TIIE COYTACT PROCESS. In the contact process the method of production is directly synthetical, and acids of any desired strength may be made in one operation. In this process the burner gas must be first rendered absolutely free from everything other than sulphur dioxide. oxygen, and inert nitrogen, the usual impurities of burner pis, such as dust, moisture, arsenic, sele nium, etc., derived from the raw material used, seriously interfering with the process. After purification, the gas is passed through a sub stance possessing the faculty of causing sulphur dioxide to oxidize into sulphur trioxide, without apparently suffering any change whatever in its own condition. This contact must be brought about under conditions as to temperature ac cording to the substance used. The contact substances used are finely divided platinum in various forms, such as platinized asbestos and pumice, etc., crusts formed of platinum and some soluble sulphate, ferric oxide, cupric sulphate, diatomaceous earth, etc. The transformation of sulphur dioxide into the trioxide takes place best between 200° and 450° C. (392°-S42° F.).
Above 450° C. the transformation grows incom plete, for at that temperature commences the dissociation of sulphur trioxide into dioxide and free oxygen—a dissociation which is complete at 900° C. The formation of sulphur trioxide hay strongest acid which can be produced by the chamber process will correspond to from 50 per cent. to GO per cent. of sulphur trioxide. Strong er acids must be produced by driving off the contained water by heat or by isolating the acid by freezing. It is thus possible to produce the concentrated acid (oil of vitriol) of commerce, containing front 93.5 to 97 per cent, of pure sul phuric acid; also distilled acid, containing up ing been attained, it simply remains either to condense it at the proper temperature in its crystalline condition or to absorb it in sulphuric acid or water until a solution of the required strength is obtained.