochemical devices have advantages over, for instance, i.r. devices, not only in size and power consumption, but also in cost.
B
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D
\
K
(Id) FIG. l(d) Photograph of chromatographic system for carbon monoxide and hydrogen in alveolar air. A—carbon monoxide scrubber tube; B—air-drying tube; C—air pump; D—air-bleed flow control; E—sample drying tube; F—sample loop; G—ganged pneumatic valves for sample injection; H—chromatographic column; I—flowmeter; J—metallised membrane electrode detector; K—bubbler for checking sample injection.
(facing page 58)
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Acknowledgements—I should like to acknowledge with thanks the devoted work of my colleagues in the Chemistry Section, of Mr R. Wilson and Mr A. Gladwin of the Special Instruments Section, and the skill of the workshops of SMRE. I should also like to acknowledge with thanks the collaboration of Dr. N. Wald of the University of Oxford, and of Mr M. Crook and his colleagues of the Yorkshire Regional Laboratory of the National Coal Board. This paper is contributed by permission of the Director of Research and Laboratory Services and Head of SMRE, Health and Safety Executive.
GRUT, A., ASTRUP, P., CHALLEN, P. J. R., and GERHARDSSON, G. (1970) Archs emir. Hlth 21, 542-544.
PETERSON, J. E. (1970) Archs emir. Hlth 21, 172-173. WALD, N., HOWARD, S., SMITH, P. G. and KJELDSEN, K. (1973) Br. med. J. 1, 761-765.
APPENDIX Reference electrodes and the amperostat system In a polarographic cell that is to be used as a continuous monitor of carbon monoxide, the potential of the working electrode with respect to the solution must be maintained at such a value that the current is small in the absence of carbon monoxide, and proportional to the partial pressure of carbon monoxide in its presence. In a normal three-electrode polarographic cell, the potential of the working electrode is maintained constant with respect to a reference electrode, while the current is carried by an auxiliary electrode. In metallised-membrane-electrode cells for oxygen monitors, the functions of reference and auxiliary electrode are combined in the form of a silver-wire anode dipping into a chloride solution. However, this electrode is unsuitable for carbon monoxide cells using a platinum working electrode, as both silver and chloride ions tend to poison such an electrode. Mercury/mercurous sulphate electrodes have been used as reference electrodes in carbon monoxide cells, but are somewhat less convenient than silver wire electrodes. The amperostat system (BERGMAN, 1971) used for most of the carbon monoxide monitors built at SMRE uses no reference electrodes; the auxiliary electrode needs mainly to resist corrosion and stay conductive. The main reason for the development of the amperostat system, however, was to minimise the variations in background current. Most of the applications for carbon monoxide monitoring require measurements in the ppm range, where the background current, that is to say the current in the absence of carbon monoxide, is an important factor. One possibility of minimising variations in background current is to use a differential system with two identical electrodes. This system can be used in laboratory analysis where the parameters of the two electrodes can be balanced before each analysis. It is not usually feasible in instruments designed to be used for long periods in the field. The 'amperostat' system (BERGMAN, 1971) uses two identical electrodes, one of which is exposed to carbon monoxide, but it is not just a differential system. In a differential system both electrodes are exposed to a particular set of conditions, differing only in the presence of carbon monoxide, and the two currents are measured and subtracted from one another. In the amperostat system, the compensating electrode, which is not exposed to carbon monoxide, actually determines the potential of the working electrode with respect to the solution by controlling the background current to a low value. A twin-metallised-membrane-electrode cell for use in an amperostat system is shown in Fig. 2. The auxiliary electrode is disposed symmetrically with respect to the working and compensating electrodes. The compensating electrode is covered by the gasket, while the atmosphere has access to the working electrode via a metal sinter, a porous polymer disc set into the gasket, and the PTFE membrane which has been metallised. A schematic circuit diagram of the amperostat system is shown in Fig. 3. The desired 'compensating current' (usually a few nA) is set by applying a voltage (usually a few mV) via a high resistance (usually IMii) to the inverting input of the compensating-electrode operational amplifier. As the compensating and working electrodes are both at 'virtual-common-line' (or earth) potential, the current carried by the working electrode in the absence of carbon monoxide should differ very little from that carried by the compensating electrode. Any small differences may be backed off by an iryection of current into the inverting input of the working-electrode operational amplifier.
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REFERENCES BERGMAN, I. (1970) The metallised membrane electrode. Brit. Pat. No. 1200595. BERGMAN, I. (1971) The amperostat system. Brit. Pat. No. 1385201. BERGMAN, I. and WINDLE, D. A. (1972) Ann. occup. Hyg. 15, 329-337. CLARK, J. C. Jr. (1956) Trans. Am. Soc. artif. internal Organs 41, 2.
