Solid State Sensor for Anhydrides

The present invention relates to a sensing device for detecting and measuring the activity of a gas by means of a solid state sensing element. In particular, the invention concerns electrochemical means useful in detecting an anhydride or gases containing anhydrides or related compounds in air or in oxygen bearing gas.

The invention is remarkable in that it permits a quantitative determination of the activity of anhydrides and related compounds which pollute surrounding air, such detection being realized by means of a solid state sensor supplying an oxy-anion of the corresponding anhydride.

Hitherto, research directed to the atmospheric pollution measurement has been oriented so as to replace so-called first generation monitors which mainly use liquid scrubbers to sample the air.

Among the atmospheric pollutants, sulfur bearing compounds and nitrogen oxydes are considered as the most harmful agents to people and among the most agressive to materials. Because of the wide concentration range that usually exists between industrial stack gases and ambient air, it is rather difficult to build a sensor able to directly monitor the ambient air and other polluting sources of high concentration. A SO.sub.2 activity measuring device based on a concentration cell principle is described in U.S. Pat. No. 3,718,546 issued on Feb. 27 1973 to Salzano et al. This device uses fused salts as electrolytes, thus cumbersome and rather difficult to transport. In addition, a great stability is required in the flow of gases to obtain realistic measurements.

A prime purpose of the present invention consists in providing an improved means for the detection of anhydrides or related compounds mixed with an oxygenated gas.

In particular, the present invention is useful for quantitavely determining extremely small and high concentrations of sulfur bearing compounds in gas phases by means of an element containing oxy-anions of the anhydride to be detected.

Another object of the invention is to provide a solid state detecting device the operation of which is independent of the flow of the gas used as a reference source or of the gas the concentration of which is to be measured.

A further object of the present invention resides in providing various types of solid state electrodes as reference sources thereby eliminating the inconveniences inherent to the use of gas as the reference sources, which substantially increases the miniaturization of the sensing device of the invention and thus decreases its cumbersomeness.

An additional object of the invention consists in providing a detecting device which may be mounted in a relatively small, compact and portable apparatus readily usable on a site where particular gases or vapors bearing anhydrides or related compounds have to be analyzed.

Another object of the invention is to provide a detector for anhydrides which is easy to build, has a relatively long useful life and a good calibration stability.

A further object of the invention consists in providing a detecting device able to detect without interference anhydride bearing compounds mixed with other vapors or gases.

A further object of the invention resides in an improved detector wherein the electro-motive force or potential difference detected signal represents the logarithm of the concentration of the anhydride and is substantially linear.

An additional object of the invention consists in providing appropriate means to compensate oxygen partial pressure variations in particular gases like stack gases.

Another object of the invention is to provide particular arrangements of inlet and outlet conduits leading the gas in contact with the detecting element in order to minimize any dead volume of gas therein and to optimize the response time of the detector. These preferred arrangements greatly enhance miniaturisation of the detector and its operating modes.

The above-mentioned objects of the present invention are actually achieved through a sensor which comprises a solid state electrolyte element containing oxy-anions of the anhydride to be detected, this solid state element being provided with electrodes to which potentiometric measuring means are connected. This detecting element is made up of one or more juxtaposed chemical compounds. Moreover, heating means are provided to increase the sensing capacity of the electrolytic element, the heating of the latter being however at a temperature such that a logarithmic variation of the activity of the anhydride to be detected will result in a proportional and substantially linear variation of the difference of potential detected at the electrodes. In any case, the heating temperature will be below the fusion temperature of the electrolyte elements.

The above and other objects will become apparent through the following description of preferred embodiments given with reference to the accompanying drawings, wherein

FIG. 1 schematically illustrates an embodiment of the present invention wherein a solid state electrolyte element is flanked with porous detection and reference electrodes, a standard anhydride-containing gas being used as a reference;

FIG. 2 shows a sensor in accordance with the present invention using the vapor pressure resulting of the thermo-decomposition of a metal salt of the anhydride to be detected, in order to fix the thermodynamical partial pressure of a reference anhydride.

