Electrochemical cell for detecting hydrogen sulphide in a gaseous mixture
The invention relates to an electrochemical cell for detecting hydrogen sulphide in a gaseous mixture, the cell having three electrodes (a work electrode, a reference electrode and an auxiliary electrode) in a polarographic arrangement in an organic, electrically conductive electrolyte. 
     Electrochemical cells for detecting a gas in a gaseous mixture are particularly important in the field of pollution control. They have to determine when dangerous concentrations are exceeded and they must therefore be sensitive without maintenance and constantly operating for a prolonged period of time. An electrochemical cell is described in our copending U.S. application Ser. No. 804,699 in which the work and reference electrodes are either silver/silver iodide electrodes or silver/silver chloride electrodes, the auxiliary electrode is made of silver, and the electrolyte is a slightly alkaline organic electrolyte. The cell described in that patent application has the disadvantage that the sensitivity of the cell diminishes with time since the auxiliary electrode is passivated as a result of hydrogen elimination and the alternation of the work electrode. 
     The object of the invention is to provide an electrochemical cell for detecting hydrogen sulphide in a gaseous mixture in which the sensitivity changes as little as possible with time. 
     According to the invention, there is provided an electrochemical cell for detecting hydrogen sulphide in a gaseous mixture, the cell comprising a work electrode, a reference electrode and an auxiliary electrode in a polarographic arrangement in an organic electrically conductive electrolyte, wherein the work electrode and the reference electrode are silver/silver sulphide electrodes and the electrolyte comprises two gel-like films one of which is adjacent the auxiliary electrode, the two films being of identical composition except that the film adjacent the auxiliary electrode contains a depolarizer. 
     In these electrochemical cells, the cathode (auxiliary electrode) is not passivated. The sensitivity is more nearly constant when using silver/silver sulphide electrodes, than when using the hitherto conventional silver iodide or silver chloride electrodes, since in the former case there is a gradual partial conversion into silver sulphide. 
     In an advantageous embodiment, the auxiliary electrode is a copper foil and it is surrounded with an electrolyte containing lead dioxide or another heavy metal oxide of high valence. The nascent hydrogen formed is oxidised to produce water and the auxiliary electrode is not passivated. This may be ascertained by the fact that the internal resistance of the cell does not increase. 
     The electrolyte preferably comprises a mixture of polyvinyl chloride, propylene carbonate, diethyl phthalate and potassium hexafluorophosphate. It has a gel-like consistency and does not separate even after a long period of operation. A suitable mixture is composed of for example 10 to 20%, preferably 16%, by weight of polyvinyl chloride, from 20 to 30%, preferably 28%, by weight of propylene carbonate, from 50 to 60%, preferably 55%, by weight of diethyl phthalate and 1% by weight of potassium hexafluorophosphate. 
     An electrochemical cell according to the invention is shown in the drawing by way of example and is described in more detail below. 
     A copper auxiliary electrode 2 rests on the base of a cylindrical container 1 made of polypropylene. An electrolyte 3 directly above the auxiliary electrode is composed of the above-mentioned mixture and additionally contains from 2 to 10%, preferably about 5%, by weight of lead oxide. 
     The electrolyte is a gel-like film having the consistency of soft rubber and is pressed against the copper auxiliary electrode. An electrolyte film 4 which is free of lead dioxide is pressed upon this electrolyte. The electrolyte film 4 has a silver/silver sulphide reference electrode 5 of spiral form. A reticular silver/silver sulphide work electrode 6 is pressed when warm into the electrolyte from above. The cell is sealed by means of a glass frit 7. A volume of gas which is about 1 mm thick remains between the frit 7 and the work electrode 6. Gas exchange takes place by diffusion and a forced gas supply is not required.