CA1045205A - Solid state sensor for anhydrides - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:
This invention relates to a method of and a system for detecting the activity of gaseous anhydrides in an oxygen-bearing gas. A solid state electrolyte element has oxy-anions of the ele-ment forming the anhydride to be detected. A reference electrode is in contact with the electrolyte element and a detection electrode is remote from the reference electrode but also in contact with the electrolyte element, the electrolyte and detection electrode being arranged such that they are free to come into contact with the anhydrous gas, whereas the reference and detection electrodes are arranged such that a difference of potential occurs therebetween when a sample of the anhydride to be detected is contacted with the detection electrode and with the electrolyte element. The electro-lyte element is heated to a temperature such that a logarithmic variation in the concentration of the anhydride to be detected causes a proportional and substantially linear variation in the difference of potential, the temperature being below the fusion temperature of the electrolyte element. Finally, oxygen content variations in the gas being measured are compensated. A potentio-metric measurement device is connected to the electrodes for mea-suring the activity of the anhydride by measuring the difference of potential.

Description

10~5Z05 This application is a divisional of parent application No. 234,646 filed on August 29, 1975.
The present invention relates to a method for detecting and measuring the activity of a gas by means of a solid state sensing element and a system to carry out this method. In parti-cular, the invention concerns a method and 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 quanti-tative determination of the activity of anhydrides and related com-pounds which pollute surrounding air, such detection being realized by means of a solid state sensor supplying an oxy-anion of the cor-responding anhydride.
Hitherto, research directed to the atmospheric pollution measurement has been oriented so as to replace so called first gene-ration 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 ambiant air, it is rather difficult to build a sen- -sor able to directly monitor the ambiant air and other polluting sources of high concentration. A SO2 activity measuring device based on a concentration cell principle is described in U.S. Patent No. 3,718,546 issued on February 27 1973 to Salzono et al. This device uses fused salts as electrolytes, thus cumbersome and ra- -ther difficult to transport. In addition, a great stability is required in the flow of gases to obtain realistic measurements.
Several detecting arrangements are also described in Canadian Patent No. 1,002,599 issued on December 28, 1976 to M. Gauthier and A.M. Chamberland.

In particular, the present invention is useful for quan--- 1 - q~

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titavely determinating extremely small and high concentrations of sulfur bearing compounds in gas phases by means of an element in contact with appropriate electrodes and containing an oxy-anion of the anhydride to be detect~
It is also a particularity of the invention to provide an activity measurement solid state sensor for carbon dioxyde and carbon bearing compounds conver~ed into CO2 in the sensor under operating conditions.
The electrochemical principle of the present invention is also applicable to the sensing of NO2 in air or in oxygenated ga-ses, by means of a nitrate element in contact with appropriate electrodes.
More specifically, the prime object of the inventionconsists in providing appropriate means to compensate oxygen par-tial pressure variations in particular gases like stack gases.
A further object of the invention resides in an improved system wherein the electro-motive force detected signal represents the logarithm of the concentration of the anhydride and is linear.
In preferred emkodiments of the invention, there are provided particu-lar arrangements d inlet and outlet conduits leading the gas in con-tact with the detecting element in order to minimize any dead vo-lume of gas therein and to optimize the response time of the de-tector. These preferred arrangements greatly enhance miniaturi-sation of the detector and its operating modes.
The above-mentioned objects of the present invention are actually achieved through a system for detecting the activity of gaseous anhydrides in an oxygen-bearing gas, comprising a solid state electrolyte element having oxy-anions of the element forming the anhydride to be detected; a reference electrode being in con-tact with said electrolyte element, a detection electrode remote from said reference electrode and also in contact with the elec-trolyte element, said electrolyte and detection electrode beingarranged such that they are free to come into contact with the anhydrous gas, said reference and detection electrodes being ar-

- 2 -.