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Sealing gasket with porous diffusion feed area
Electrolyte filler screw
Thermistor
Auxiliary ~ electrode
Dialysis membrane
Gasket clamping plate and sintered disc
FIG. 2. Twin-metallised-membrane-electrode cell for use in amperostat system. This amplifier acts as a current-voltage converter. The gain is set with a thermistor (usually of about IMil resistance at ambient temperature) which has been trimmed, usually with a series resistance, to have a temperature coefficient of resistance identical to the temperature coefficient of the current output of the working electrode per unit partial pressure of carbon monoxide. The voltage output will therefore be independent of temperature. With the amperostat system, adjustments in the potential of the working electrode with respect to the solution are made by means of the current carried by the compensating electrode. As this current is set to a more anodic value, so the potential of both metallised membrane electrodes (tied via the virtual common line of the operational amplifiers) will move to a more anodic value. Another way of moving the potential to a more anodic value is to use a more acid electrolyte. This will tend to suppress oxygen evolution and require a more anodic potential to achieve a particular value of background current. Compensating electrode
Working electrode
FIG. 3. Amperostat system. The metallised membrane electrode is based on a non-porous polymer membrane. The rate of diffusion of carbon monoxide through this membrane will be much lower than its rate of diffusion through a gas. When the electrode is used in a polarographic gas sensor, the membrane will normally act as a barrier limiting the rate of diffusion of carbon monoxide to the electrode. If the electrocatalytic activity of the electrode is sufficiently high, the current output of the cell will be determined by this rate of diffusion. If the membrane is physically and chemically stable, the cell output per unit partial pressure of carbon monoxide will also be stable. For most of our work we have used membranes made of PTFE because of its chemical inertness, but also because the temperature coefficient of permeability isrelativelystable, and lies between 2 and 3 percent/°C. This value gives cell output currents which are easily compensated for temperature variations over several decades of °C by means of commercially-available thermistors.
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Dialysis membrane recess with electrolyte
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With metallised membrane electrodes of platinum spluttered on to PTFE and used in dilute aqueous electrolyte solutions at temperatures up to 35°C the membrane appears to be diffusion limiting for carbon monoxide oxidation down to a membrane thickness of 12 tun. After the cell has settled down in an amperostat circuit for several days, the response to carbon monoxide is stable for periods of several months at least. When thinner membranes are used, the response is initially higher, but falls roughly exponentially over a period of months; it may then settle down. The activation procedure described below can increase the carbon monoxide response, sometimes to its original value. This means, however that accidental variations in the voltages or currents applied to the cell can lead to sudden changes in carbon monoxide sensitivity. This is much less likely to occur with membranes of thickness 12 (im and above.
Activation of electrocatalytic activity The amperostat system has another advantage with sensors for active species that require electrodes with a high electrocatalytic activity, for instance for carbon monoxide. Electrocatalysis, like most catalysis, is still relatively imperfectly understood, and many individual and only vaguely recorded pre-treatment procedures have been used by workers in the field to 'activate' their electrodes. In basic research it is not unusual for an activation step to precede each test. Sometimes activation procedures are relatively fierce, for instance brief exposure to a hydrogen/oxygen flame. The more usual procedure is to swing the electrode voltage to a value at least several hundred mV more anodic than the working value, then a value cathodic by a similar amount, and so on in a manner, usually arbitrary, which has been found to be useful in the particular laboratory with the particular electrodes for the particular procedure. Often large currents flow during the procedure, and in some cases may result in deactivation rather than activation. In any case the large currents are likely to mean that the cell takes a long time to 'settle down'. There are some indications that electrocatalytic sites may be 'burned out' by large currents. The amperostat system permits electrodes to be activated in a controlled way, with little risk of unduly high currents. The voltage on the compensating and working electrodes is at the 'virtual common line' potential of the operational amplifiers. This potential is set with respect to the solution by the choice of the current at which the compensating electrode is run. The activation procedure is also achieved via the compensating current. The present design of cell used with the amperostat system has two electrodes, each a disc of 10 mm dia. The compensating currents used for activation have been in the range of 0-5-2-5 (iA. If a sputtered platinum electrode, freshly made up or after some use, gives a response to carbon monoxide which is small and slow, the compensating current is set to say 0-5 (iA cathodic, with the working electrode exposed to nitrogen (if the working electrode were left exposed to air, large currents owing to oxygen reduction would result). A 'back-off' current is applied to the current-amplifier (the second stage in the amperostat system), to reduce the output as much as possible, and the gas fed to the working electrode is changed from pure nitrogen to nitrogen containing carbon monoxide (500 ppm) and back again once the response to carbon monoxide has been measured. Because the compensating and working electrodes are not exposed to exactly the same environment, the system output may drift for some time, and it may be desirable to back-off this output before each carbon monoxide response test. This can be done automatically by means of a combined timeT, solenoid gas valve, and auto-zero system developed at SMRE. The response to carbon monoxide may increase with the first cathodic treatment; in general it will tend to slow down rather than become more rapid. When the compensating current is then switched to anodic, the response will tend to become more rapid. Eventually the response will tend to decrease
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Working-electrode potential With an electrocatalytic system such as that for the oxidation of carbon monoxide, the electrode potential has to be chosen not only to have a minimal effect on output current, but also to lie in a potential region in which the electrocatalytic activity of the electrode is maintained at a level necessary to cope with the molecules of carbon monoxide diffusing through the particular polymer membrane used as diffusion barrier. It is normally undesirable that a device designed to monitor a pollutant should be too sensitive to oxygen deficiency (although such deficiency will mimic the presence of an oxidisable material such as carbon monoxide). The compensating current of the amperostat system for carbon monoxide is normally adjusted to remove all but a trace of oxygen sensitivity. This sets the working electrode potential to a value at which the background current has a negligible temperature coefficient near ambient temperature or one that can easily be compensated, and at which the electrocatalytic activity of the platinum electrodes is maintained at a reasonably high level.