FIG. 3 shows another sensor in accordance with a further embodiment of the present invention using a solid electrode as a reference.

FIG. 4 shows another sensor in accordance with an embodiment of the present invention wherein the sensing element is formed of two juxtaposed compounds, one being an oxy-anion-bearing compound, at the anhydride measuring electrode, and the other containing an oxygen-bearing electrolyte at the reference electrode, the latter electrode generating a known potential when exposed to air or to oxygen having a given partial pressure.

FIG. 5 shows a graph relating to the potential difference experimentally obtained for various concentrations of SO.sub.2 --Air, NO.sub.2 --Air, CO.sub.2 --Air, COS--Air, PH.sub.3 --Air, AsH.sub.3 --Air and SeH.sub.2 --Air mixtures. The sensors illustrated in FIGS. 1-4 were used to compile those results.

FIG. 6 shows an arrangement in accordance with an embodiment of the present invention to compensate for the oxygen partial pressure variations. The oxygen partial pressure in the gaseous sample is measured by means of a known oxygen sensor whereas the anhydride concentration is measured by anyone of the sensors described in FIGS. 1 to 4. The emf produced by both sensors are electronically corrected and substracted in order to produce an oxygen compensated signal of the anhydride concentration in the sampled gas.

FIG. 7 shows graphs obtained from experiments carried out with the arrangement illustrated in FIG. 6.

FIG. 8 shows another arrangement to compensate oxygen partial pressure variations in a gaseous sample. Compensation is achieved by injection of a fixed amount of oxygen rich gas into the stream of a sample gas entering the detector in order to increase its oxygen partial pressure to a nearly constant level.

FIG. 9 schematically illustrates three preferred arrangements of the anhydride detectors. Gas circulation chambers are formed by a system of small diameter parallel holes, inside a quartz or alumina rod. One end of this rod is mechanically pressed against the solid electrolyte in such a way as to seal the gas chambers by thermal deformation of the solid electrolyte. A gas circulating path is achieved between the inlet and the outlet chambers by perforating the common wall of these chambers in the vicinity of the solid electrolyte electrode.

FIGS. 9a and 9b show a gas circulating system in which a measuring electrode wire runs through the outlet gas chambers.

FIG. 9c shows a double independent gas circulating system, each of which being similar to the one shown in FIG. 9a. One of the gas conduits is used as the measuring system and the other as the gas reference system in which a known anhydride concentration is maintained.

FIG. 10 shows the curve obtained from SO.sub.2 concentration measurements effected by means of K.sub.2 SO.sub.4 /ZrO.sub.2 --CaO arranged as shown in FIG. 4 and using gas circulating system of FIG. 9a. This graphic representation shows the evolution of the emf signal at different SO.sub.2 concentrations in function of the time.

FIG. 1 illustrates a first snesor. This sensor comprises a detecting element 1 constituted of an electrolyte containing oxy-anions of the gaseous anhydride to be analyzed. The element 1 is made up of an alkali metal salt or an alkaline earth metal salt.

That element 1 is preferably pellet shaped, but, of course, any other form is also quite acceptable. Each end of the element 1 is in contact with an electronically conductive porous material 2 and 3 such as silver, platinum, gold or other.

The electrolyte element 1 is tightly inserted into a tube 4 made of alumina so as to hermetically separate a measure compartment "A" from a reference compartment "B". Each end of the tube 4 is sealed with any appropriate material.

A sample "C" of the anhydride the concentration of which is to be determined is introduced into the measure compartment "A" through a conduit 5. Similarly, a corresponding anhydride "D" of known concentration is introduced into the reference compartment "B" via a conduit 6. These gas supply conduits 5 and 6 are suitably disposed axially and at the center of the alumina tube 4 so as to provide a better contact for each of the gases with the corresponding metal surface and the electrolyte. The anhydride gases are thereafter exhausted through outlet tubes 7 and 8 respectively extending from each of the compartments to the outside.