~045Z~?5 ranged such that a difference of potential occurs between said re-ference and detection electrodes when a sample of said anhydride to be detected is contacted with said detection electrode and with said electrolyte element; heating means for heating said electro-lyte element to a temperature such that a logarithmic variation in the concentration of the anhydride to be detected causes a propor-tional and substantially linear variation in said difference of potential, said temperature being below the fusion temperature of said electrolyte element; compensation means for compensating for oxygen content variations in the gas being measured; and a poten-tiometric measurement device connected to said electrodes for measuring the activity of said anhydride to be detected by measuring said difference of potential.

The present invention also relates to a method of detect-ing gaseous anhydrides in an oxygen-bearing gas.
As non-limitative examples, the following sensing elements constituting the detector in accordance with the present invention are able to detect a particular anhydride:
1) a sintered electrolyte sensor containing oxy-anions of sul-fur, the reference potential being a gas or a solid, is advanta-geously used for measuring the activity of compounds such as SO3, SO2, H2S, CH3S, COS or other sulfur bearing compounds, in particu-lar, which may be transformed into the corresponding anhydride.
2) a sintered electrolyte sensor containin~ oxy-anions of car-bon, permits to determine the amount of CO2, in particular, or other carbon bearing compounds in air or in mixtures of gases containing oxygen.

3) a sintered electrolyte sen or containing oxy-anions of ni- -trogen is advantageously used for measuring, in particular, the presence of nitrogen dioxyde ~n air or in oxygen-bearing gases.

1~45Z~5 The above and other objects will become apparent through the following description of preferred embodiments given with refe-rence to ~he accompanying drawings, wherein Figure 1 schematically illustrates a sensor described in the above-mentioned Canadian patent, using a standard gas mixture as a reference;
Figure 2 shows a sensor using the vapor pressure result-ing of the thermo-decomposition of a metal salt of the anhydride to be detected, in order to fix the thermodynamical partial pressu-re of a reference anhydride;
Figure 3 shows another sensor described in the above Canadianpatent, and using a solid metal electr.ode f.or establishing a referer.ce potential.
Figure 4 shows another sensor 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 referen-ce electrode, the later electrode generating a known potential when exposed to air or to oxygen having a given partial pressure.
Figure 5 shows a graphic relating to the electromotive force experimentally obtained for various concentrations of SO2 -Air, NO2 ~ Air, CO2 - Air, COS - Air and H2S - Air mixtures. The sensor illustrated in figure 3 was used to compile those results. ~-Figure 6 shows an arrangement in accordance with the em- -bodiment 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 where-as the anhydride concentration is measured by a sensor described in figures 1 to 4. The emf produced by both sensors are electro-nically corrected and substracted in order to produce an oxygen compensated signal of the anhydride concentration in the sampled gas;
Figure 7 shows graphs obtained from experiments carried ~045Z05 out with the arrangement illustrated in figure 6;
Figure 8 shows another arrangement to compensate oxygen partial pressure variations in a gaseous sample. Compensation is achieved by injection of a predetermined 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.
Figure 9 schematically illustrates three preferred ar-rangements 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 pres-sed 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.
Figures 9a and 9b show a gas circulating system in which a measuring electrode runs through the outlet gas chamber.
Figure 9c shows a double independent gas circulating system, each of which being similar to the one shown in figure 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 con-centration is maintained.
Figure 10 shows the curve obtained from SO2 concentration measurements effected by means of K2SO4/ZrO2-CaO arranged as shown in figure 4 and using gas circulating system of figure 9a. This graphic representation shows the evolution of the emf signal at different SO2 concentrations in function of the time.
Figure 1 illustrates a sensor described in Canadian patent No. 1,002,599. This sensor comprises a detecting element 1 constituted of an electrolyte containing an oxy-anion of the gaseous anhydride to be analyzed. The element 1 is made up of an alkali metal salt or an alkali-earth metal salt.

.... ... .. .