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in size because of the deactivation of the working electrode. When the current is then switched to cathodic again, the size of response will increase, probably beyond that obtained during the previous cathodic part of the cycle. The speed of response may not slow down for some time. Of course if the cathodic treatment is carried out for too long, or at too high a current, the response will also decrease in size. This will be not because of any deactivation, but because the working electrode potential has moved to a value which is too cathodic to allow efficient carbon monoxide oxidation. After the first or second cycles of current treatment it is probably a good idea to return to zero or a low compensating current between the cathodic and anodic parts of the cycle, to check what the effect of the treatment has been on the normal operating mode of the cell. Whether the cathodic or anodic part of the cycle is last in the activating procedure will depend on how well the cell maintains the amplitude of response during the anodic part and the speed of response during or after the cathodic part. When the electrode metal is platinum alone, as in most of our recent work, the activation procedure will probably involve the oxidation or reduction of impurities, as well as the achievement of a surface in which an optimal arrangement of platinum and oxidised platinum sites exists.
ELECTROCHEMICAL CARBON MONOXIDE SENSORS BASED ON THE METALLISED MEMBRANE ELECTRODE* I. BERGMAN
Safety in Mines Research Establishment, Sheffield S3 7HQ
people are exposed to potentially harmful concentrations of carbon monoxide it is often useful to know what the variation in concentration is with time and in space. The primary consideration may be to protect a number of men with individual alarms. A variety of instruments can be useful: recorders, alarms and meters, some preferably compact enough to be carried in a pocket. A number of possible means of detecting and measuring carbon monoxide have been utilised in commercial instruments; these range from chemical detector tubes utilising the reaction between carbon monoxide and periodic acid to sophisticated infrared spectrophotometers. This paper describes the development at SMRE of an electrochemical system, the essential feature of which involves the oxidation of carbon monoxide at a metallised membrane electrode. The "metallised membrane electrode" (BERGMAN, 1970), is made by evaporating or sputtering metal on to a non-porous, but gas-permeable, polymer membrane. In the "amperostat" system (BERGMAN, 1971), an electrochemical cell with two such electrodes and an auxiliary electrode in a control circuit is used to minimise the effect of temperature on the background current. It also allows the electrocatalytic activation of carbon monoxide sensors to be controlled, and the activity to be maintained. WHERE
CURRENT-GENERATING ELECTROCHEMICAL MONITORS Setting the potential of an electrode to a particular value with respect to an electrolyte solution may be thought of as giving the electrons available at the electrode a particular energy. In a current-generating electrochemical monitor, this energy is chosen to be sufficient for electron transfer to take place efficiently to or from the species to be monitored, but not to the electrolyte solution. In a "polarographic" • Crown Copyright 1975. 53
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Abstract—The 'metallised membrane electrode' is made by evaporating or sputtering metal on to a non-porous but gas-permeable polymer membrane. The 'amperostat' system consists of an electrochemical cell with two such electrodes and an auxiliary electrode in a control circuit. The system minimise the effect of temperature on the background current. It also allows the electrocatalytic activation of carbon monoxide sensors to be controlled, and the activity to be maintained. Such sensors, made at SMRE or by collaborators, have given useful results in the field and in the laboratory over a number of years. Preliminary results suggest that, with ancillary equipment, they can make a significant contribution to the measurement of carbon monoxide at concentrations of 10 ppm upwards, and in a wide variety of gases: internal combustion exhaust gases, flue gases, cigarette smoke, exhaled air, polluted air, and air from environments that are subject to spontaneous combustion.
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monitor the rate of arrival of the active species at the electrode, and hence the output current, depends on the permeability of a diffusion-limiting layer.