It is to be noted that the metal surfaces or electrodes 2 and 3 are at such a porosity as to permit and to carry out the triple contact between the electrolyte element 1, the gas "C" or "D" and the respective electrodes 2 and 3. In practice, however, those metal electrodes 2 and 3 need not entirely cover the corresponding electrolyte element surfaces, but are rather made up of metal dots deposited thereon. In the latter case, the metal electrodes present a non-porous structure.

Each of the metal surfaces 2 and 3 are connected to the terminals of a potentiometric measuring instrument 9, such as a voltmeter, by means of conductive wires 10 and 11. The measuring instrument 9 operates to indicate the difference of potential existing between the electro-motive forces built-up on each of the conductive surfaces 2 and 3 when in contact with the sampled gas, and the reference gas, respectively.

In order to increase the sensing capacity of the element 1, by increasing its ionic conductivity, thereby allowing occurrence of rapid electrochemical reactions at the electrodes between the gaseous anhydrides and the oxy-anions of the electrolyte, the tube 4 is introduced into an electrical oven (not shown). The heating temperature of the oven is however below the fusion point of the electrolyte element, and of such a value that a logarithmic variation in the anhydride concentration will produce a proportional and substantially linear variation in the difference of potential detected at the electrodes 2 and 3, as graphically illustrated in FIG. 5.

A modified arrangement of the embodiment shown in FIG. 1 is presented in FIG. 2 wherein a block D' of a metal salt of the anhydride to be detected in placed inside the then hermetically closed reference compartment "B". When heated, that metal salt D' evolves a metal oxyde and an anhydride identical to the one to be analyzed. For instance where CO.sub.2 is the anhydride which is fed at "C", the corresponding metal salt chosen may then be MCO.sub.3 which, when heated, will give MO+CO.sub.2, the latter defining a partial pressure which will therefore produce a fixed reference potential at the reference electrode 3. Therefore, the thermo-decomposition of a salt of the anhydride to be detected sets at the reference electrode a stable partial pressure which results in a fixed potential at that electrode, thereby allowing detection and measurement of the anhydrides to be analysed. The arrangement of FIG. 2 permits to avoid the reference gas circulating arrangement of FIG. 1. It is to be noted that by setting the metal salt block D' close to the reference electrode 3, the concentration of the reference anhydride evolved from D' remains stable, and then the compartment B does not need to be hermetically closed, but actually is open to air.

FIG. 3 illustrates a variant of the arrangement shown in FIG. 1. To the reference gas source "D" of FIG. 1 is substituted a solid state reference element. Then, the conduits 6 and 8 used for supplying and exhausting the reference gas from compartment "B" become superfluous and are eliminated. The use of a solid state reference is rendered possible owing to the use of a detecting element 1'. This element 1' is constituted through the sintering of a pure electrolyte compound 1a made of an alkali metal salt or an alkaline earth metal salt, which corresponds to the oxy-anions of the anhydride to be detected, and a second compound 1b made of the compound 1a to which a small amount of a metal salt has been added. The electrode 12 is then formed of a metal corresponding to the metal salt added by doping or vice-versa. For instance, if K.sub.2 SO.sub.4 is used as compound 1a, the compound 1b will be constituted of K.sub.2 SO.sub.4 doped with about 1% of Ag.sub.2 SO.sub.4 or of AgCl, provided the electrode 12 is made of silver. The other numeral references indicated in FIG. 2 represent the same elements as those to which they refer in FIG. 1.

Although the arrangement of FIG. 3 is preferred, outstanding and useful results were obtained by solely using an electrolyte element made up of an homogeneous compound 1b only, that is without compound 1a. The reference electrode 12 is, in this case, always made of a metal identical to the metal of the doping metal salt, whereas the detection electrode is of any other metal.

FIG. 4 illustrates a further embodiment of a sensor having a solid state reference. In this embodiment, to the oxy-anion bearing compound 1 made of an alkali metal salt or alkaline earth metal salt is juxtaposed an oxygen-ion bearing electrolyte compound 13. A stable reference potential is thus produced at the reference electrode 3 whenever this electrode is exposed to ambient air or to oxygen, provided the oxygen partial pressure in air is constant, or when used in the arrangement of FIG. 6, described below.

