INDIGOTIN.

443

anthranilic acid (C 7 H 7 N0 2 ), and if heated to 300 °, along with solid causticpotassa, indigotin yields salicylic acid (C 7 H 60 3 ), and when submitted to drydistillation along with caustic potassa, aniline (CgH 7 N) is formed.

Without entering here into lengthy details which belong stridtly to thedomain of theoretical organic chemistry, we may observe that these readtionsevidently denote an intimate relation between indigotin and the phenylgroup; salicylic acid, indeed, may be formed by the combination of carbonicacid with phenate of soda, whereby salicylate of soda is obtained, whileanthranilic acid can be considered as phenyl-carbamic acid, and aniline asrepresenting phenylamin. Dry indigotin is not adted upon by either chlorineor bromine, but if water is simultaneously present oxidation takes place,and isatin is first formed, but entering into combination with the haloids theultimate result is the formation of substitution products, such as chlorisatinand bromisatin. Iodine has no adtion upon indigotin, either in the dry ormoist state. Concentrated sulphuric acid yields, with indigotin, sulpho-conjugated acids and sulphuric acid derivatives, the production of whichdepends upon the nature of the acid (fuming or ordinary), the duration of thecontact, and the temperature applied. There are thus formed either sulpho-purpuric acid (C I 6H I oN 2 0 2 ,S0 3 ) or sulphindigotic acid (C8H 5 NO,S0 3 ), or thehyposulphindigotic acid, which is described only by Berzelius, and the com-position of which is not accurately known. Sulphopurpuric acid, also knownas sulphophenicic acid, phenicin, and indigo-purple, is the product of the firstmoments of the action of sulphuric acid upon indigotin, and even when thecontact of the acid and the indigotin is very short, especially if fuming acidbe employed, it is very difficult to obtain sulphopurpuric acid perfectly freefrom sulphindigotic acid.

M. Camille Koechlin* states, contrary to the observations of Berzelius andGerhardt, that when fuming acid is employed, though the contact of the acidand indigotin be only for a few minutes before the addition of water, nosulphopurpuric acid at all is obtained. He advises the use of strong commonsulphuric acid. This last-named acid converts indigotin readily into sulpho-purpuric acid; the best mode of preparing the latter consisting in mixingfinely ground indigo with the acid, so as to form a homogeneous mass, whichis to be poured at once into at least from fifty to sixty times its bulk of coldwater, in order to prevent gradual conversion into sulphindigotic acid.The former acid is sparingly soluble in an acidified liquid; it is thereforedeposited from the large bulk of water in the shape of a red flocculent body,which is collected on a filter, and washed with water slightly acidified withhydrochloric acid. This washing, however, is not sufficient to obtain the acidin a pure state, for which purpose wool-yarn or worsted should be immersedin a bath of it, and next washed in a solution of carbonate of soda, wherebythe sulphindigotic acid which might have been present is washed out, sulpho-purpuric retained, and may be removed by washing in acidulated water. Abetter method of purifying sulphopurpuric acid is to apply the unequal solu-bility of the potassa or soda salts of the two conjugate acids; the sulpho-purpurate being far less soluble, it is clear that after careful neutralisation ofthe acid mixture, previously diluted, however, to some extent with water, theeffedt of a prolonged washing will be the elimination of the blue, while the

“ Bulletins de la Societe Industrielle de Mulhouse,” vol. xxiv., p. 331.