Electrocatalytic activity Since carbon monoxide is not easily oxidised (it is not a good reducing agent at room temperature), an electrode at which it is to be oxidised needs to have some electrocatalytic activity for the process in question. The electrode must have the appropriate degree of affinity for carbon monoxide and its reaction products. The carbon monoxide must normally be adsorbed on to the electrode surface strongly enough to be converted into an activated form, that will give up an electron to the electrode. Once this electron transfer, and perhaps other reactions with the solvent have taken place, the reaction product must be desorbed, to allow further molecules of carbon monoxide to adsorb on to the electrode; otherwise the reaction is selfinhibiting. Further technical details concerning the metallised membrane electrodes and the amperostat system may be found in the Appendix. APPLICATIONS Carbon monoxide sensors incorporating metallised membrane electrodes and the amperostat system have been on trial at SMRE for a number of years. Five examples of applications in which they have been found to be of value are: 1. Spontaneous combustion in coal mines: a pocket "sniffer" Spontaneous combustion occurs in some coal mines. At one time, the only warning in advance of over-heating and "sweating" of the strata was the "gob-stink" that some miners claimed to be able to detect. Carbon monoxide is produced in the spontaneous combustion of coal at a fairly early stage. In recent years the National Coal Board has been installing "tube-bundle" systems in some pits. Air from a number of selected points in the pit is pumped to the surface to a carbon monoxide
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The metallised membrane electrode A non-porous, but gas-permeable, polymer membrane is used to separate the carbon monoxide-containing air from an electrolyte solution, as in the earlier method of CLARK (1956). Consequently only those materials that can diffuse through the membrane (usually of PTFE) can gain access to the electrolyte; particulate materials have no effect. However, in contrast to previous devices such as that of Clark, in which a film of electrolyte separates the PTFE membrane from a solid electrode, the metallised membrane electrode (BERGMAN. 1970; BERGMAN and WINDLE, 1972) consists of a porous or otherwise permeable metal film evaporated or sputtered on to the inner surface of the polymer membrane. This is supported and protected by a "salt bridge", usually a dialysis membrane, which is permeated by the electrolyte solution; it acts virtually as a solid electrolyte. The disposition of the electrode and the geometry of diffusion of carbon monoxide to the electrode are kept stable more easily with the metallised membrane electrode than with earlier devices. The carbon monoxide diffuses from the atmosphere (via high-porosity sintered metal and sintered polythene discs) through the PTFE film which acts as a diffusion barrier, and immediately encounters wet electrode material in intimate contact with the whole of the diffusion barrier.
Electrochemical carbon monoxide sensors
55
2. Carbon monoxide recorder with periodic zero checks A carbon monoxide monitor to spot the upward trend in concentration that indicates that spontaneous combustion has flared up in a pit should probably cover the range 0-20 ppm, discriminate to 0-2 ppm or better, and reliably show a rise of about 1 ppm per day. The biggest problems for a metallised membrane carbon monoxide detector working in this range are noise and zero drift. However, the noise can be reduced by capacitative damping, and the zero drift can be compensated for by periodic exposure of the cell to air from which the carbon monoxide has been removed. Indeed the latter function has been carried out in preliminary trials by a pump of power consumption less than 10 mW. This makes even a portable recorder to cover the range feasible. 3. Industrial hygiene: meters and alarms for instantaneous or integrated exposures Another application of carbon monoxide monitors is in industrial hygiene; to prevent workers suffering harmful exposures. The limit currently recommended by the Department of Employment for industrial exposure to carbon monoxide is an average of 50 ppm for a week of five days (seven or eight hours per day). This value is derived from the list issued by the ACGIH (American Conference of Governmental Industrial Hygienists) in which a "threshold limit value" of 50 ppm is associated with a "peak" limit of 75 ppm. This latter value is probably inappropriate for carbon monoxide, which is thought to act almost exclusively on the haemoglobin of the blood in a relatively cumulative manner, so that peak concentrations are less important than with corrosive materials, for instance. The United States National Institute for
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monitor, usually based on a non-dispersive i.r. transducer. If an upward trend in carbon monoxide concentration suggests that spontaneous combustion has started in a district, the problem arises of localising the combustion. What is needed is a carbon monoxide meter that is compact enough to be easily carried, and that responds quickly enough for a source of carbon monoxide in the strata to be located. It was originally suggested that the meter should be calibrated for a range of concentrations of 0-500 ppm in order to avoid large swings of the meter being caused by the carbon monoxide from diesel engine exhaust gases or shot-firing fumes. However, these other sources of carbon monoxide do not occur in all pits, and in some circumstances a more sensitive scale may be useful. A variety of prototype carbon monoxide "combustion sniffers" based on the metallised membrane electrode have been constructed over a period of years, and have received extensive laboratory trials and some trials underground. One form is of dual range: normally 0-100 ppm, but with a push-button to give a range of 0-500 ppm. The metallised membrane electrode transducer does not react to methane, which can be encountered in coal mines in concentrations of 1 % or more. The sensitivity to ethylene is normally about 20 % of that to carbon monoxide. The sensitivity to hydrogen can be reduced to this magnitude by choice of an anodic potential for the electrode. Ethylene and hydrogen will usually be encountered in significant concentrations in coal mines, emanating from spontaneous combustion sites that are already giving off even larger concentrations of carbon monoxide, although some coal-mine methane does contain hydrogen.
