SOLID STATE GALVANIC-CELL SENSORS FOR MEASURING GASEOUS SULFUR DIOXIDE

Abstract: An improved gaseous sulfur dioxide galvanic cell (10) is formed by utilizing a solid electrolyte (12) composed of a mixture of lithium sulfate and silver sulfate. A reference electrode (14) is formed by placing a solid mixture of the electrolyte mixture and metallic silver in contact with a portion of the surface of the solid electrolyte. A gas-type detector electrode is provided by contacting the solid electrolyte with a gold lead (20) at a point spatially separated from the contact point between the reference electrode and electrolyte. Catalyst means (34) are preferably provided to convert the sulfur dioxide to an equilibrium mixture of sulfur dioxide and sulfur trioxide near the electrolyte detector electrode surface. The electrolyte is preferably heated to a temperature at which it is a two-phased mixture of solid solutions for increased long term stability.

Description: SOLID STATE GALVANIC-CELL SENSORS FOR MEASURING GASEOUS SULFUR DIOXIDE BACKGROUND OF THE INVENTION FIELD OF THE INVENTION . The invention relates to improved solid state galvan ic-cell sensors for measuring the concentration of gaseou inorganic acid anhydrides, especially sulfur dioxide. Sulfur and sulfur dioxide (SO2) are important chemi cal species in many high temperature systems. The detectio and measurement of the presence and concentration of sulfu dioxide and other inorganic sulfur-containing gaseous com pounds which may, for example, be generated and released a effluents in the operation of these high-temperature system has, in recent years, generated considerable interest wit the growing concern regarding atmospheric pollution. Concen trations of sulfur dioxide as low as about 1 part per millio (ppm) are believed by some to be injurious to plant lif while concentrations of 400 to 500 ppm may be fatal t humans. Additionally, in the presence of moisture, SO gas reacts to sulfurous and eventually sulfuric acid causin considerable corrosion problems and the phenomenon known a "acid rain". Over the years, various techniques, includin electrochemical techniques have been developed for detectin the presence of Sθ2» Among the earliest of the electrochemical technique was the bubbling of air believed to be contaminated with sulfu dioxide through deionized water and detecting the presence o the gas through changes in the water's conductivity. The use of different electrolytes and electrode materials has provided improved response time and accuracy. For example, U.S. Patent 3,622,488 to Chand et al discloses the use of a liquid acidic electrolyte such as sulfuric acid with an inert (noble) metal sensing electrode and a solid, counter electrode. The counter electrode comprises an electroactive material capable of being reduced when interconnected with the sensing electrode and -.mm placed in contact with the electrolyte. The presence and con¬ centration of sulfur dioxide at the sensing electrode is indi¬ cated by the magnitude of current between the sensing and coun- ter electrodes. H. Dahms in U.S. Patent 3 756,923 describes a system utilizing a thin layer of liquid electrolyte containing silver ions held to the surface of a sensor electrode together with a counter electrode immersed in the liquid. The sample gas is exposed to " the liquid electrolyte and the concentration of SO2 determined by the change in current between the electrodes. Saϊzano et al in" U.S. Patent 3,718,546 describe a potentiometric determination of sulfur dioxide using an elec¬ trochemical cell having a molten, fused salt electrolyte composed of a combination of lithium sulfate (Li2S04), potassium sulfate (K2SO ) and sodium sulfate (Na2Sθ ). A reference gas electrode containing a known concentration of sulfur dioxide in air and a sample gas electrode contain¬ ing an unknown concentration of sulfur dioxide in air are placed in contact with the electrolyte. The difference of electromotive potential between the two electrodes can be related to the concentration of sulfur dioxide in the sample gas. A solid electrolyte and solid reference electrode may be combined to yield a compact, simple solid state galvanic- cell detector. M. Gauthier et al describe in "Solid-State Detectors for the Potentiometric Determination of Gaseous Oxides", Journal of the Electrochemical Society; SOLID-STATE SCIENCE AND TECHNOLOGY, October 1977 (pp. 1579-1583) two galvanic cells employing potassium and silver sulfates for the measurement of SO2 concentrations in air. In a subsequent article entitled "Progress in the Development of Solid-State Sulfate Detectors for Sulfate Oxide", Journal of the Electro¬ chemical Society: SOLID STATE SCIENCE AND TECHNOLOGY, Febru¬ ary 1981 (pp. 371-378), Gauthier et al disclose yet a third potassium sulfate-silver sulfate solid state galvanic-cell for the detection and measurement of sulfur dioxide, but reveal that the potential measurements of each of these ^BUR£ three described Gauthier potassium sulfate based solid state cells drift when used over a period of weeks or even days. These cells are also believed to be the subject of Canadian Patent, 1,002,599. It would be highly desirable to provide a simple, solid state detector for SO2 and similar inorganic acid anhy¬ drides which detector would