As mentioned previously, the sensors shown in FIGS. 1 to 4 are introduced into an electric oven (not shown) so as to increase the sensing capacity of the electrolyte element. However, the temperature of the oven should not go beyond the melting point temperature of the electrolyte, and should be such that a logarithmic variation in the activity of the anhydride to be detected will result in a proportional and substantially linear variation of the potential detected. Therefore, the operating temperature having been selected, correct and useful measurement results are obtained, even if the activity or concentration of the anhydride varies from one measurement to another.

It is to be noted that the sensors illustrated in FIGS. 1 to 4 are able to produce potential differences in a range running from a few millivolts to several hundred of millivolts when a gaseous state compound is put into contact with the detecting part thereof.

Experiments were carried out by means of the arrangements illustrated in FIGS. 1 to 4 and certain results of which have been plotted on FIG. 5, which results will be discussed in connection with specific examples given hereafter.

FIG. 6 shows an arrangement to compensate for any variations in the partial pressure of the oxygen gas in the gaseous sample C. Actually, the anhydride detector is influenced both by a variation in the partial pressure of the anhydride and by a variation in the partial pressure of the oxygen in the sample. This phenomena does not interfere in environmental measurements since the oxygen partial pressure remains constant in air, but such variations are to be taken into account in stack gas analysis, for example, and other gases where the oxygen partial pressure fluctuates. To compensate for the oxygen partial pressure variations in a stack gas C, a portion C1 of this gas is fed to an oxygen sensitive detector 13'a made up of an oxygen-ion bearing electrolyte 13a having a reference electrode 3a and a measuring electrode 2a, these two electrodes being of any electrically conductive material. Another portion C2 of the gaseous sample C is forwarded toward a second detector 13' which is identical to the one shown in FIG. 4. The reference electrodes 3 and 3a of detectors 13' and 13'a, respectively, are exposed to ambient air. In addition, the two solid state detectors may be placed into the same electrical oven 16, as shown, or in two independent ovens in order to meet the operating temperature requirements of each detector. The potentials built-up at each electrode are sent to an analyser 14 which differentiates the signals from both detectors, thereby cancelling the variation effects of oxygen in the measurement of the anhydride concentration in the sample C. The sample gas is exhausted from both detectors through conduits E.sub.1 and E.sub.2, respectively, by means of a pump 15.

Although the arrangement illustrated in FIG. 6 has been described above with reference to detector 13', similar to the one shown in FIG. 4, it should be understood that anyone of the anhydride detectors of FIGS. 1 to 3 may as well be used. The use of the detector 13' in the arrangement of FIG. 6 being given by way of example only. On the other hand, the oxygen detector 13'a may be of any known type, and the one described in U.S. Pat. No. 3,400,054 issued on Sept. 3, 1968, to Ruka et al, may for instance, be advantageously utilized.

Conclusive results have been obtained with the arrangement shown in FIG. 6, specific experimental results for SO.sub.2 and CO.sub.2 being presented on FIG. 7. An experiment carried on with CO.sub.2 is further given below in example 7.

FIG. 8 shows another arrangement suitable to compensate variations of the partial pressure of oxygen contained in a gaseous sample. Compensation is achieved by injecting a predetermined amount of an oxygen rich gas "F" into the incoming stream of a sample gas C, the flow of the oxygenated gas "F" being regulated by means of a flow-meter 17. Thus, the partial pressure of oxygen is increased to a level such that the potential difference measured by the voltmeter remains substantially unaffected by the oxygen concentration variations that may occur in the anhydride-containing sample gas, thereby enabling a true determination of the concentration of the anhydride to be detected by the detector 18, the latter being of the type described in anyone of FIGS. 1 to 4. It is therefore noted that the concentration of oxygen at the measuring electrode of detector 18 is substantially stable and proportional to the ratio F/C. A pump 19 controls the flow of the gas mixture, which flow value may be observed by means of the flow-meter 20.