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4. Carbon monoxide in alveolar air Wald and his co-workers (WALD et al. 1973; WALD and HOWARD, this issue pp. l-14)have shown that smokers whose blood haemoglobin is combined with carbon monoxide to the extent of 5 % (5 % COHb) or more, are 21 times as likely to be affected by atherosclerotic diseases, including ischaemic heart disease, as similar smokers with a COHb level of less than 3 %. GRUT et al. (1970) in their support for a threshold limit value of 50 ppm for carbon monoxide, relate this value to a COHb level of 8-10 % after several hours of exposure. PETERSON (1970) found that alveolar air analysis could be used to estimate the post-exposure COHb saturation accurately. He found that 3 % COHb corresponded to about 15 ppm carbon monoxide in alveolar air, 5 % to about 25 ppm, 8 % to about 35 ppm, and 10% to about 50 ppm. During some early attempts to determine carbon monoxide in breath, we discovered that humans exhale some hydrogen. This is a phenomenon that appears to be familiar only to gastroenterologists, and has some clinical significance in malabsorption in the gut. The experience in gas chromatography of Mr G. Vizard and his colleagues at the National Coal Board Yorkshire Regional Laboratory was of great assistance in the development of an apparatus for the determination of both hydrogen and carbon monoxide in breath. A column of an activated molecular sieve was used at ambient temperature and with air as carrier gas to separate carbon monoxide and hydrogen; a metallised membrane electrode was the detector. Figures l(a) and l(b) show the apparatus for sampling alveolar air, suggested by Wald. Figures l(b) and l(c) show schematic diagrams of the chromatographic system. A small pump provides the air flow through the column. A ganged pair of pneumatic valves serve to inject the sample into the gas flow. The signal peak height, accurately linear with concentration for both hydrogen and carbon monoxide, may be measured with a meter or with a recorder. Figure l(d) is a photograph of the chromatographic system; the commercially-available components are extremely inexpensive.
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Occupational Safety and Health (NIOSH) has recently promulgated limits of 35 ppm mean and 200 ppm peak exposure concentrations for carbon monoxide. Audible and flashing light alarms have beenfittedto a variety of oxygen deficiency monitors based on the metallised membrane electrode. Preliminary studies have also been carried out on peak concentration alarms set at 200 ppm or even 100 ppm carbon monoxide. Their accuracy and reproducibility depend on how extreme are the variations in temperature, and how frequently the zero reading can be checked, e.g. by sucking air through a small carbon monoxide-scrubber tube. Silver microcoulometers are now available for long-term integration of currents. The output of the metallised membrane electrode sensor is linear with partial pressure over a wide range, and can be integrated in such a device. Thus the integrated exposure of a man to carbon monoxide over a shift could be measured. An integratedexposure alarm has been suggested by Mr R. Wilson of SMRE. An amount of silver equivalent to the desired maximum exposure is plated on to the coulometer electrode; the output of the carbon monoxide cell is then used to dissolve the silver again. When all the silver has been dissolved, the alarm is activated.
Electrochemical carbon monoxide sensors
(c)
57
Air pump-
Ganged pneumatic l ^
Sample loop Chromatographic ~ j l column
-Flowmeter
5. High range meters Metallised membrane electrode sensors may prove useful for monitoring carbon monoxide in the concentration range 0-1-10% in, for instance, cigarette smoke or petrol engine exhaust gases. If a metallised membrane electrode carbon monoxide sensor is exposed to pure carbon monoxide for more than a few seconds, the cell current rises to a peak and then falls ; presumably carbon monoxide or an oxidation product tends to inhibit oxidation. There is also sometimes an effect on the background current of the cell. Both these effects disappear some time after the exposure of the cell to the high concentration of carbon monoxide has ceased. The linearity of cell current with continuous exposures to carbon monoxide has an upper limit dependent on the thickness of the PTFE membrane. With a 3 (xm thick membrane, the limit is somewhere between 0 05 % and 0 1 % ; with 25 (xm thick membranes it is between 0-4% and 1 per cent. However, linearity can be achieved, even up to one atmosphere of carbon monoxide, by means of sample injection techniques such as are used in gas chromatography. The equipment may consist of simple and cheap pneumatic valves such as those shown in Fig. 1. Indeed the gas chromatograph shown in Fig. 1 may be used for high concentrations merely by using a small injection loop. If carbon monoxide is the only component of the gas to be monitored that is oxidisable or reducible at the metallised membrane electrode and is present in significant concentrations, the chromatographic column may be unnecessary. Alternatively, a chemical scrubbing layer may be used to remove unwanted components from the gas mixture, probably before it is injected into the air stream that is fed to the monitor.
