GB850379A - Improvements in lead dioxide electrodes - Google Patents

Abstract

850,379. Electrodeposition of lead dioxide; electrolytic production of per-salts and chromic acid. PENNSALT CHEMICALS CORPORATION] Dec. 12, 1956 [Dec. 14, 1955], No. 37935/56. Class 41 Lead dioxide is electrodeposited on a metal screen or grid made of wire of diameter from 3 to 80 mils or by perforating a thin metal sheet and presenting a large surface area relative to its volume so as to fill completely the screen interstices thereby forming an electrode for electrochemical processes, e.g. an anode for oxidation of chlorates to perchlorates, chromic sulphate to chromic acid, sodium chloride to sodium perchlorate, and of sulphates to persulphates. The metal grid may be of steel, nickel, "Monel" (Registered Trade Mark) metal, tin, magnesium, aluminium, copper, or tantalum and prior to plating is freed from oxide by chemical or mechanical means. The lead dioxide may be anodically deposited using a carbon cathode from any of the following aqueous lead salt solutions containing (a) 50 to 700 gms. lead nitrate, 0À5 to 30 gms. copper nitrate trihydrate with or without 0À5 to 10 gms. of an alkyl aryl polyether alcohol, e.g. an alkyl phenoxy polyoxyethylene ethanol per litre of solution; (b) 50 to 500 gms/litre lead perchlorate to give a non- alkaline solution, preferably of pH 0À8 to 5; (c) an alkaline lead tartrate solution, e.g. of pH 12, formed by a water-soluble tartrate other than that of lead, e.g. Rochelle salt, a base such as sodium, potassium or ammonium hydroxide, and lead oxide, the molecular ratio of alkali metal hydroxide to lead tartrate being in the range 2À7 to 8À1 and the Rochelle salt being up to 50% molecular in excess of the lead oxide content which provides a lead concentration of 10 to 93 gms/litre. Preferably deposition is effected at temperatures of 60 to 75‹C. A resist may be applied to desired areas of the grid to prevent deposition at these points and plastic shielding may be provided near the edges to prevent treeing. When the electrode is used in processes. in which the supporting grid metal is soluble, e.g. nickel or iron grids in chlorateperchlorate reduction, under slightly alkaline or neutral conditions some of the contaminant metal plates out at the cathode while some precipitates as the hydroxide but if acid conditions are employed the contaminated solution is rendered alkaline to precipitate the metals as hydroxides. The electrode may have a copperover-silver contact applied to it as described in Specification 797,644 or a block of Woods metal cast on to its top to form a contact.