Referring to FIGS. 9a, 9b and 9c, there are shown particular arrangements of the inlet and outlet conduits suitable to bring the anhydride to be detected and/or the reference gas in close contact with the corresponding electrode. Although these embodiments may appear quite simple, they have proven to be highly effective in hermetically sealing the contact points with the surface of the solid state sensors. As illustrated in FIGS. 9a and 9b, two substantially parallel channels 22 are pierced in a rod-like material 21, and thereafter the extreme portion 23 of the rod, that is the portion facing the measuring electrode 2, is cut off in order to provide a free gas flow path for sample C between the two channels. Sealing is effected by heating the electrolyte element to a temperature in the vicinity of its sintering temperature and then by pressing the extremity of the rod-shaped material provided with the opening 23 against the surface of the electrolyte element so as to slightly embedding the peripherical extremities thereof into the electrolyte element. A highly hermetical sealing is thus produced. Although in FIGS. 9a and 9b the particular conduit arrangement is used in connection with the sensors shown in FIGS. 4 and 3, respectively, it is understood that the above-described sealing method may be readily applied to any other types of solid state sensors, particularly those illustrated in FIGS. 1 and 2. In this respect, utilizing the solid state sensor of FIG. 1, an arrangement of a particular interest, being highly compact, is presented in FIG. 9c in which a plurality of substantially parallel channels 22' have been pierced through the rod-like material 21' and openings 23 provided at the rod extremity and in alignment with the respective measuring electrode 2 and reference electrode 3 to bring the sample gas C and the reference gas D in intimate contact with the corresponding electrodes. It is to be noted that with such arrangement both electrodes may be set at the same side of the solid state electrolyte element 1, thereby greatly increasing the compactness of the detector. A separating wall 24 prevents the intermixing of the gas sample and the reference gas, this separating wall being also embedded into the element 1 in accordance with the sealing method mentioned above so as to sealingly separate the sample gas channel from the reference gas channel.

The rod-like material may be of ceramic, alumina, mullite, quartz or even of an abrasion resistant alloy.

The arrangement shown in FIG. 9 offers several advantages over hitherto known gas inlet and outlet conduits. Indeed, the present arrangement allows a direct contact of the gases with the different electrodes of the solid state electrolyte element and ensures the complete sealing of the various channels with respect to the environment and to other adjacent channels. Since the diameters of the channels are relatively small, their volume are reduced to a strict minimum and the gases, after having contacted the electrode, are promptly exhausted. Thus, the system response time, always influenced by gas dead volumes, is substantially increased and the true values of the potential corresponding to the anhydride concentration in the sample gas become available for analysis at the very start of the detection proceedings. Moreover, it is to be noted that additional hermetically sealed channels may be provided for the leads connecting the various electrodes to the voltmeter 9, rather than running through the gas channels, as shown in FIG. 9c for instance.

We will hereinafter give some examples of experiments carried out by means of the arrangements illustrated in the above-described figures with particular reference to the graphs shown in FIGS. 5, 7 and 10 on which experimental results have been quoted.

A series of tests were conducted using the arrangement of FIGS. 1 and 2 to monitor the amount of SO.sub.2 in air. Various SO.sub.2 --Air mixtures were prepared and tested. In a concentration range running from 0.1 to 20,000 ppm, it was established that a linear relationship existed between the logarithm of the SO.sub.2 content of the sampled gas and the electro-motive force recorded on the voltmeter.

A series of tests were conducted using the arrangement of FIG. 3 for samples such as SO.sub.2, H.sub.2 S, CH.sub.3 S and COS in air.

A pellet similar to that shown in FIG. 3 was used as a sensor. The pellet was of pure K.sub.2 SO.sub.4 in contact with a porous platinum electrode and of K.sub.2 SO.sub.4 doped with 1% of Ag.sub.2 SO.sub.4 in contact with a porous silver electrode.

