Indigo, B.P. Indigo.
Synonyms.—Indigo Blue; Indigotin.
Indigo is a colouring matter obtained chiefly from Indigofera tinctoria, Linn. (N.O. Leguminosae), I. Anil, Linn., and other species, in Bengal, Java, and Guatemala. For the production of the colouring matter, which does not exist ready formed in the plant, the leafy shoots are collected and macerated with water, often with addition of lime. The greenish-coloured infusion is stirred so as to bring it into contact with air, and the indigo which is rapidly formed allowed to deposit, washed with water, pressed into cakes and dried. Commercial indigo is usually seen in the form of brick-shaped cakes, about 6 centimetres long. It is of an intensely blue or bluish-violet colour, and assumes a fine bronze sheen when rubbed with a hard, smooth substance. It is close in texture, but not hard or granular, and is light in weight. It burns easily, producing purplish vapours and leaving a reddish ash. Many other plants belonging to a variety of natural orders yield indigo. The chief genus, however, is Indigofera. Some appear to contain indoxyl. Indigo can also be produced synthetically from indoxyl, isatin chloride, orthonitrophenylpropiolic acid, and other bodies, and most of the indigo of commerce is so prepared. Prussian blue, a mixture of ferric ferrocyanide with ferrous ferricyanide, though closely resembling indigo in appearance, can readily be distinguished from it by warming with dilute solution of potassium hydroxide, filtering, acidifying, and adding a trace of ferric chloride, when a blue precipitate indicates Prussian blue.
Constituents.—The plant contains a yellowish glucoside, indican, which is readily hydrolysed in aqueous solution by dilute mineral acids or by ferments into indigo blue and a sugar, indiglucin. The indigo blue is insoluble in water and deposited as a blue sediment. Good indigo consists chiefly of indigo blue (from 90 down to as little as 20 per cent. in poor qualities), water (3 to 5 per cent.), and inorganic matter (5 to 10 per cent.). Small quantities of other substances, such as indigo brown and indigo red, etc., are also present. Indigo blue (or indigotin), C16H10N2O2, can be obtained in deep blue or coppery rhombic crystals, which are insoluble in water, diluted acids, or alkalies, slightly soluble in chloroform, but more easily in glacial acetic acid. Diluted nitric acid converts it into yellowish-red isatin.
Action and Uses.—Indigo is rarely used in pharmacy except as a test. Dissolved in strong sulphuric acid it forms indigotin-disulphonic acid which after suitable treatment is sold in the form of a paste as indigo extract. B.P. solution of indigo sulphate is prepared by heating 0.1 of dry indigo, in fine powder, with one of sulphuric acid, at 100°, for one hour, then pouring the mixture into 99 of sulphuric acid, shaking, and decanting the clear liquid. This solution is decolourised by free chlorine and by nitrates or nitratic acid. The sodium salt of indigotin-disulphonic acid is used under the name "Indigo Carmine" as a staining agent in microscopy. The sodium salt of orthonitrophenylpropiolic acid (Nitropropiol) is used as a test for sugar in urine; in the presence of glucose indigo blue is formed by reduction on boiling.