104S2~)5 That element l is preferably pellet shaped, but, of course, any other form is also quite acceptable. Each end of the element l is in contact with an electronically conductive material 2 and 3 such as silver, platinum, gold or other.
The electrolyte element l is tightly inserted into a tube 4 made of alumina so as to hermetically separate a measure compartment "A" from a reference compartment "s". 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 pre-ferably 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. The anhydride gases are thereafter exhausted through outlet tubes 7 and 8 respectively extending from each of the compartments to-the outside.
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 exis-.
ting 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 the ionic conductivity of the electrolyte, the tube

4 is introduced into an electrical oven (not shown). The heating temperature of the oven is however not to exceed the fusion point of the electrolyte element.
A modified arrangement of the embodiment shown in figu-re 1 is presented in figure 2 wherein a block 12 of any metal salt ~
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lV45205 of the anhydride to be detected is placed inside the then hermeti-cally closed reference compartment "s". When heated, that metal salt 12 evolves a metal oxyde and an anhydride identical to the one to be analyzed. For ins~ance, where CO2 is the anhydride fed at "C", the corresponding metal salt chosen will then be MCO3 which, when heated, will give MO + CO2, 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 poten-tial at that electrode, thereby allowing detection and measurement - of the anhydrides to be analysed. The arrangement of figure 2 per-mits to avoid the reference gas circulating arrangement of figure 1.
It is to be noted that by setting the metal salt block 12 close to the reference electrode 3, the concentration of the reference anhy-dride evolved from 12 remains stable, and then the compartment "B"
does not need to be hermetically closed, but might be open air.
Figure 3 illustrates a variant of the arrangement shown in figure 1. To the reference gas source "D" of figure 1 is subs-tituted a solid state reference element, this is an electrode 12. ~ -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 the solid state electrode 12 which lS
made of a metal, is rendered possible owing to the use of a detect-ing element 1'. This element 1' is constituted through the sinter- ~ ~ -ing of a pure electrolyte compound la made of an alkali metal salt or an alkali-earth metal salt, which corresponds to the oxy-anion of the anhydride to be detected, and a second compound lb made of the compound la to which a small amount of a metal salt has been added. The electrode 12 must be formed of a metal corresponding to the metal salt added by doping or vice-versa. For instance, if K2SO4 is used as compound la, the compound lb will be constituted 104S'~05 of K2SO4 doped with about 1% of Ag2SO4 or of AgCl, provided the electrode 12 is made of silver. The other numeral references indicated in figure 2 represent the same elements as those to which they refer in figure 1.
Figure 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 alkali-earth metal salt is juxtaposed an oxygen ion bearlng 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.
As mentioned previously, the sensors shown in figures 1 to 4 may be 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. ~-It is to be noted that the sensors illustrated in figu-res 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 figures 1 to 4 and certain results of which have been plotted on figure 5, which results will be discussed in con-nection with specific examples given hereafter. ;;
Figure 6 shows an arrangement to compensate for any va~riations in the partial pressure of the oxygen gas of a 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 interfer in environmental measurements since the 1045'~,05 oxygen partial pressure remains constant in air, but such varia-tions are to be taken into account in stack gas analysis, for exam-ple, and other gases where the oxygen partial pressure fluctuates.
To compensate for the oxygen partial pressure variations in a stack gas C, a portion Cl of this gas is fed to an oxygen sensitive de-tector 13'a made up of an oxygen ions bearing electrolyte 13a havir.g a reference electrode 3a and a measuring electrode 2a, these two electrodes being of any electrically conductive material. Another portion C2 of the gaseous samplé "C" is forwarded toward a second detector 13' which is identical to the one shown in figure 4. The reference electrodes 3 and 3a of detectors 13' and 13'a, respecti--vely, are exposed to ambient air. In addition, the two solid state detectors are placed into the same electrical oven 16 to achieve uniformity of operating temperature for both of them. The poten-tials 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 El and E2, respectively, by means of a pump 15.
Although the arrangement illustrated in figure 6 has - been described above with reference to a detector 13' similar to the one shown in figure 4, it should be understood that anyone of the anhydride detectors of figures 1 to 3 may as well be used. The use of the detector 13' in the arrangement of figure 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. Patent No. 3,400,054 issued on September 3, 1968 to Ruka et al, may, for instance,be advantageously used.
Conclusive results have been obtained with the arrangement shown in figure 6, specifical experimental results for SO2 and C2 being presented on figure 7. An experiment carried on with ~045205 C2 is further given below in example 6.
Figure 8 shows another arranc3ement 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 nearly constant level, which enables 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 figures 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 figures 9a, 9b and 9c, there are shown par-ticular 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 embodi-ments may appear quite simple, they have proven to be highly ef- -fective in hermetically sealing the contact points with the surface of the solid state sensors. As illustrated in figures 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 tempe-rature and then by pressing the extremity of the rod-shaped mate-rial 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 herme-; : . : , , : -.''' ~ : : ' ' ~04S;~()5 tical sealing is thus produced. Although in figures 9a and 9b the particular conduit arrangement is used in connection with the sensors shown in figures 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 illus-trated in figures l and 2 In this respect, utilizing the solid state sensor of figure l, an arrangement of a particular interest, being highly compact, is presented in figure 9c in which a plurali-ty 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 re-ference 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 se-parating wall 24`prevents the intermixing of the gas sample and the reference gas, this separating wall being also embedded into the element l 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, or quartz.
The arrangement shown in-figure 9 offers several advan-tages 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 contac-ted the electrode, are promptly exhausted. Thus, the system res-, - . . : -, :
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- , 1045'~5 ponse time, always influenced by gas dead volumes, is substantially increased and the true values of the potential corresponding to the concentration of 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 pro-vided for the leads connecting the various electrodes to the volt-meter 9, rather than running through the gas channels, as shown in figure 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 figures 5, 7 and 10 on which experlmental results have been quoted.
EXAMPLE 1:
A series of tests were conducted using the arrangement of figures 1 and 2 to monitor the amount of SO2 in air. Various S2 ~ Air mixtures were prepared and tested. In a concentration range running from 0.1 to 20,000 ppm, it was established that a linear relationshlp existed between the logarithm of the SO2 con-tent of the sampled gas and the electro-motive force recorded on the voltmeter.
EXAMPLE 2:
A series of tests were conducted using the arrangement of figure 3 for samples such as SO2, H2S, CH3S and COS in air.
A pellet similar to that shown in figure 3 was used as a sensor. The pellet was of pure K2SO4 in _ontact with a platinum electrode and of K2SO4 dopèd with 1% of Ag2SO4 in contact with a silver electrode.
Each series of tests of a sulfur-bearing molecule in air at different concentrations gives a linear relationship bet-ween the log of the concentration of the sulfur-bearing compound and the electro-motive force recorded on the voltmeter. The re-'.: .', ~' ' , ~' ' :

