Harper's new monthly magazine 258, 1871
Messrs. A. A. Aguiar and A. Bayer publish as new a simple process by which they obtain pure indigotine, or the dyeing principle of the indigo of commerce, using aniline as a solvent. Pulverized indigo and pure aniline are to be heated to boiling in a flask, when the organic base almost instantaneously dissolves the coloring matter, and becomes a deep blue liquid, very much like a concentrated solution of indigo in sulphuric acid. This is filtered, and he residue treated with aniline as long as coloring matter is dissolved. Most of the indigotine crystallizes in the cooling solution within a few hours, and the remaining liquid becomes black from the foreign substances it retains. On redissolving these crystals, and removing all traces of aniline by means of alcohol, indigotine of the utmost purity will be obtained. The aniline here used is an organic basis produced by the decomposition of indigo by heat, or, better, by the simultaneous action of hydrate of potash and heat.
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