have greater potentiometric sta¬ bility than heretofor achieved. While the more recent Gauth¬ ier et al article (1981) describes several other solid state sulfur dioxide detectors, each is mechanically more compli¬ cated and difficult to manufacture than the potassium sulfate- silver sulfate ' cells referred to above. All of the potassium sulfate galvanic cell sulfur dioxide detectors described by Gauthier et al also operate at temperatures between about 700 and 900°C, apparently to provide sufficient ionic conductivity for acceptable detector response times. It is also highly desirable to provide a solid state detector which provides adequate response time while operating at lower temperatures whereby less expensive and less heat resistent materials may be employed in the construction of the detector and less heating needs to be provided in the measure¬ ment of sulfur dioxide in the atmosphere. Jacob and Rao in an article entitled "A Solid-State Probe for SO2/SO3 Based on Na2Sθ4~I Electrolyte", Journal of the Electrochemical. Society: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY, pp. 1842-1847 (Vol. 126, No. 11, Nov. 1979) describe the -possibility of a sulfur dioxide detecting cell employing sodium sulfate and silver sulfate. It is believed however, that such cell would suffer the drawbacks of the Gauthier et al potassium sulfate-silver sulfate cells in that it would be necessary to operate the sodium sulfate cell at a temperature above about 700 β C and the potentiometric stability of the cell would not be adequate for accurate long term gas surveillance. OBJECTS OF THE INVENTION It is an object of this invention to provide detec tors and detecting systems which are suitable for the detectio of gaseous inorganic acid anhydrides. More particularly, i is an object of this invention to provide detectors fo sulfur dioxide having improved stability over time. Anothe object is to provide such detectors which are compact an which may be fabricated easily. Yet another object is t provide compact systems for SO2 detection which may b operated at lower temperatures than previously' possible Yet another object is to provide compact galvanic cells fo SO2 detection which produce higher voltage responses tha previously possible. Yet another object is to provide suc detectors having ' voltage outputs very close to predicte theoretical values for the detector. These and other object will be apparent from a review of the following description SUMMARY OF THE INVENTION This invention provides apparatus for detecting th activity of gaseous inorganic acid anhydrides in oxygen-con taining gas streams. More particularly, this inventio provides detection apparatus which are useful for the detec tion of sulfur dioxide, sulfur trioxide and mixtures of th two. According to one aspect of the invention such detectio apparatus comprises a galvanic cell having a solid electrolyt element formed from a pair of metallic salts, at least on of the salts providing an oxy-anion of the inorganic aci a.rihydride to be detected. The two salts are provided i proportions such that a two-phased, solid electrolyte mixtur forms therefrom at operating temperatures of the cell. Refer ence and detector electrodes are provided in contact wit the solid electrolytic element, the detector electrode bein spatially separated from the reference electrode. The syste is adapted to provide a difference in electrical potentia between the electrodes upon exposure of the detector electrod to the gas. According to a second aspect of the invention, galvanic cell is provided having a solid electrolyte forme from a mixture of lithium sulfate and silver sulfate, first reference electrode and a second gas-type electrode, each electrode being in electrical contact with the electrolyt and spatially separated from one another. Means are als provided to convert the gaseous .sulfur anhydride into sulfu dioxide or sulfur trioxide to produce a potential betwee the electrodes. In a preferred embodiment of both aspects,of the in¬ vention, a sulfur dioxide detector is provided having an in¬ tegral sensor element comprising a solid electrolytic portion and a solid reference electrode portion. The electrolytic portion is formed from a mixture of lithium sulf te and silver sulfate while the reference electrode portion is formed from a mixture of metallic silver, silver sulfate and lithium sulfate. A gas-type detector electrode is provided at a surface of the electrolytic portion of the integral sensor element spatially separated from the solid reference electrode portion of the element. Catalyst means are further provided for converting an incoming gas stream containing oxygen and sulfur dioxide into an equilibrium mixture of sulfur dioxide, sulfur trioxide and oxygen at the detector electrode. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of a solid state gal¬ vanic cell of the present invention; FIG. 2 is a diagrammatic partially, sectioned view of a test detection apparatus incorporating the solid state gal¬ vanic cell of FIG. 1 for the measurement of SO2 FIG. 3 depicts graphically the measured potentiomet¬ ric response of two embodiments of the galvanic cell sensors of FIGS. 1 and 2 for indicated concentrations of sulfur dioxide in equilibrium with sulfur trioxide and oxygen; and FIG. 4 depicts graphically the least square values of the measured potentiometric response of a preferred, two-phase U RE; OΛΪPI lithium sulfate-silver sulfate electrolyte galvanic cell for indicated SO2 equilibrium concentrations in air tested on a daily basis by the same galvanic cell. DETAILED DESCRIPTION OF THE INVENTION It is believed that one source of long-term potentio¬ metric instability of the previously referred to sodium and potassium sulfate-silver sulfate solid-state detectors is the result of reaction of the " silver with sulfur anhydrides in the air. Another plausible source to the potentiometric instability is ' the formation of pyrosulfates due to reaction between the sulfate. electrolyte and SO3 in the gas mixture. Either of these two sources could create a concentration dif¬ ferential in the electrolyte and extraneous potentials in the cell which would change over time and thus change the potentio¬ metric response of the sensor. It has been found that the creation of a concentration differential in the solid electro¬ lyte can be minimized or eliminated by forming the electrolyte from a pair of metallic salts which are in a solid, two-phase state at the operating temperature of the cell such that reac¬ tion of the metal or other components of the reference elec¬ trode will not cause a change in the potentiometric response of the galvanic cell. Solid state detectors similar in construction and function to those described for the detection of SO2 may be used for the detection of a variety of gaseous anyhydride compounds; the principle of our invention may be extended to those types of detectors as well. The novel apparatus for detecting.the presence of SO2 comprises a reference electrode, a solid electrolytic element in electrical contact with the reference electrode and a detec¬ tor electrode in electrical contact with the electrolytic ele- ment and spatially separated from the reference electrode. The active components of the electrolytic element are a pair of metallic salts, at least one of the salts providing the electro¬ lytic element with oxy-anions of the particular anhydride being detected. The pair of metallic salts are provided at relative proportions to form a two-phase, solid mixture at the cell's operating temperatures. The reference electrode provides fixed electrical potential. The detector electrode provides an electrical potential when the anhydride to be detected is in contact with the detector electrode and the electrolytic element. This potential changes with changes in the concentra¬ tion of the anhydride. A potentiometric detector connected between the electrodes permits the measurement of the concen¬ tration of the anhydride to be detected by measuring the dif¬ ference in potential between the electrodes. It may be neces- ' sary or desirable to further provide catalyst means ,to convert the gaseous anhydride desired to be detected into another gaseous compound of the anyhydride which will react with the detector electrode to provide a potential across the reference and. etector electrodes. - According to one important aspect of the invention, sulfuric anhydrides such as SO2 may be detected using a solid electrolytic element formed from lithium sulfate. The second component of the electrolytic element is preferably silver sulfate which allows the use of a silver-silver sulfate-lithium sulfate solid reference electrode. Several two-phase solid mixtures of the lithium sulfate and silver sulfate exist at temperatures below 700 β C. A two-phase system consisting of solid solutions of c ~Li2S04 and (Ag,Li)2Sθ4, which exists between about 510" and 560 β C for proportions of lithium sul¬ fate between about 80 to 64 mole percent with proportions of silver sulfate between about 20 and .36 mole percent, is preferred. According to yet another important feature of the invention a solid reference electrode is formed for use with a solid electrolyte by mixing the electrolyte mixture with a me¬ tal of one of the metallic salts forming the electrolyte. Embedding silver in the described lithium sulfate-silver sulfate electrolyte mixtures minimizes or virtually eliminates sulfation of the silver by the gaseous atmosphere. According to yet another important feature of the in¬ vention, the electrolytic element and reference electrode may preferably be formed as a single integrated element so as to ' BU E O P provide good electrical contact and to assure that the electri¬ cal potential at the electrode-electrolyte interface is es¬ tablished solely- y the reference electrode. A layer of the mixed silver sulfate and lithium sulfate components of the electrolytic element and a layer of mixed silver, lithium sulfate and silver sulfate components of the solid reference electrode may be compressed together in a single operation. The compressed, integrated element is subsequently sintered in an atmosphere containing sulfur dioxide. I -is envisioned that the solid sensor elements may similarly be formed from other components for the detection of sulfuric and other anhydrides in which the electrolytic portion of the integrated solid sensor is two-phased at its operating temperature and- is substantially uneffected by reactions occurring with¬ in the reference electrode. Catalyst means for transforming the inorganic acid