1~)4521)S
sults obtained from samples of SO2 - Air, H2S - Air and COS - Air are presented on figure 5.
It was also demonstrated that the presence of a high concentration of NO2 in a tested sample does not affect the mea-surement of SO2.
The detector of figure 3 was kept into operation for more than 5 weeks and proved to be stable and the results reproducible.
EXAMPLE 3:
Another series of tests were effectuated by using the arrangement of figure 3 for determining the amount of CO2, CO, COS, HCHO CH OH (CH3)2CO, CH4, C2H6, C2H4 and C2 2 The sensing element used for those tests consisted of pure K2CO3 near a platinum electrode and of K2CO3 doped with 1% of Ag2SO4 near a silver electrode. The results obtained with diffe-rent mixtures CO2 - Air are summarized on figure 5 and demonstrate well the linearity of the detected values.
EXAMPLE 4:
A series of tests were also performed with NO2 - Air, using a nitrate pellet. Each pellet was of pure Ba(NO3)2 near a platinum electrode and of Ba(NO3)2 doped with 1% of AgCl near a silver electrode. The operating temperature of the -system was in the range of 450C.
The results obtained from different mixtures of nitrogen oxyde in the air indicate that the mixtures NO2 - Air gave rise to a linear emf signal through the pellet of nitrate. Those results with mixtures of nitrogen dioxyde-Air are reproduced on figure 5.
EXAMPLE 5:
A series of tests were performed with the arrangement of figure 9a with various SO2 - Air samples.
A sensing element similar to the one shown on figure 4 was used as a sensor. The sensor was made of pure K2SO4 in contact with a (Zr2) 85(CaO) 15 electrolyte, the latter being in contact --. . ~:
. ' ' ~: ' 3.945Z05 with a platinum electrode in air.
The series of tests obtained with the detector of figure 4, when using the oxygen of air as a reference, gave a linear rela-tionship between the log of the concentration of SO2 and the elec-tromotive force recorded on the voltmeter.
The anhydride detector of figure 4 has proven to be highly stable and reproductible for a continuous operation of more than a month.
EXAMPLE 6:
As mentioned above with reference to figure 6, the anhy- ~ ~ ' dride 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 measurement 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 aprtial pressure may fluctuate.
With CO2, for example, the above experimental observation may be translated by the following electrode reaction:
C2 t 1/2'O2 t 2e ~ CO3 " , which explains the form of the signal obtained at the electrode:

