A blue dye-stuff obtained from several species of Indigofera.
Nat. Ord.—Leguminosae.
SYNONYMS: Indicum, Pigmentum indicum.

Source and History.—The plants furnishing indigo are quite numerous; chief among them are Indigofera tinctoria, Linné, cultivated in India and the East India Islands; Indigofera Anil, Linné, growing in the West Indies, Central and South America, and Indigofera argentea, Linné, cultivated in Egypt and the French colonies in Africa. To these sources is to be added the commercial product called Woad, prepared from the leaves of Isatis tinctoria and I. lusitanica (Nat. Ord.—Cruciferae), plants which were formerly much cultivated in France and Germany, and supplied the demand for indigo before it was introduced into Europe from foreign countries.

Indigo-blue does not pre-exist as such in these plants, It is developed by the decomposition of the bitter glucosid indican (C26H31NO17), a colorless chromogene existing therein. This substance also occurs sometimes in pathological urine. When acted upon by diluted acids or ferments, it is said to absorb water and to be hydrolyzed into indigo-blue (C16H10N2O2), and a saccharine principle indiglucin (C6H10O6), which undergoes further decomposition by fermentation. However, oxidation also plays an important part in the production of indigo-blue. The plants are collected during the flowering season, and are kept immersed in water in soaking vats or cisterns. At a temperature of about 30° C. (86° F.), fermentation soon sets in, lasting from 12 to 15 hours. When the liquid becomes of a yellowish-green color, it is drawn off and stirred briskly for about 3 hours. The liquid now turns deep blue, and the indigo, being insoluble in water, and of a greater specific gravity, soon falls to the bottom. The supernatant liquid is removed, the thick indigo sediment is heated to the boiling point of water to prevent further fermentation, then strained, pressed, and cut into cubical blocks and carefully dried. Three hundred kilograms of the indigo plant yield about 1 kilogram of indigo (S. P. Sadtler, Handbook of Industrial Organic Chemistry, 2d ed., 1895).

Description and Chemical Composition.—Commercial indigo occurs in hard, porous, brittle lumps or cubes of a dark-blue color and devoid of taste or odor. Indigo which is firm, dense, not easily broken, and which has a dull or greenish or grayish hue, is of inferior quality. Its specific gravity varies between 1.32 and 1.45. Commercial indigo contains from 20 to 80 per cent of indigo-blue, the average being about 45 per cent; the remainder consists of indigo-brown, indigo-red, indigo-gluten, water, and varying quantities of mineral matters. The ash should not amount to more than 8 per cent. When indigo is rubbed with a hard and smooth body, the surface of friction assumes a copper-like lustre. Heated to 287.7° C. (550° F.), indigo sublimes as a violet-colored vapor, although not without decomposition, and condenses on cool surfaces in the form of small, acicular crystals of a copper hue. Indigo-blue is not affected by the ordinary solvents, such as water, alcohol, ether, diluted acids and alkalies. It can be crystallized, however, from aniline and oil of turpentine, and is also soluble in chloroform, glacial acetic acid, paraffine, castor oil, nitrobenzene phenol, and similar solvents. Concentrated, especially fuming sulphuric acid, readily dissolves indigo, forming deep-blue solutions. According to the relative quantities of indigo and sulphuric acid, and the temperature maintained in the reaction, either indigo-mono-sulphonic (sulpho-purpuric) acid (C16H9[SO3H]N2O2), or indigo-disulphonic (sulphindigotic) acid (C16H8[SO3H]2N2O2), may be obtained. The monosulphonic acid is insoluble in water; its sodium salt is soluble, and is called indigo-purple or red-indigo carmine. The disulphonic acid is soluble in water, and is the substance commercially termed Saxony blue or Chemnitz blue. The sodium or potassium salt of this acid is the true indigo-carmine or soluble indigo of commerce, and occurs in the form of a pasty mass or as a powder; in the latter case it obtains the commercial name indigotine (S. P. Sadtler, Handbook, 1895, p. 447). The name indigotin is also often given in text-books to the pure indigo-blue (C16H10N2O2).

Indigo-blue is converted by oxidizers, e. g., nitric acid, into yellow-red, crystallizable isatin (C16H10N2O4), which, when distilled with caustic potash, yields aniline (C6H5NH2). When indigo is acted upon by reducing agents in alkaline solution, e. g., by dextrose, or sulphurous acid, or hydrogen sulphide, zinc, etc., a solution of a yellow color is yielded, containing alkali-soluble indigo-white (C16H12N2O2). This solution, exposed to the air, precipitates indigo-blue again. This reaction is made use of in dyeing; the cloth, being saturated with the alkaline solution containing indigo-white, is exposed to the air, whereby the indigo-blue formed is firmly fixed in the fiber.

Action, Medical Uses, and Dosage.—Indigo and its preparations when administered, have imparted a blue color to the cutaneous and renal secretions. Large doses cause gastro-intestinal irritation, debility, and nervous derangements. The sulphate is reputed an active emmenagogue, and indigo was several years ago tested as a remedy for epilepsy, but has been abandoned as inefficient and even detrimental. The dose may be stated at from 1 to 20 grains.

SYNTHETICAL INDIGO.—Within the last 20 years, several syntheses of indigo-blue and its derivatives have been accomplished. An important stepping-stone to this feat was the synthesis of Indol (C8H7N) (the skeleton substance of indigo), by Baeyer and Emmerling, in 1869. The first synthesis of indigo by Baeyer (Ber. d. Deutsch. Chem. Ges., 1880, p. 2254), was accomplished by converting ortho-nitro-cinnamic acid (C6H4NO2.CH:CH.COOH) into ortho-nitro phenyl propiolic acid (C6H4[NO2].CC:COOH.), and this into indigo-blue through the reducing agency of sugar in alkaline solution. The graphic formula of indigo-blue (C6H4.NH.CO:C=C:CO.NH.C6H4), elucidated by the researches of Prof. Baeyer and his co-workers, suggests to him an analogy with that of the Azo dyes (which see). More recently additional syntheses of indigo were effected simultaneously by L. Lederer and K. Heumann (see Chemiker Zeitung, 1890, Oct. 1st and Oct. 8th; also see Amer. Jour. Pharm., 1890, p. 614); likewise some methods for the direct synthesis of indigo-carmine are on record (see A. Haas, in Amer. Jour. Pharm., 1891, p. 406, and B. Heymann, in Prof. Sadtler's Handbook, 1895, p. 452).

Related Species and Product.—In addition to the plants above mentioned, indigo is probably yielded also by the following plants: Gymnema tingens, Sprengel, Hindustan; Polygonum tinctorium, Linné, China; Wrightia tinctoria, Robert Brown, Hindustan; and Galega tinctoria. Prof. H. Molisch (Amer. Jour. Pharm., 1893, p. 494), detects the presence of indican in plants by boiling some fragments of the latter in a test-tube with a diluted solution of aqua ammoniae, and in another experiment with diluted hydrochloric acid, filtering, cooling, and agitating with chloroform, which assumes a blue color if indican is present.

Crescentia cujete.—South America. The fruit of this plant contains a blue body resembling indigo, and crescentinic acid (G. Peckolt, Pharm. Rundschau, 1884).

King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.