anhydride into a second acid anhydride, such as the transforma¬ tion of SO2 into SO3, is generally useful in the practice of the described embodiments of the invention. The detector electrodes of the lithium sulfate-silver sulfate cells of this invention are believed to respond to the activity of sulfur trioxide. Therefore, catalyst means are preferably provided for converting the sample gas stream containing SO2 and O2 into an equilibrium mixture of O2, SO2, and SO3 near the detector electrode. Where the preferred, two-phased em¬ bodiment of the lithium and silver sulfates and other single phased embodiments of that cell are operated at temperatures less than about 600 β C, vanadium pentoxide may be successfully employed to create the equilibrium mixture of SO2, SO3 and 02- Other catalysts will be understood by those skilled in the art to be similarly useful. Solid mixtures of metals and metallic compounds, es¬ pecially oxides or sulfates M χ 0-M x S0 (M representing metal), may be used as solid reference electrodes for solid galvanic cells. However, the silver-silver sulfate (Ag-Ag2Sθ ) com¬ bination is preferable because it not only gives fixed sulfur trioxide potentials, but also good electric contact at the electrolyte/reference electrode interface. Silver is unique metal that is stable in oxidizing atmospheres at tempe atures over about 200 β C; its sulfate ( g2Sθ4) , is also stabl A comparison of different binary systems of Ag2≤ with alkali and alkali earth metal sulfates indicates that t Li2S04 and Ag2S04 binary system is preferred for detection SO2 in gaseous streams. A phase diagram of this bina system is provided at page 315 of PHASE DIAGRAMS FOR CERAMIST 1975 SUPPLEMENT, E.M Levin et al, The American Ceramic Societ Inc. (1975), which is incorporated herein by -referenc The Li2Sθ4 has an FCC structure at high temperature a g2≤θ4 a rhombic structure. There are two high conductivi phases in this system,c?^-Li2S04 and (Ag, Li) 2 S0 4' " bot solid solutions. These two high conductivity phases co-exis with good conductivity between about 510 β C and 560 β C. Th activity of g2Sθ4 remains constant in this two phas region, no matter how the. composition of the binary syste changes. In comparison, Na2Sθ4~Ag2Sθ4 and K2Sθ4~Ag2Sθ systems each forms a continuous solid solution (single phase over its entire range of compositions at high temperatures Thus galvanic cells formed using these solid solutions a electrolytes are more susceptible to variations in th concentration of the silver sulfate. Variations in concentra tions may be caused either by reaction of the silver in th reference electrode with SO3 to form. g2Sθ4 or by decompo sition of the Ag2S04. Another advantage of the Li2Sθ system over th Na2Sθ4 and K2SO systems is the higher conductivity due t the smaller ionic radius and higher mobility of the lithiu ions, which provides adequate sensor response times at lowe temperatures. Also the two phase electrolytes have les absorption of water vapor and improved mechanical properties The operation of our lithium sulfate-silver sulfate cell with silver-lithium sulfate-silver sulfate solid referenc electrode is believed to be as follows. In Li2Sθ4~ g2Sθ electrolytes, the lithium ions are the predominant mobile species. The electromotive force ("emf") across the electrolyte is given by the equation (2) where E is the emf and u" _,± , ' j ± are the chemical potentials of lithium on each side of the electrolyte. Rewriting the chem¬ ical potentials in terms of activities, and with t 3 _ on = 1 (ionic transference number) , (3) It is believed that the activities of lithium are fixed by the composition of the gas according to the following reaction at detector electrode i 2 S0 4 = 2Li SO. + 1/2 0. and at reference electrode 2Ag Li S0. = Ag S0 4 + 2Li 2 4 *Li for two phase eletrolyte, a-^i so ano - a A g SO are constant 2 4 2 4 At high temperatures and in the presence of a cata lyst, SO2 SO3 and O2 are in equilibrium. The partial pressur of SO2 (Pso ) can >e substituted for the partial pressure of SO 2 (PSO )' yielding 3 The partial pressure of O2 of air is known and the unknow partial pressure of SO2 or SO3 can be calculated from th measured emf. The emf response generated for various SO concentrations by a preferred, two-phase electrolyte embodi ment of this invention and by a single-phase electrolyt embodiment, are depicted graphically in FIG. 3. As can be seen, the cells exhibit a proportionate and substantiall linear variation for a logarithmic variation of the concen¬ tration of the SO2, SO3 equilibrium mixtures. An exemplary embodiment of a solid state galvanic cell 10 according to the present invention is depicted diagram- matically in-FIG. 1. The cell 10 comprises a solid electrolyte 12 formed from a mixture of lithium sulfate..and silver sulfate and a solid reference electrode 14 formed .from a mixture of silver and the mixture of the electrolyte components (i.e. silver sulfate and lithium sulfate in the same proportions as the electrolyte) and having a lead 16 for electrical contact extending therefrom. Although the electrolyte 12 and reference electrode 14 may be formed separately and placed in contact with one another, preferably they are formed as a single inte¬ gral sensor element 18 to provide good electrical contact