Each series of tests of a sulfur-bearing molecule in air at different concentrations gives a linear relationship between the log of the concentration of the sulfur-bearing compound and the difference of potential recorded on the voltmeter. The results obtained from samples of SO.sub.2 --Air, H.sub.2 S--Air and COS--Air are presented on FIG. 5.

It was also demonstrated that the presence of a high concentration of NO.sub.2 in a tested sample does not affect the measurement of SO.sub.2.

The detector of FIG. 3 was kept into operation for more than 5 weeks and proved to be stable and the results reproducible.

Another series of tests were effectuated by using the arrangement of FIG. 3 for determining the amount of CO.sub.2, CO, COS, HCHO, CH.sub.3 OH, (CH.sub.3).sub.2 CO, CH.sub.4, C.sub.2 H.sub.6, C.sub.2 H.sub.4 and C.sub.2 H.sub.2 in air samples.

The sensing element used for those tests consisted of pure K.sub.2 CO.sub.3 in contact with a porous platinum electrode and of K.sub.2 CO.sub.3 doped with 1% of Ag.sub.2 SO.sub.4 in contact with a porous silver electrode. The results obtained with different mixtures CO.sub.2 --Air are summarized on FIG. 5 and demonstrate well the linearity of the measured values.

A series of tests were also performed with N.sub.2 O--Air, NO--Air and NO.sub.2 --Air mixtures using a nitrate pellet. Each pellet was of pure Ba(NO.sub.3).sub.2 in contact with a platinum electrode and of Ba(NO.sub.3).sub.2 doped with 1% of AgCl in contact with a silver electrode. The operating temperature of the system was in the range of 450.degree. C.

The results obtained from different mixtures of nitrogen oxyde in the air indicate that the mixtures NO.sub.2 --Air gave rise to a linear emf signal with the pellet of nitrate. Those results with mixtures of nitrogen dioxyde-Air are reproduced on FIG. 5.

A series of tests were performed with the arrangement of FIG. 9a with various SO.sub.2 --Air samples.

A sensing element similar to the one shown on FIG. 4 was used as the sensor. The sensor was made of pure K.sub.2 SO.sub.4 in contact with a (ZrO.sub.2).sub.0.85 (CaO).sub.0.15 electrolyte, both being in contact with porous platinum electrodes in air.

The series of tests obtained with the detector of FIG. 4, when using the oxygen of air as a reference, gave a linear relationship between the ln of the concentration of SO.sub.2 and the electro-motive force recorded on the voltmeter.

The anhydride detector of FIG. 4 has proven to be highly stable and reproducible for a continuous operation of more than a month.

A series of tests were performed with the arrangement of FIG. 9c with various AsH.sub.3 --Air, PH.sub.3 --Air and SeH.sub.2 --Air samples. The sensing element used was similar to the one shown in FIG. 1.

It is to be noted that, in the vicinity of the detection electrode, each of those gases is readily converted into the corresponding anhydride, as shown by the following relations:

In the case of AsH.sub.3, an electrolyte made of Ca.sub.3 (AsO.sub.4).sub.2 was heated to a temperature of 800.degree. C. and the reference gas contained 100 p.p.m. of arsine. The results are plotted in FIG. 5.

For PH.sub.3 an electrolyte of Ca.sub.2 P.sub.2 O.sub.7 was heated at 600.degree. C. and 100 p.p.m. of phosphine was contained in the reference gas. The various resulting data are also computed in the graph of FIG. 5.

Concerning SeH.sub.2, a K.sub.2 SeO.sub.4 electrolyte was heated at 860.degree. C. whereas only 5 p.p.m. thereof was contained in the reference gas. The potential differences for various concentrations of SeH.sub.2 are indicated in FIG. 5.

Consequently, the curves shown in FIG. 5 clearly illustrate the substantially linear relationship between the ln of the concentration of each gas to be detected and the potential differences measured by the voltmeter.

As mentioned above with reference to FIG. 6, the anhydride detector is influenced both by a variation in the partial pressure of the anhydride and of the oxygen in the sample. This phenomena does not however interfer in environmental measurements since the oxygen partial pressure is constant in air, but it has to be taken into account in stack gas analysis and other gases where the oxygen partial pressure may fluctuate.