signal Eo ~ 4F log PO2 t 4F log PCO2 ' '' E = the signal of ,the anhydride detector in volts Eo= a constant fixed by the composition of the reference electrode R = gas constant T = temperature in K
F = Faraday number PO2 = oxygen partial pressure pCO2= 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 detec-tor which produces a following signal: -, .
:, ., . ~ . . -~045Z05 E E ' ~ RT log pO

where Epo - signal proportional to PO2 Eo' - a constant characterized by the material of the refe-rence electrode.
The arrangement of figure 6 was used in order to verify the proposed mechanism of oxygen compensation for the Co2 detector. -The experimental results of figure 7, confirm such a mechanism and indicate that the electronic compensation of the signal from the oxygen detector by the signal from the CO2 detector leads,to a resulting signal which is independent of the concentration varia-tions of the oxygen contained in the analysed gas sample.
With CO2, 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 de- ,, tector. For examFle, in compensating for the signal from a SO2 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 SO2 signal. This fact can be mathemati-cally demonstrated by introducing the equilibrium constant of the reaction (SO2 + 1/2 2 = SO3) in the calculation of the signal from the SO2 detector in the appropriate low concentration range of oxygen ( C 20~ 2) EXAMPLE 7:
Another series of tests was made in order to demonstrate the performance and the reliability of the particular arrangement of figure 9a. The solid state detector used was a sensing element as described on figure 4 and in example 5. Sealing through thermal deformation was easily obtained at temperatures near the sintering -~045Z05 temperature of the element. The curve shown in figure 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 gas in the inlet and outlet channels and to the reduced adsorption surface.
EXAMPLE 8: -Moreover, in order to demonstrate that the illustrated geometrical forms of the sensor 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 em-bodiments 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 suita-ble to compensate for the oxygen variations in this sample gas (as mentioned in example 6 above) was built up. An oxygen ion ;
electrolyte tube (one end closed), the electrolyte being for ins-tance stabilized zirconia, is interiorly platinized in order to pro-vide a common reference electrode with air (PO2= 0.21 atm). A
portion of outside surface of the tube is covered with a K2SO4 electrolyte and another portion with a K2CO3 electrolyte. These electrolyte layers are obtained by wetting the tube with the molten electrolyte or by any powder deposition technique, and are then sintered.
Then, two platinum wires are turned around respectively both electrolyte layers covering the tube outside surface to cons-titute the measuring electrodes for the two anhydrides to be detec-ted whereas a third platinum wire is simply wound around the zir- -conia tube to constitute the measuring electrode used to compensate for oxygen partial pressure variations.