between them and to prevent the intrusion of air or other [ O contaminant into their interface, which is indicated diagram¬ matically by the dashed line 18a. The lead 16 is of a nonreactive material such as gold. The use of silver in the lead 16 is to be avoided as its reaction with SO2, SO3 or both in the sample gas may lead to failure of the lead 16. A gas type detector electrode is formed by providing a second, nonreactive lead 20 in electrical contact with the electrolyte 12 at a surface 12a of the electrolyte 12 which is spatially separated from the reference electrode 14 and passing the sample gas, represented by the arrow 22, over the lead 20 and electrolyte surface 12a. A platinum mesh 23 has been embedded in the electrolyte 12 and is partially exposed at a surface 12a of the electrolyte spatially separated from the solid reference electrode 14. In the depicted embodiment, the platinum mesh 23 is welded to the lead 20 to provide a large electrode-electrolyte surface area with which the active anhydride of the sample gas may react. The platinum mesh also assists in maintaining an equilibrium mixture of oxygen, sulfur dioxide and sulfur trioxide at the detector electrode surface 12a of the cell 10. It is believed that the reaction at that electrode involves preferably sulfur trioxide rather than sulfur dioxide. Tests made without a catalyst produced emf values lower than predicted for the SO2 concentrations involved. Powdered, anhydrous 99.999% pure Li2Sθ and g2Sθ4 were used to make the electrolytes. Solid reference electrodes were made by * mixing 2/3 (by weight) of powdered 99.99% pure silver and 1/3 (by weight) of the Li2Sθ4~Ag2Sθ4 electrolyte powder. Each component was first passed through a U.S. Standard Sieve 325 mesh. The electrolyte components were throughly mixed and then compressed with a pressure of about 20,000 psi. The resulting aggregate was reduced to a size which allowed it to pass through a U.S. Standard Sieve No. 50 mesh. The solid reference electrode components were simi¬ larly passed through a U.S. Standard Sieve No. 325 mesh, throughly mixed, compressed at a pressure of about 20,000 psi and broken up sufficiently to pass through a U.S. Standard Sieve No. 50 mesh. The galvanic cells 10 were additional provided with a platinum wire mesh at the gas electro surface 12a of the electrolyte 12. The gold lead 20 of t gas electrode was welded to an exposed portion of the mes 23. The platinum mesh 23 and welded gold wire 20 were plac in the bottom of a rubber mold. The -50 mesh electrolyt powder mix was added to the mold and tamped by hand. Th -50 mesh reference electrode mix was added to the mold an . a gold wire lead inserted therein. The mold was placed in hydraulic isostatic press and subjected to a pressure o approximatetly 80,000 pounds per sq. in. The cell 10 wa removed from the mold and the gas electrode surface 12 cleaned to expose the platinum mesh 23. The cell 10 wa thereafter sintered for approximately 50 hours in an atmos phere of about 100 parts per million sulfur dioxide in air ' he preferred electrolytes in the two-phase £?<-Li2S0 an (Ag,Li)2S04 solid solutions range were fabricated by usin a mixture of approximately 23 mole percent Ag2S04 and 7 mole percent Li2S04« Single-phase electrolytes, one abou 5 mole percent Ag2S04 and 95 mole percent Li2Sθ4 in th <= χ -Li2Sθ4 solid solution region and the other about 55 mol percent Ag2S04 and 45 mole percent Li2S0 in (Ag, LD2SO solid solution region, were also prepared for comparison Sintering temperatures were about 730 β C for 95 mole percen Li2Sθ4~5 mole percent g2S04 electrolytes and about 530 to 540°C for the preferred 77 mole percent Li2S04-23 mol percent Ag2SC>4 and the remaining 45 mole percent Li2S04~5 mole percent Ag2Sθ4 electrolytes. The former cell was operate at a temperature of about 700°C. while the latter two cells, including the preferred type, were operated at a temperatur of about 530°C. An apparatus for detecting SO2 using the integra solid cells 10 of FIG. 1, was fabricated as is depicted i FIG. 2. Sample gas, again indicated by arrows 22, was cir culated to the detector electrode surface 12a of the cel 10, where platinum mesh 23 was again exposed, by means of pair of concentric silica (Siθ2) tubes 24 and 25 wh: O where centered on the surface 12a. Silica was preferred for its inertness, thermal shock capability and price, but other materials and other means to channel sample gas to and from the detector electrode surface 12a of the cell may be used. The tubes 24 and 25 were held against the surface 12a of the cell by an arrangement of tension springs and wires (not depicted) which resulted in an essentially gas type seal between the surface 12a and the end of the tubes 24 and 25. Again, a gold detector electrode lead 20 was welded to the mesh 23 and run along the innermost tube 25 and through an opening (not depicted) . A second, gold electrode 16 was again embedded in the solid reference electrode portion of the cell 10. The apparatus was placed in an electric furnace, depicted illustratively by its furnace tube 28. The leads 16 and 20 were connected electrically with a potentiometric measuring device 26 located outside the furnace 28. The temperature of the integral cell 10 was monitored by a thermocouple 30. A sample gas 22 