Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Patent
1992-09-28
1995-04-04
Niebling, John
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
20415314, 20415316, 204412, 204425, 204426, 204428, G01N 2726
Patent
active
054034523
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for determining the concentration of gases in a mixture of gases by means of a gas sensor having a solid electrolyte.
A plurality of embodiments of electrochemical gas sensors is based on watery electrolytes or polymeric solid electrolytes (U.S. Pat. No. 4,227,984). The materials as per the latter are water containing proton conductors.
The use of a watery or water containing electrolyte limits severely the field of use for the sensors. Because the water in the electrolyte evaporates even at temperatures below 100.degree. C. and the, conductivity of the electrolyte drops sharply.
Another drawback of these gas sensors using a water containing electrolyte is the considerable expenditure needed for miniaturization and realization of a compact design ( British patent application 22 28 327).
The measuring effect of such sensors is based on the electrochemical oxidation or reduction of the gas component to be detected and on the formation of protons or water at a measuring or a counter electrode. The selectivity of these sensors is thus limited by the oxidation or the reduction potential of the gas species to be detected.
It is known that the concentrations of gases in a mixture can be measured by means of ion-conductive materials provided certain conditions are met ( see H. Rickerr "solid ion conductors principles and application" in a journal titled Angew. Chem. (applied chemistry) vol. 90 pages 38-48, 1978). For that purpose a galvanic chain is constructed being comprised of two electrodes and a solid electrolyte wherein one electrode, the so called measuring electrode, is exposed to the gas whose composition is unknown and the other electrode is held to constant conditions through appropriate features. In order to obtain a suitable measuring signal essentially two measuring principles are used: without any current flow if at all possible or (potentiostatic or amperic measurement)
A known example for the principle a) is the lambda probe using zirconium dioxide ZrO.sub.2 as solid electrolyte. This probe is used for measuring the concentration of oxygen. The counter electrode is exposed to the relatively constant partial pressure of oxygen in air.
Such measurements can also be conducted when the solid electrolyte cannot directly transmit the ions of the gas to be detected (German patent 29 26 172 C2). Furthermore it is not always necessary to expose the counter electrode to a reference gas, but the latter electrode may be comprised of a material which contains the ions of the solid electrolyte either as a chemical compound or in elemental form. The partial gas pressure is determined by means of such a galvanic chain and uses the chemical reaction of the gas to be detected with an electric-active ion of the solid electrolyte. For this an electric voltage (EMF) is measured between the reference and the measuring electrodes, a reaction takes place at the latter. The higher the partial pressure of the gas reacting with the ion the larger is the electric voltage.
The dependency of the partial pressure is determined by the Nernst equation:
The parameter E.sub.r contains the portion of the counter electrode that is independent from the partial pressure. and the parameter Z combines all terms that are attributable to other gas components. The quantity E.sub.0 contains the Gibbs reaction enthalpy to be calculated for the reaction of the gas with the ions of the solid electrolyte by means of the equation G/(n*F)=-E.sub.0.
The quantity G informs about the thermodynamic stability of a compound. The ions of the solid electrolyte may combine with different gases in differently stabile compositions.
For example, sodium ions can be transferred through the solid electrolyte Na-.beta.-alumina and together with sulfur-dioxide in the presence of oxygen it will form sodium sulfate or together with carbon-dioxide it will form sodium carbonate. In the most simple case the compound with the largest reaction enthalpy will form under conditions given
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patent: 4269684 (1981-05-01), Zick
patent: 4400260 (1983-08-01), Stahl et al.
patent: 4820386 (1989-04-01), La Conti et al.
patent: 4900425 (1990-02-01), Sasayama et al.
patent: 5215643 (1993-06-01), Kusanagi et al.
Hielscher Bernd
Horn Berthold
Schmah Martin
Bell Bruce F.
Hartmann & Braun
Niebling John
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