SULPHANILIC ACID, Para-Aminoben , 411 zene Sulphonic Acid, C4I-14 sa Hp., or, / N H, C4114 \ /, was first prepared by Gerhardt in 1845. It is usually obtained by heating ani line with fuming sulphuric acid at 180° C., for several hours. Upon pouring the reaction mix ture into cold water a fairly pure product is ob tained, which may be further purified by con verting into the sodium salt and by treat ing the latter with animal charcoal. Sul phanilic acid has also been prepared by a patented process (Ger. 205150), which consists in heating chlor-benzene-p. sul phonic acid with ammonia under pressure in the presence of cupric chloride. Under this treatment the chlorine atom is readily replaced by the NH, group. Sulphanilic acid crystallizes from hot water in plates with one molecule of water of crystallization. It has no definite melting point; upon heating it decomposes be tween 280°-300° C. One part of the acid dis solves in 112 parts of water at 15° C. Upon oxidation with manganese dioxide and sulphuric acid it forms quinone. Fused with caustic
potash it yields aniline and with bromine water it is readily converted into tribromaniline.
Sulphanilic acid is of special importance as an intermediate in the manufacture of azo dyes. When its sodium salt is treated with sodium nitrite in the presence of sulphuric acid, diazobenzene sulphonic acid is formed, which may he readily coupled with a number of reagents forming important azo-dyes. With phenol the diazo compound forms Tropaeolin Y. With resorcin it yields Resorcin Yellow. With dimetbyl aniline it forms Helianthine, the alkaline salt of which is the well-known indi cator Methyl Orange. With alpha-naphthol the diazo compound forms Orange I, one of the seven food colors, and with beta-naphthol it forms Orange II. Sulphanilic acid has also been used for the manufacture of Resorcin Brown, Azofuchsine G, Hessian Brown MM, Benzo Brown G, Columbia Green, and Conogo Brown G.