Sulfate solid electrolyte sensors

Also for sensors with sulfate electrolytes in the region of application above 8(X)°C no stable Ag/Ag reference electrodes could be found (37). Better results were achieved with the solid state reference systems Mg0,MgS04 and Mn0,MnS04 Again, in the case of sulfate solid electrolyte sensors the most stable and lelieable reference electrodes were those with flowing reference gas. 

For direct contacts between oxide-ion-conducting and sulfate solid electrolytes equations analogue to Equations (25-75) and (25-76) are valid. The sensitivity of such contacts were examined in detail with and without catalysts (34] because without a metallic electrode phase a better long-term stability of the sensors with sulfate solid electrolytes is expected. 

Electrolytes are used in electrochemistry to ensure the current passage in -> electrochemical cells. In many cases the electrolyte itself is -> electroactive, e.g., in copper refining, the copper(II) sulfate solution provides the ionic conductivity and the copper(II) ions are reduced at the - cathode simultaneous to a copper dissolution at the - anode. In other cases of -> electrosynthesis or - electroanalysis, or in case of - sensors, electrolytes have to be added or interfaces between the electrodes, as, e.g., in case of the -> Lambda probe, a high-temperature solid electrolyte. 

Thermodynamic equilibria are also of principal importance, if one wishes to determine carbon dioxide or sulfur dioxide with carbonates or sulfates which show conductance for sodium or potassium ions. It is not the ion migrating through the solid, but the electrochemical equilibrium between molecules in the gas phase, particles in the solid electrolyte and electrons in the electrical conductor that determine the electrode potential utilizable in sensors. 

For research into the manufacture of gas sensors of a potentiometric type, we have tested different solid electrolytes. The unique property of these sensors is that the two different metalhc electrodes ate located in the same gaseous phase. This property has prompted us to study particularly the beta-alumina and calcium sulfate. 

A solid-state sensor employs a soHd alkaH sulfate electrolyte at an elevated temperature (800°C). 

A similar situation occurs with sensors based on several types of solid electrolytes (Fergus 2008). For example, carbonate and sulfate electrolytes could be used with CO and SO sensors. However, those electrolytes generally do not provide adequate stability (see Chap. 6 (Vol. 1)), and therefore the most promising sensors use common electrolytes, such as Nasicon, P-alumina, and yttria-stabilized zirconia (YSZ). These electrolytes require auxiliary electrodes to provide the desired response, but they provide good stability and long operating lives. Therefore, while optimizing the reactions responsible for a gas sensor s sensitivity, one should also aim to maximize the chemical, structural and time stability of the device. 

In these solid electrolytes, the species migrating in the solid phase are sodium ions and not suliiir oxides. Therefore, by using this chemistry, we can devise an appropriate sensor arrangement. The gas to detect is SO2 and the ionic state of SO2 gas is SO . As Na+ is the mobile cation in the solid electrolyte, sodium sulfate, which is the combination of both Na and SO ", should be formed on the solid electrolyte surface. In the sulfur dioxide gas sensor with those solid electrolytes, sodium sulfate is used to detect SO2 gas. 

Shiokawa, 1985b, J. Electrochan. Soc. 132,2519. Imanaka, N., G. Adachi and J. Shiokawa, 1986a, A solid electrolyte for sulfur dioxide detection, sodium sulfate mixed with rare earth sulfates and silicon dioxide, ACS Symposium on Chemical Sensors -Fundamentals and Applications (American Chemical Society) p. 121. 

Salto Y, Maruyama T, Matsumoto Y, Kobayashi K, Yano Y (1984) Applicability of sodium sulfate as a solid electrolyte for a sulfur oxides sensor. Solid State Ion 14 273-281... 

A new solid state chemical sensor for sulfur dioxide utilizing a sodium sulfate/rare earth sulfates/silicon dioxide electrolyte has been developed. The addition of rare earth sulfates and silicon dioxide to the sodium sulfate electrolyte was found to enhance the durability and electrical conductivity of the electrolyte. The electrolyte exhibits a Nernstian response in the range of SC gas concentrations from 30 ppm to 1 %. 

Worrell, W.L. and Liu, Q.G. (1984) A new sulfur dioxide sensor using a novel two-phase solid-sulfate electrolyte. 

As a practical test for on-site application of the sensor for SO2 detection, a prototype probe sensor was fabricated. A sectional view of the probe sensor is illustrated in fig. 57 (Adachi and Imanaka 1991). In this type of probe, a mullite tube is utilized instead of the more expensive quartz tube. In order to prevent the chemical reaction between the sulfate electrolyte and the solid reference, an alumina plate was placed between them, as for the case of the SO2 sensor with the solid reference reservoir (sect. 5.6). The measurement was... 

Worrell, W.L., 1983, The application of solid-sulfate electrolytes in SO2/SO3 sensors, in Proc. Int. Meeting on Chemical Sensors, eds T. Seiyama, K. Fueki, J. Shiokawa and S. Suzuki (Kodansha/... 

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