ADVANTAGES AND LIMITATIONS OF METALLISED MEMBRANE ELECTRODE CELLS Electrochemical transducers such as cells using the metallised membrane electrode have a number of advantages over other devices; the predominant one is low power
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FIG. 1. Simple chromatographic system for carbon monoxide and hydrogen in alveolar air using the metallised membrane electrode detector, (a) Carbon monoxide sample collection; (b) Flushing-out sample loop (c) Injecting sample into chromatographic flow.
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CONCLUSIONS
There are a variety of situations in which individuals may be exposed to potentially harmful concentrations of carbon monoxide. In some circumstances, protection (or surveillance) may best be provided by continuous monitoring of their environment by a fixed recording instrument; in others individual alarms and/or meters, compact enough to be carried in the pocket may be desirable. As a general method of detecting and measuring carbon monoxide, electrochemical sensors have a number of advantages, predominant among which is low power consumption. Such sensors, made at SMRE or by collaborators, have given useful results in the field and in the laboratory over a number of years. Preliminary results suggest that, with ancillary equipment, they can make a significant contribution to the measurement of carbon monoxide at concentrations of 10 ppm upwards, and in a wide variety of gases: internal combustion exhaust gases, flue gases, cigarette smoke, exhaled air, polluted air, and air from environments that are subject to spontaneous combustion.
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consumption and, consequently, that instruments should need infrequent servicing. For example, a simple pocket carbon monoxide meter with an amperostat control system and a current-voltage converter will run continuously for six to twelve months on four hearing-aid batteries. Moreover, because of the low power requirements, "intrinsic safety" is easily achieved for instruments to be used in coal mines and in other places where explosive gases such as methane may be encountered. Furthermore, the cells are robust, yet can be made very compact and lightweight. They are current generators; the output of a cell will normally bear a linear relationship with the partial pressure of carbon monoxide over several orders of magnitude. The only part of the cell that is consumed during its life is the electrolyte solvent; normally water. If the electrocatalytic activity of the cell falls significantly after its specified life, this activity can be regenerated by means of the cyclic activation procedure used when a cell is brought into operation (see Appendix). For a single design of cell, variations may be made in the thickness and material of the non-porous but gas-permeable polymer membrane, the composition of the metal layer, the solvent, the acidity and ionic strength of the electrolyte, electrocatalytic poisons or activators, activation or deactivation procedures, and the eventual potential of the working electrode with respect to the solution. Because the transducer has so many variables, it is veryflexible,and can be designed to meet potential problems. However this flexibility means that much work is necessary to ensure that a particular device is the optimum for a particular application. So far, metallised membrane electrode transducers are available commercially only for oxygen. Quite apart from commercial considerations, the problems that arise in the conversion of a prototype device into one that can be mass produced are very difficult to predict. Unless the manufacture of an instrument is commissioned by a potential user, the format of the device will depend on the manufacturer. The cost of the instrument is entirely in the hands of the manufacturer. The gas-monitoring instruments currently on the market suggest that electrochemical devices have advantages over, for instance, i.r. devices, not only in size and power consumption, but also in cost.
B
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D
\
K
(Id) FIG. l(d) Photograph of chromatographic system for carbon monoxide and hydrogen in alveolar air. A—carbon monoxide scrubber tube; B—air-drying tube; C—air pump; D—air-bleed flow control; E—sample drying tube; F—sample loop; G—ganged pneumatic valves for sample injection; H—chromatographic column; I—flowmeter; J—metallised membrane electrode detector; K—bubbler for checking sample injection.
(facing page 58)
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Acknowledgements—I should like to acknowledge with thanks the devoted work of my colleagues in the Chemistry Section, of Mr R. Wilson and Mr A. Gladwin of the Special Instruments Section, and the skill of the workshops of SMRE. I should also like to acknowledge with thanks the collaboration of Dr. N. Wald of the University of Oxford, and of Mr M. Crook and his colleagues of the Yorkshire Regional Laboratory of the National Coal Board. This paper is contributed by permission of the Director of Research and Laboratory Services and Head of SMRE, Health and Safety Executive.
GRUT, A., ASTRUP, P., CHALLEN, P. J. R., and GERHARDSSON, G. (1970) Archs emir. Hlth 21, 542-544.
PETERSON, J. E. (1970) Archs emir. Hlth 21, 172-173. WALD, N., HOWARD, S., SMITH, P. G. and KJELDSEN, K. (1973) Br. med. J. 1, 761-765.