With CO.sub.2, for example, the above experimental observations may be translated by the following electrode reaction:

which explains the form of the signal obtained at the electrode:

E=the signal of the anhydride detector in volts

E.sub.o =a constant fixed by the composition of the reference electrode

R=gas constant

T=temperature in .degree.K.

F=Faraday number

pO.sub.2 =oxygen partial pressure

pCO.sub.2 =partial pressure of the carbon dioxyde in the sample

On the other hand, it is known that the oxygen partial pressure can be measured with a zirconia stabilized oxygen detector which produces a following signal:

where

E.sub.pO.sbsb.2 =Signal proportional to pO.sub.2

E.sub.o '=a constant characterized by the material of the reference electrode.

The arrangement of FIG. 6 was used in order to verify the proposed mechanism of oxygen compensation for the CO.sub.2 detector. The experimental results of FIG. 7, confirm such a mechanism and indicate that the electronic compensation of the signal from the CO.sub.2 detector by the signal from the O.sub.2 detector leads to a resulting signal which is independent of the concentration variations of the oxygen contained in the analysed gas sample.

With CO.sub.2, compensation is obtained by directly opposing the signals from the two detectors. This same signal opposition principle may be applied in connection with any other anhydride detectors.

In certain cases, the oxygen detector signal has to be modified before being opposed to the signal from the anhydride detector. For example, in compensating for the signal from a SO.sub.2 detector, it has been experimentally observed that the signal from this latter detector may be compensated by the signal from the oxygen detector if the oxygen signal is multiplied by two (2) before opposing it to the SO.sub.2 signal. This fact can be mathematically demonstrated by introducing the equilibrium constant of the reaction (SO.sub.2 +1/2O.sub.2 .revreaction.SO.sub.3) in the calculation of the signal from the SO.sub.2 detector in the appropriate low concentration range of oxygen (.ltoreq.20% O.sub.2).

Another series of tests was made in order to demonstrate the performance and the reliability of the particular arrangement of FIG. 9a. The solid state detector used was a sensing element as shown on FIG. 4 and described in example 5. Sealing through thermal deformation was easily obtained at temperatures near the sintering temperature of the element. The curve shown in FIG. 10 clearly shows the short response time obtained with that arrangement since 90% of the signal is available in about 30 seconds from the start of the measurement. Such performances are mainly due to the short time spent by the gas at the measuring electrode, to the small dead volumes of the gas circuit and to the reduced adsorption surface.

Moreover, in order to demonstrate that the illustrated geometrical forms of the sensors are not limitative and that any form respecting the electrochemical principle of the sensor can be used and that multiple combination of the different preferred embodiments described above may be used to achieve a multi-function detector, tests were performed on a combination detector.

A combination detector made up of a sulfur anhydride detector, a carbon anhydride detector and an oxygen detector suitable to compensate for the oxygen variations in this sample gas (as mentioned in example 7 above) was built up. An oxygen ion electrolyte tube (one end closed), the electrolyte being for instance stabilized zirconia, was interiorly platinized in order to provide a common reference electrode with air (pO.sub.2 =0.21 atm). A first portion of the outside surface of the tube was covered with a K.sub.2 SO.sub.4 electrolyte and another portion remote from the first portion with a K.sub.2 CO.sub.3 electrolyte. These electrolyte layers were obtained by wetting the tube with the molten electrolyte or by a powder deposition technique, and were then sintered.

Then, two platinum wires were respectively turned around both electrolyte layers covering the tube outside surface to constitute the measuring electrodes for the two anhydrides to be detected whereas a third platinum wire was simply wound around the zirconia tube to constitute the measuring electrode used to compensate for oxygen partial pressure variations.

Therefore, when samples containing SO.sub.2 and CO.sub.2 were passed over this tube, emf signals identical to those shown in FIG. 7 have been obtained, which signals are proportional to the anhydride concentrations when the oxygen partial pressure varies.