INDIGO.

463

Industrially-Prepared Sulphuric Acid Derivatives of Indigo; IndigoCarmine ; Purple-Blue, also known as Boiley Blue.

For the purposes of dyeing and printing woollen and silk fabrics and yarns,and likewise for the purpose of imparting a blue hue to white cottons, indigois frequently dissolved in sulphuric acid, whereby blue soluble compounds areobtained which exhibit a shade different from that of indigo, and are morereadily applicable in dyeing. About thirty or forty years ago the solution ofindigo in sulphuric acid was only known and used after having been properlydiluted with water, being technically designated Saxon blue, or “ chemic.” Itis clear, however, that this liquid, besides being very acid, contains also thebrown and resinous matters present in native indigo. The preparation, more-over, of this solution was subjedt to variation, since in almost every dye-worksthe respedtive proportions of acid and indigo differed, while some manufadturersused ordinary strong sulphuric acid only, others again fuming acid, and somea mixture of both ; it accordingly depended entirely on the doses of acid andindigo, the duration of contadt, and the temperature, whether sulphindigotic,sulphophenicic acid, or a mixture of the two, was obtained ; and since theshade produced by these acids is not the same, it is clear that, according tothe tint preferred by the different makers, the choice of the prescription formaking the dye-material differed also.

M.Persoz* advises the following receipt for preparing the above solution :—•I kilo, each of finely ground indigo and fuming and of ordinary, but strong,sulphuric acid ; leave the mixture standing for forty-eight hours, and next heaton a water-bath until a drop of the fluid, taken out by means of a glass rod,dissolves in water without producing a precipitate; after the mixturehas cooled, it is diluted with water so as to form a fluid of i'i34 sp. gr.;18° Baume. M. Haussmann used 6‘5 parts of fuming sulphuric acid for 1 partof indigo.

Another mode of preparation is to add gradually, and by small quantities ata time, 1 part of pulverised indigo to 5 or 6 parts of very concentrated fumingacid, or from 10 to 12 parts of ordinary concentrated sulphuric acid : thismixture is left standing for twenty-four to forty-eight hours, and is next pouredinto twenty times its bulk of cold water, and filtered.

There is a very great difference, not to say discrepancy, between the differentreceipts for dissolving indigo, in respedt of the proportion of indigo and acidprescribed to be used. According to Dr. G. J. Mulder, who investigated thecause of this difference, it is chiefly due to the fadt that various kinds of indigoand the produdts of various countries do not behave alike in this respedt; thesuperior sorts of the drug are less easily adted upon, and, as is also ob-served by M. Meyer, a really good and ready solution can only be obtained, ina comparatively short time, by the use of very strong fuming acid, and eventhen heat is required for the richer kinds: with inferior kinds the adtion of theacid is often weakened by the presence of lime compounds, but, if these arefirst removed by boiling with weak hydrochloric acid, the adtion of fumingsulphuric acid dissolves these inferior kinds rapidly, and with less expenditure.

M. Marnas recommends to withdraw colour from the dilute acid solution bymeans of woollen or worsted yarn. This fibre, having taken up the colouring

* “ Impression des Tissus,” vol. 1., p. 454-