APPENDIX Reference electrodes and the amperostat system In a polarographic cell that is to be used as a continuous monitor of carbon monoxide, the potential of the working electrode with respect to the solution must be maintained at such a value that the current is small in the absence of carbon monoxide, and proportional to the partial pressure of carbon monoxide in its presence. In a normal three-electrode polarographic cell, the potential of the working electrode is maintained constant with respect to a reference electrode, while the current is carried by an auxiliary electrode. In metallised-membrane-electrode cells for oxygen monitors, the functions of reference and auxiliary electrode are combined in the form of a silver-wire anode dipping into a chloride solution. However, this electrode is unsuitable for carbon monoxide cells using a platinum working electrode, as both silver and chloride ions tend to poison such an electrode. Mercury/mercurous sulphate electrodes have been used as reference electrodes in carbon monoxide cells, but are somewhat less convenient than silver wire electrodes. The amperostat system (BERGMAN, 1971) used for most of the carbon monoxide monitors built at SMRE uses no reference electrodes; the auxiliary electrode needs mainly to resist corrosion and stay conductive. The main reason for the development of the amperostat system, however, was to minimise the variations in background current. Most of the applications for carbon monoxide monitoring require measurements in the ppm range, where the background current, that is to say the current in the absence of carbon monoxide, is an important factor. One possibility of minimising variations in background current is to use a differential system with two identical electrodes. This system can be used in laboratory analysis where the parameters of the two electrodes can be balanced before each analysis. It is not usually feasible in instruments designed to be used for long periods in the field. The 'amperostat' system (BERGMAN, 1971) uses two identical electrodes, one of which is exposed to carbon monoxide, but it is not just a differential system. In a differential system both electrodes are exposed to a particular set of conditions, differing only in the presence of carbon monoxide, and the two currents are measured and subtracted from one another. In the amperostat system, the compensating electrode, which is not exposed to carbon monoxide, actually determines the potential of the working electrode with respect to the solution by controlling the background current to a low value. A twin-metallised-membrane-electrode cell for use in an amperostat system is shown in Fig. 2. The auxiliary electrode is disposed symmetrically with respect to the working and compensating electrodes. The compensating electrode is covered by the gasket, while the atmosphere has access to the working electrode via a metal sinter, a porous polymer disc set into the gasket, and the PTFE membrane which has been metallised. A schematic circuit diagram of the amperostat system is shown in Fig. 3. The desired 'compensating current' (usually a few nA) is set by applying a voltage (usually a few mV) via a high resistance (usually IMii) to the inverting input of the compensating-electrode operational amplifier. As the compensating and working electrodes are both at 'virtual-common-line' (or earth) potential, the current carried by the working electrode in the absence of carbon monoxide should differ very little from that carried by the compensating electrode. Any small differences may be backed off by an iryection of current into the inverting input of the working-electrode operational amplifier.
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REFERENCES BERGMAN, I. (1970) The metallised membrane electrode. Brit. Pat. No. 1200595. BERGMAN, I. (1971) The amperostat system. Brit. Pat. No. 1385201. BERGMAN, I. and WINDLE, D. A. (1972) Ann. occup. Hyg. 15, 329-337. CLARK, J. C. Jr. (1956) Trans. Am. Soc. artif. internal Organs 41, 2.
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Sealing gasket with porous diffusion feed area
Electrolyte filler screw
Thermistor
Auxiliary ~ electrode
Dialysis membrane
Gasket clamping plate and sintered disc
FIG. 2. Twin-metallised-membrane-electrode cell for use in amperostat system. This amplifier acts as a current-voltage converter. The gain is set with a thermistor (usually of about IMil resistance at ambient temperature) which has been trimmed, usually with a series resistance, to have a temperature coefficient of resistance identical to the temperature coefficient of the current output of the working electrode per unit partial pressure of carbon monoxide. The voltage output will therefore be independent of temperature. With the amperostat system, adjustments in the potential of the working electrode with respect to the solution are made by means of the current carried by the compensating electrode. As this current is set to a more anodic value, so the potential of both metallised membrane electrodes (tied via the virtual common line of the operational amplifiers) will move to a more anodic value. Another way of moving the potential to a more anodic value is to use a more acid electrolyte. This will tend to suppress oxygen evolution and require a more anodic potential to achieve a particular value of background current. Compensating electrode
Working electrode
FIG. 3. Amperostat system. The metallised membrane electrode is based on a non-porous polymer membrane. The rate of diffusion of carbon monoxide through this membrane will be much lower than its rate of diffusion through a gas. When the electrode is used in a polarographic gas sensor, the membrane will normally act as a barrier limiting the rate of diffusion of carbon monoxide to the electrode. If the electrocatalytic activity of the electrode is sufficiently high, the current output of the cell will be determined by this rate of diffusion. If the membrane is physically and chemically stable, the cell output per unit partial pressure of carbon monoxide will also be stable. For most of our work we have used membranes made of PTFE because of its chemical inertness, but also because the temperature coefficient of permeability isrelativelystable, and lies between 2 and 3 percent/°C. This value gives cell output currents which are easily compensated for temperature variations over several decades of °C by means of commercially-available thermistors.
