Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-07-13
2001-06-26
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S431000, C204S432000, C205S794500, C205S775000
Reexamination Certificate
active
06251244
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an electrochemical measuring cell for detecting hydride gases and especially arsine and phosphine. The measuring cell includes at least one working electrode made of a catalytically inactive material and a reference electrode in an electrolyte chamber filled with an electrolyte. The electrolyte chamber is closed off with respect to the gas to be detected by a diffusion membrane.
BACKGROUND OF THE INVENTION
An electrochemical measuring cell for detecting hydride gases such as phosphine and arsine is disclosed in U.S. Pat. No. 5,128,018. In this known measuring cell, a measuring or working electrode, a reference electrode and an auxiliary electrode are mounted in an electrolyte chamber of a measuring cell housing. The electrolyte chamber is closed off with a gas permeable diffusion membrane toward the ambient which contains the gas sample to be measured having the hydride gas to be detected. Sulphuric acid with catalyzing additives is used as an electrolyte.
Although the known measuring cell is characterized by a high sensitivity, cross sensitivities are present relative to a series of other gases. In specific applications, these cross sensitivities are especially disturbing or can lead to false alarms. In addition to phosphine and arsine, the known measuring cell also reacts to NO
2
, H
2
, C
2
H
2
, SO
2
, H
2
O
2
and O
3
.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electrochemical measuring cell of the kind described above which is improved with respect to the cross sensitivity to other toxic gases.
The electrochemical measuring cell of the invention is for detecting hydride gases including arsine and phosphine. The electrochemical measuring cell includes: a housing having an opening directed toward the hydride gas to be detected and defining an electrolyte chamber; an electrolyte disposed in the electrolyte chamber; the electrolyte containing sulphuric acid and silver sulphate as an electrolyte additive; a working electrode configured as a carbon electrode; a reference electrode mounted in the electrolyte chamber so as to be in spaced relationship to the working electrode; and, a diffusion membrane permeable to the hydride gas mounted in the housing to define an interface between the hydride gas and the electrolyte chamber.
The advantage of the invention lies in the surprising realization that the selectivity of the detection of arsine and phosphine compared to other gases, such as H
2
, C
2
H
2
and NO
2
, is significantly improved by a combination of a working electrode made of carbon with an electrolyte containing sulphuric acid to which silver sulphate is added as an electrolyte additive. Phosphoric acid can also be used as an electrolyte as an alternative to sulphuric acid with the phosphoric acid containing silver phosphate as an electrolyte additive.
The electrochemical reaction within the measuring cell takes place in such a manner that the phosphine or arsine to be detected reacts selectively with the electrolyte additive within the electrolyte chamber and the reaction product is then detected on the working electrode. By utilizing a working electrode made of carbon, a measuring cell of this kind supplies no or a very slight measuring signal in the presence of toxic gases such as NO
2
and SO
2
. An electrochemical measuring cell, which contains sulphuric acid as an electrolyte and silver sulphate as electrolyte additive is distinguished by a high sensitivity and short response time with respect to the detection of arsine and phosphine.
The measuring cell according to the invention can be configured as a two-electrode measuring cell or also as a three-electrode measuring cell having an additional auxiliary electrode. The electrodes are then connected to a potentiostat as known per se.
It is especially purposeful to use, as an electrolyte, a mixture of phosphoric acid and sulphuric acid with silver phosphate as an electrolyte additive.
It is advantageous to use the electrolyte additive in saturated solution. Measuring cells configured in this manner have an especially high long-term stability. A saturated solution of the electrolyte additive is advantageously achieved in that the electrolyte additive is present as a solid phase in the electrolyte chamber. In this way, consumed electrolyte additive can be continuously replaced by the solid phase. The solid phase can be, for example, a sediment or a precipitate.
The carbon electrode is purposefully configured as a thick film electrode. The carbon electrode can be cut from a plate-shaped material. The preferred thickness of the carbon electrode lies between 1 to 2 mm. Carbon paper, glassy carbon or porous glassy carbon can also be used as a material for the working electrode.
A sintered carbon electrode has been shown to be especially advantageous and comprises a mixture of carbon and PTFE (polytetrafluoroethylene). An electrode of this kind can be applied to the porous PTFE diffusion membrane with a casting process or with a screen printing process.
The thickness of the sintered carbon electrode preferably lies between 100 &mgr;m and 1,000 &mgr;m.
A carbon electrode made of a sintered mixture of carbon and gold provides especially good results.
The reference electrode and the auxiliary electrode can be made of gold, platinum, iridium, silver, ruthenium, rhodium or palladium.
REFERENCES:
patent: 4571292 (1986-02-01), Liu et al.
patent: 5128018 (1992-07-01), Kiesele
patent: 5316648 (1994-05-01), Kühn et al.
patent: 5997706 (1999-12-01), Kiesele et al.
patent: 3923717 (1991-01-01), None
patent: 0239190 (1987-09-01), None
patent: 0400488 (1990-12-01), None
patent: WO93/10444 (1993-05-01), None
patent: WO96/33404 (1996-10-01), None
Kiesele Herbert
Mett Frank
Drager Sicherheitstechnik GmbH
Ottesen Walter
Tung T.
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