was conducted to the detection electrode face 12a of the cell 10 in the gap formed between the concentric silica tubes 24 and 25 passed through openings 25a at the base of the innermost tube 25 and along the length of the tube 25 away from the cell 10 to a venting apparatus (not depicted) . The vanadium pentoxide powder 34 is provided in the gap between the two tubes 24 and 25. Quartz wool 36a and 36b was packed, around the gap at either end of the powder 34 to keep the powder in position and prevent it from contact¬ ing the cell 10. The vanadium pentoxide powder 34 was the primary catalyst used to obtain equilibrium mixtures of SO2, SO3 and O2 in the preferred and other cell 10 embodiment operated below 600°C. (specifically, at 530°C). In the one exemplary embodiment cell 10 operated at about 700°C. (5 mole percent silver sulfate-95 mole percent lithium sulfate) a large mass of platinum wire (not depicted) was substituted for the vanadium pentoxide catalyst 34 in the gap between the concentric tubes 24 and 25 upstream from the . detector electrode surface 12a. The test furnace 28 provided an essentially constan (_+ 1°C.) temperature zone within which the cell 10 was posi tioned. Test gases were supplied with SO2 concentration of 20, 100, 204, 500, 1000 ppm and 10,000 ppm (1%) by volum and were mixed with air in a mixing chamber (not depicted) before being passed into the tube 24 to vary the compositio of tested gases. The emf was measured by a high impedanc potentiometric electrometer coupled with a chart recorder. The presence of a catalyst was necessary to obtai equilibrium between SO2, SO3 and O2 for the detection o SO2 in the tested gases. When a gold mesh was substitute for platinum mesh 23 of FIG. 1 the values of the emf wer less than theoretical values when the vanadium oxide catalyst was also removed. It can be concluded therefore that the electrochemical reaction involves preferably SO3 rather than SO2 and that conversion of SO2 into SO3 is required for pre¬ ferred use of the sensor. Two kinds of catalysts were success¬ fully employed as described, a 0.2 mm diameter platinum wire and vanadium pentoxide powder. The- emf of the cell 10 is stable and . reproducible, especially for the preferred, two-phase lithium sulfate-silver sulfate electrolyte sensors. In order to check the reversi¬ bility of their emf, a small external current (6 microamps) was passed through a two-phase electrolyte cell in both directions for 30 seconds. The emf was restored to the original values (within + 0.5 its/) in 2 minutes. The results of the emf dependence ' of two embodiments of the galvanic cells 10 on SO2 compositions " of inlet gases are depicted graphically in FIG. 3. The horizontal axis of the fig¬ ure represents the inlet gas SO2, ppm concentrations (by vol¬ ume) in air, which is proportional to the partial pressures of SO3 if equilibrium among SO2, SO3 and O2 is achieved. The solid lines are the measured emf responses for the preferred, embodiment cell (77 mole percent lithium sulfate-23 mole percent silver sulfate electrolyte with a solid reference electrode formed from. 2/3 by weight powdered silver with τ /^U by weight electrolyte) and a second, single phase cell embodi¬ ment (45 mole percent lithium sulfate-55 mole percent silver sulfate solid electrolyte with a solid reference electrode) formed from 2/3 by weight silver and 1/3 by weight electrolyte mixture. Both cells were fabricated in the manner previously described and operated using a device comparable to that depicted in FIG. 2 at 530°C. The preferred, two-phase embodi¬ ment was used to measure concentrations from a few ppm to more than 10,000 ppm SO2 in equilibrium with O2 and SO3. It is believed that an even greater range of measurement can be achieved using the preferred, two-phase system. The measured emf responses of the cells appear to be within about.5 millivolts of the theorectical values across the range of concentrations depicted. Interference in the SO2-SO3 response of the preferred two-phase electrolyte sensors was tested by using the aforesaid two-phase (77 mole % lithium sulfate-23 mole % silver sulfate) electrolyte sensor at 530°C with various concentrations of carbon dioxide gas (between about 8 and 24% by volume) added to concentrations of between 10 and 1000 part per million (by volume) of SO2 in air. Variations between the measured emf response of the galvanic cell 10 and the theoretical values for the cell were less than 3 millivolts. The preferred two-phase electrolyte cell 10 was also tested for sensitivity to water vapor. Cell 10 using the preferred, • two-phase 77% lithium sulfate-23% silver sulfate electrolyte was again tested at 530 C C with concentrations of about 110 part per million SO2 (by volume) and between about 0.5% and 7.3% (by volume) H2O in air. The maximum variation of the cell 10 produced emf from predicted theoretical emf values with added water was about one millivolt. It is suggested, however, for longevity of the sensor that water vapor content of the sample gas be reduced to the lowest possible levels as pitting was noticed on the detector electrode surface 12a of the cell 10. FIG. 4 depicts the results of a long term stability test to which a preferred, two-phase electrolyte (77% lithium sulfate-23% silver sulfate) cell 10 