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1C)45205 Therefore, when samples containing S02 and C02 are pas-sed over this tube, emf signals identical to those shown in figure 7 are obtained, which signals are proportional to the anhydride con-centrations when the oxygen partial pressure varies.

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Claims (17)

The embodiments of the invention in which an exclusive property or privilege is claimed, are defined as follows:

1. A system for detecting the activity of gaseous anhy-drides in an oxygen-bearing gas, comprising a solid state elec-trolyte element having oxy-anions of the element forming the anhy-dride to be detected; a reference electrode being in contact with said electrolyte element, a detection electrode remote from said reference electrode and also in contact with the electrolyte ele-ment, said electrolyte and detection electrode being arranged such that they are free to come into contact with the anhydrous gas, said reference and detection electrodes being arranged such that a difference of potential occurs between said reference and detec-tion electrodes when a sample of said anhydride to be detected is contacted with said detection electrode and with said electrolyte element; heating means for heating said electrolyte element to a temperature such that a logarithmic variation in the concentration of the anhydride to be detected causes a proportional and substan-tially linear variation in said difference of potential, said temperature being below the fusion temperature of said electrolyte element; compensation means for compensating for oxygen content variations in the gas being measured; and a potentiometric measure-ment device connected to said electrodes for measuring the activity of said anhydride to be detected by measuring said difference of potential.

.2. A detecting system as claimed in claim 1, wherein said compensation means comprise means for admixing an oxygen-rich gas with the gas sample containing the anhydride to be detected.

3. A detecting system as claimed in claim 1, wherein said compensation means comprise an oxygen ions bearing electro-lyte for sensing said oxygen content variations, two electrodes remote from one another and operatively contacting said oxygen ions bearing electrolyte, means for heating the oxygen ions bear-ing electrolyte to a temperature such as to achieve detection of said oxygen content variations, and means to connect said both electrodes to said potentiometric measurement device.

4. A detecting system as claimed in claim 1, 2 or 3, wherein said solid state electrolyte element is tightly inserted inside a support means so as to define a measure compartment and a reference compartment, the sample of the anhydride to be detect-ed being introduced into said measure compartment while a standard anhydride gas of said anhydride to be detected is introduced in said reference compartment.

5. A detecting system as claimed in claim 1, 2 or 3, wherein said solid state electrolyte element is tightly inserted inside the support means so as to define a measure compartment and a reference compartment, the sample of the anhydride to be detected being introduced into said measure compartment, and a metal salt material being placed in said reference compartment, said metal salt material evolving an anhydride gas identical to the anhydride to be detected, at said reference electrode, when heated at said solid state electrolyte temperature.

6. A detecting system as claimed in claim 1, 2 or 3, wherein said solid state electrolyte element is constituted of two parts, one part being an alkali metal salt or an alkali-earth metal salt containing the oxy-anions of the anhydride to be detect-ed; and the other part being a mixture of said alkali or alkali-earth metal salt and a metal salt of the metal of said reference electrode with which said other part is in contact.

7. A detecting system as claimed in claim 1, 2 or 3, wherein said solid state electrolyte element is constituted of two parts, one part being an alkali metal salt or an alkali-earth metal salt containing the oxy-anions of the anhydride to be detected;
and the other part being an oxygen ions bearing electrolyte mate-rial, said reference electrode being in contact with said other part of the solid state electrolyte element.

