Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/403—Cells and electrode assemblies
  • G01N27/406—Cells and probes with solid electrolytes
  • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
  • G01N27/4073—Composition or fabrication of the solid electrolyte
  • G01N27/4074—Composition or fabrication of the solid electrolyte for detection of gases other than oxygen

Definitions

• 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 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.
• SO gas reacts to sulfurous and eventually sulfuric acid causin considerable corrosion problems and the phenomenon known a "acid rain”.
• various techniques, includin electrochemical techniques have been developed for detectin the presence of S ⁇ 2»
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• galvanic cell 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.
• a sulfur dioxide detector 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.
• 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;
• FIG. 4 depicts graphically the least square values of the measured potentiometric response of a preferred, two-phase
• O ⁇ PI lithium sulfate-silver sulfate electrolyte galvanic cell for indicated SO2 equilibrium concentrations in air tested on a daily basis by the same galvanic cell.
• 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.
• 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 ⁇ C.
• a two-phase system consisting of solid solutions of c ⁇ Li2S04 and (Ag,Li)2S ⁇ 4, which exists between about 510" and 560 ⁇ 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.
• 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.
• the electrolytic element and reference electrode may preferably be formed as a single integrated element so as to
• 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.
• 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.
• 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 may be used as solid reference electrodes for solid galvanic cells.
• the silver-silver sulfate (Ag-Ag2S ⁇ ) 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 ⁇ C; its sulfate ( g2S ⁇ 4) , is also stabl
• g2S ⁇ 4 its sulfate
• 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 Li2S ⁇ 4 has an FCC structure at high temperature a g2 ⁇ 4 a rhombic structure.
• Li2S ⁇ system over th Na2S ⁇ 4 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.
• 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.
• 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.
• 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
• 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.
• 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 Li2S ⁇ and g2S ⁇ 4 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 Li2S ⁇ 4 ⁇ Ag2S ⁇ 4 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.
• the cell 10 wa removed from the mold and the gas electrode surface 12 cleaned to expose the platinum mesh 23.
• 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).
• 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 was less than theoretical values when the vanadium oxide catalyst was also removed.
• The- emf of the cell 10 is stable and . reproducible, especially for the preferred, two-phase lithium sulfate-silver sulfate electrolyte sensors.
• 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 ⁇
• 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.
• 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.
• 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.
• 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.

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