having a solid reference electrode of the same two-phase electrolyte mixture (one-thir by weight) and silver (two-thirds by weight) is currentl being subjected. Solid lines indicate the emf response of th two-phase system for identified concentrations of SO2 (b volume) in air plotted for the days when each such indicate concentration was measured by the sensor. The emf response and the calculated, theoretical emf values for the cell 1 are indicated in parentheses . The sensor has been operated continuously for 121 consecutive days to date. The vertical lines denote when changes have been made in the SO2 concen¬ trations. The . emf responses of the sensor have been within about 5 millivolts of the calculated, theoretical response of the cell for the gas concentrations employed. The emf stabil¬ ity achieved to date by this two-phase galvanic cell detector is far superior to that reported by Gauthier et al in the a- forereferenced article "Progress in Development of Solid-State Sulfate Detectors for Sulfur Dioxide" (1981) for three SO2 detectors using a potassium sulfate electrolyte with silver and/or silver sulfate solid reference electrodes. The results are also superior to those reported in the same article for other, more complicated SO2 detectors employing magnesium sul- fate-magnesium oxide (MgS04~MgO) or maganese sulfate-manganese oxide (MnSθ4~MnO) solid reference electrodes or potassium sul- fate-zirconia based electrolytes. Based upon the encouraging results to date it is envisioned that the preferred two-phased lithium sulfate-silver sulfate detector ( ~ - ' ±2SO^ and ( g,Li)2 SO4 solid solutions) will produce emf responses within a few millivolts of predicted theoretical values for at least 6 months or more. It is further envisioned that the described sulfur dioxide detectors may be used with an oxygen detector in the manner described in yet another Gauthier et al article entitled "Solid-State Dectectors for the Potentiometric Determination of Gaseous Oxides", Journal of the Electrochem¬ ical Society: SOLID-STATE SCIENCE AND TECHNOLOGY, October 1977 (pp. 1584-1587), for the measurement of SO2, SO3 or other gaseous anhydrides in oxygen variable gases. _, , ^ OM It will be appreciated by one skilled in the ar that although the described, single—phase lithium sulfate silver sulfate electrolyte solid state galvanic cells d not provide the long-term stability of the preferred, two- phase systems, each does, however, provide response and stability at operating temperatures significantly less than about 700°C to rival the use of the previously referred to potassium sulfate-silver sulfate and sodium sulfate-silve sulfate galvanic cells, which require operating temperatures in excess of 700°C for short term measurements. - Moreover, it will be appreciated by one skilled in the art that although the described galvanic cells have been described as being used with a potentiometric device to measure SO2 concentra¬ tions, the potential difference of the cell may be used in other ways such as through an alarm circuit or light to merely indicate the presence of SO2 (or SO3) or a minimu concentration of those gases. While preferred embodiments of the invention have been described, changes to the composition, form and compo¬ nents of the solid state integral sensors and the detection apparatus of this invention may be made by one skilled in the art without departing from the scope of the invention which is more fully defined in the following claims. 'BU

Claims: WHAT IS CLAIMED: 1. An apparatus for detecting the presence of gaseous inorganic acid anhydrides in a gas containing oxy¬ gen comprising: a galvanic cell, having a solid electrolytic element formed from a - pair of metallic salts, at least one of the metallic salts providing the electrolytic element with oxy-anions of the particular inorganic acid anhydride being detected; a reference electrode in electrical con¬ tact with said solid electrolyte element; a detector electrode spatially separated from said reference electrode and in electrical contact with, said electrolytic element, said detector electrode being adap¬ ted to provide a difference in potential when an inorganic acid anhydride is in contact with the detector electrode and the electrolytic element; the pair of metallic salts of the electro¬ lytic element being provided in relative proportions to form a two-phase solid mixture substantially throughout the volume of the electrolytic element at operating temperatures of the galvanic cell; and means electrically connected to said reference and detector electrodes for signalling the presence of the potential difference of the galvanic cell. 2. The apparatus of claim 1 wherein said means com¬ prises a potentiometric detector connected between said refer¬ ence electrode and said detector electrode permitting measure¬ ment of the concentratration of said anhydride to be detected by the measurement of said difference in potential. O 3. The apparatus of claim 1 further comprising cata lyst means for converting the gaseous inorganic acid anhydrid to be detected into a second inorganic acid anhydride 4. The apparatus of claim 1 wherein said gaseou inorganic acid anhydride is sulfur trioxide or sulfur dioxid and one of said pair of metallic salts of the electrolyti element is lithium sulfate. 