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Dialysis membrane recess with electrolyte
Electrochemical carbon monoxide sensors
61
With metallised membrane electrodes of platinum spluttered on to PTFE and used in dilute aqueous electrolyte solutions at temperatures up to 35°C the membrane appears to be diffusion limiting for carbon monoxide oxidation down to a membrane thickness of 12 tun. After the cell has settled down in an amperostat circuit for several days, the response to carbon monoxide is stable for periods of several months at least. When thinner membranes are used, the response is initially higher, but falls roughly exponentially over a period of months; it may then settle down. The activation procedure described below can increase the carbon monoxide response, sometimes to its original value. This means, however that accidental variations in the voltages or currents applied to the cell can lead to sudden changes in carbon monoxide sensitivity. This is much less likely to occur with membranes of thickness 12 (im and above.
Activation of electrocatalytic activity The amperostat system has another advantage with sensors for active species that require electrodes with a high electrocatalytic activity, for instance for carbon monoxide. Electrocatalysis, like most catalysis, is still relatively imperfectly understood, and many individual and only vaguely recorded pre-treatment procedures have been used by workers in the field to 'activate' their electrodes. In basic research it is not unusual for an activation step to precede each test. Sometimes activation procedures are relatively fierce, for instance brief exposure to a hydrogen/oxygen flame. The more usual procedure is to swing the electrode voltage to a value at least several hundred mV more anodic than the working value, then a value cathodic by a similar amount, and so on in a manner, usually arbitrary, which has been found to be useful in the particular laboratory with the particular electrodes for the particular procedure. Often large currents flow during the procedure, and in some cases may result in deactivation rather than activation. In any case the large currents are likely to mean that the cell takes a long time to 'settle down'. There are some indications that electrocatalytic sites may be 'burned out' by large currents. The amperostat system permits electrodes to be activated in a controlled way, with little risk of unduly high currents. The voltage on the compensating and working electrodes is at the 'virtual common line' potential of the operational amplifiers. This potential is set with respect to the solution by the choice of the current at which the compensating electrode is run. The activation procedure is also achieved via the compensating current. The present design of cell used with the amperostat system has two electrodes, each a disc of 10 mm dia. The compensating currents used for activation have been in the range of 0-5-2-5 (iA. If a sputtered platinum electrode, freshly made up or after some use, gives a response to carbon monoxide which is small and slow, the compensating current is set to say 0-5 (iA cathodic, with the working electrode exposed to nitrogen (if the working electrode were left exposed to air, large currents owing to oxygen reduction would result). A 'back-off' current is applied to the current-amplifier (the second stage in the amperostat system), to reduce the output as much as possible, and the gas fed to the working electrode is changed from pure nitrogen to nitrogen containing carbon monoxide (500 ppm) and back again once the response to carbon monoxide has been measured. Because the compensating and working electrodes are not exposed to exactly the same environment, the system output may drift for some time, and it may be desirable to back-off this output before each carbon monoxide response test. This can be done automatically by means of a combined timeT, solenoid gas valve, and auto-zero system developed at SMRE. The response to carbon monoxide may increase with the first cathodic treatment; in general it will tend to slow down rather than become more rapid. When the compensating current is then switched to anodic, the response will tend to become more rapid. Eventually the response will tend to decrease
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Working-electrode potential With an electrocatalytic system such as that for the oxidation of carbon monoxide, the electrode potential has to be chosen not only to have a minimal effect on output current, but also to lie in a potential region in which the electrocatalytic activity of the electrode is maintained at a level necessary to cope with the molecules of carbon monoxide diffusing through the particular polymer membrane used as diffusion barrier. It is normally undesirable that a device designed to monitor a pollutant should be too sensitive to oxygen deficiency (although such deficiency will mimic the presence of an oxidisable material such as carbon monoxide). The compensating current of the amperostat system for carbon monoxide is normally adjusted to remove all but a trace of oxygen sensitivity. This sets the working electrode potential to a value at which the background current has a negligible temperature coefficient near ambient temperature or one that can easily be compensated, and at which the electrocatalytic activity of the platinum electrodes is maintained at a reasonably high level.
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in size because of the deactivation of the working electrode. When the current is then switched to cathodic again, the size of response will increase, probably beyond that obtained during the previous cathodic part of the cycle. The speed of response may not slow down for some time. Of course if the cathodic treatment is carried out for too long, or at too high a current, the response will also decrease in size. This will be not because of any deactivation, but because the working electrode potential has moved to a value which is too cathodic to allow efficient carbon monoxide oxidation. After the first or second cycles of current treatment it is probably a good idea to return to zero or a low compensating current between the cathodic and anodic parts of the cycle, to check what the effect of the treatment has been on the normal operating mode of the cell. Whether the cathodic or anodic part of the cycle is last in the activating procedure will depend on how well the cell maintains the amplitude of response during the anodic part and the speed of response during or after the cathodic part. When the electrode metal is platinum alone, as in most of our recent work, the activation procedure will probably involve the oxidation or reduction of impurities, as well as the achievement of a surface in which an optimal arrangement of platinum and oxidised platinum sites exists.