8. A system for detecting the activity of gaseous anhy-drides in an oxygen-bearing gas, comprising a first sensor cons-tituted of a solid state electrolyte element having oxy-anions of the element forming the anhydride to be detected, a first electrode in contact with said electrolyte element and said anhydride, a second electrode remote from said first electrode and also in contact with the electrolyte element, the second electrode acting as a reference electrode in said first sensor; a second sensor constituted of an oxygen ions bearing electrolyte for sensing oxygen content variations in said oxygen-bearing gas, the second sensor having a reference electrode and a measuring electrode both in contact with the oxygen ions bearing electrolyte; said electrodes of said first and second sensors being respectively arranged such that a difference of potential occurs therebetween when a sample of said anhydride to be detected is contacted with said first electrode and said solid state electrolyte, and with said measuring electrode and said oxygen ions bearing electrolyte;
means for heating said first and second sensors to a temperature below the melting point of said solid state electrolyte element and such that a logarithmic variation in the concentration of the anhydride to be detected causes a proportional and substantially linear variation in said difference of potential; and a potentio-metric measurement device connected to said electrodes for detect-ing and measuring the activity of said anhydride to be detected by measuring said difference of potential from said first sensor and second sensor, respectively.

9. A method of detecting and measuring the activity of gaseous anhydrides in an oxygen-bearing gas, comprising the steps of:
forming a solid state electrolyte element having oxy-anions of the element forming the anhydride to be detected, a reference electrode being in contact with said electrolyte element, a detection electrode remote from said reference electrode and also in contact with the electrolyte element, said electrolyte element and detection electrode being arranged such that they are free to come into contact with the gaseous anhydride to be detect-ed, said reference and detection electrodes being arranged such that a difference of potential occurs therebetween when a sample of said anhydride to be detected is contacted with said detection electrode and with said electrolyte element;
heating said electrolyte element to a temperature below the fusion temperature thereof and such that a logarithmic varia-tion in the concentration of the anhydride sample to be detected causes a proportional and substantially linear variation in said difference of potential;
passing the sample of said anhydride to be detected into contact with said detection electrode and said electrolyte element so as to result in said difference of potential between said two electrodes;
compensating for oxygen content variations in said sample of anhydride gas to be measured; and measuring said difference of potential across the refe-rence and detection electrodes with a potentiometric measurement device.

10. A method as claimed in claim 9, characterized in that the compensating step comprises admixing an oxygen-rich gas with the gas sample containing the anhydride to be detected.

11. A method as claimed in claim 9, characterized in that the compensating step comprises detecting said variations by passing the gas sample containing the anhydride to be detected into contact with an oxygen ions bearing electrolyte having two electrodes in contact therewith so as to create another difference of potential representative of said oxygen content variations, and by feeding said another difference of potential to said potentio-metric measurement device.

12. A method as claimed in claim 9, 10 or 11, characte-rized in that the solid state electrolyte element is formed by sintering an alkali salt or an alkali-earth salt.

13. A method as claimed in claim 9, 10 or 11, characte-rized in that the solid state electrolyte element is formed by sintering a composition constituted of an alkali salt or an alkali-earth salt and a mixture of said composition to which is added a small amount of a salt corresponding to the metal of said reference electrode with which said mixture is in contact.

14. A method as claimed in claim 9, 10 or 11, characte-rized in that the solid state electrolyte element is formed by sintering two compounds, one compound being an alkali metal salt or an alkali-earth metal salt containing the oxy-anions of the anhydride, and the other compound being an oxygen ions bearing electrolyte material, said reference electrode being applied onto said other compound.

15. A method as claimed in claim 9, 10 or 11, characte-rized in that a reference gas is evolved at the reference electrode by heating a metal salt material containing an anhydride identical to the anhydride to be detected at said solid state electrolyte temperature.

16. A method as claimed in claim 9, 10 or 11, characte-rized in that the sample gas of the anhydride to be detected is passed into contact with said detection electrode and said solid state electrolyte by means of supply and exhaust conduits connected to a compartment hermetically insulated from outside.

17. A method as claimed in claim 9, 10 or 11, characte-rized in that conduits are formed to pass the sample gas of the anhydride to be detected into contact with said detection elec-trode by boring substantially parallel channels through a solid body, by forming a passage at one extremity of the solid body so as to allow a gas communication between said channels, by register-ing said passage with the detection electrode and heating the electrolyte element at a temperature so as to embed slightly the solid body into the electrolyte element.