5. The apparatus of claim 4 wherein the remainin metallic salt of the electrolytic element is silver sulfate. 6. The apparatus of claim 5 wherein said operatin temperature is less than about 700 β C. 7. The apparatus of claim 6 wherein said operatin temperature is between about 515°C and 560 β C. 8. The apparatus of claim 1, 2, 3, 4, 5, 6, or wherein the electrolytic element is composed of between abou 64 and about 80 mole percent lithium sulfate and about 36 t about 20 mole percent silver sulfate. 9. The apparatus of claim 1 wherein said referenc electrode comprises a solid mixture of a first compound of metal mixed with the same metal or with a second compound o the same metal. 10. The apparatus of claim 1 wherein said referenc electrode comprises a solid mixture of said pair of electrolyt metallic salts and the metal of one of said pair of metalli salts. 11. The apparatus of claim 5 wherein the said ref¬ erence electrode comprises a solid mixture of said lithiu sulfate and silver sulfate electrolytic element salts and silver. 12. The apparatus of claim 11 wherein said electro lytic element and reference electrode are formed as a singl integrated element by combining powdered silver, silver sulfat and lithium sulfate under pressure. 13. The detection apparatus of claim 12 wherein sai integrated element is sintered in an atmosphere containing sul fur dioxide. 14. The apparatus of claim 4 further .comprisin catalyst means for converting gaseous sulfur dioxide into a equilibrium mixture of sulfur dioxide, sulfur trioxide an oxygen near the detector electrode. 15. The apparatus of claim 14 wherein said catalys means comprises platinum wire embedded in the electrolytic ele ment and exposed at the detector electrode surface of th electrolyte. 16. The apparatus of claim 14 or 15 wherein sai temperature value is less than about 600°C and said catalys means comprises vanadium pentoxide. . - -- > 17. In a method of measuring the concentration o an organic acid anhydride in the form of a gaseous compoun present in an oxygen containing gas comprising the steps of providing a solid electrolyte containing oxy-anions of th particular anhydride being detected; providing a referenc electrode and electrical contact with said solid electrolyte; providing a detector electrode and electrical contact wit said solid electrolyte and spatially separated from said refer ence electrode, said electrolyte and two electrodes providin a difference in potential across said two electrodes whe said anhydride to be detected is in contact with said detecto electrode and said solid electrolyte; stabilizing said soli electrolyte at an operating temperature .below the meltin point σf said solid electrolyte; and measuring the potentia between said two electrodes, the improvements comprising th steps of: forming said electrolyte from a mixture com prising two metallic salts, at least one of said salts con taining oxy-anions of the particular anhydride being detected and said two salts being provided in suitable proportions t form a two— hase solid mixture substantially throughout th volume of the electrolyte at said operating temperature 18. The improvement of claim 17 wherein said gaseou anhydride compound is sulfur dioxide or sulfur trioxide an said step of- forming said electrolyte comprises forming sai electrolyte from a mixture of lithium sulfate -and silve sulfate. 19. The improvement of claim 18 further comprising catalyzing said sulfur dioxide or sulfur trioxide into a equilibrium mixture of sulfur dioxide, sulfur trioxide an oxygen at said detector electrode. 20. In a galvanic cell comprising a solid electro lyte formed from a mixture of two metallic salts and a pai of electrodes in electrical contact with said solid electro lyte, the improvement wherein one of said electrodes i formed from a solid mixture of said two electrolyte metalli salts and the metal of one of said two metallic salts 21. The improved galvanic cell of claim 20 wherei * said two metallic salts are provided in relative proportion s . as to form a two-phase solid electrolyte at the operatin temperature of the cell and said relative proportions of th two metallic salts to one another are the same in the electro lyte and said solid reference electrode. 22. An improved galvanic cell comprising: a soli ' electrolyte formed from a mixture of silver sulfate and lithiu sulfate; a first solid electrode formed from a solid mixture of silver, silver sulfate and lithium sulfate in electrical contact with said solid electrolyte, .and a second electrode in electrical contact with said electrolyte electrochemically responsive to sulfur dioxide. 23. An improved galvanic cell comprising: a solid electrolyte formed from a mixture of silver sulfate and lithium sulfate; a first solid electrode formed from a solid mixture of silver, silver sulfate and lithium sulfate in electrical contact with said solid electrolyte, and a second electrode in electrical contact with said electrolyte electrochemically responsive to sulfur trioxide. -.- r r ' - R £ -iU ΪΓ -Γ - S a * 'J - L-. C .-- l f OM

Inventors: QING-GUO LIU, WORRELL WAYNE L, QING-GUO, LIU, WORRELL, WAYNE, L.
Applicants: UNIVERSITY PATENTS INC, UNIVERSITY PATENTS, INC.
Application year: 1982
Main classification: G01N27
IPC codes: G01N27