INDIGO′TIN. Syn. Cerulin, Indigo blue. This is the pure blue principle of indigo. It appears to be the oxide of the same organic radical of which indigo white is probably the hydrate.
Prep. 1. Indigo (in fine powder) is digested successively in dilute hydrochloric acid, solution of potassa, and alcohol; the dried residuum is crude indigotin.
2. Indigo (in fine powder), 1 part; green sulphate of iron, 2 parts; hydrate of lime, 3 parts; water, 15 parts; mix, agitate occasionally until the colour is destroyed, then decant the clear portion, precipitate with dilute hydrochloric acid, and wash the powder first with water, and then with boiling alcohol, until the latter ceases to acquire a yellow colour.
3. Caustic soda and grape sugar, of each 1 part; water, 20 parts; powdered indigo, 5 parts; mix, and proceed as last. The above are essentially the same as the indigo vat, but on the small scale.
4. The process for estimating the value of indigo given under Indigo is a good process for obtaining Indigotin.
Obs. The product from all the above exceeds 50% of the indigo operated on.
5. (Taylor.) Powdered indigo, 2 parts; plaster of Paris, 1 part; water, q. s. to reduce the mixture to a thin paste; spread the mass evenly upon an oblong iron plate to the depth of about 1⁄8 inch, and dry it by a gentle heat. It must then be held over the flame of a spirit lamp, when a disgusting odour will be evolved, the mass will begin to smoke, and in a few minutes will be covered with a heavy purple vapour, which will condense into brilliant flattened prisms or plates of an intense copper colour, forming a thick velvety coating over the surface immediately exposed to the heat. Should the mass catch fire, it may be instantly extinguished by a drop of water let fall upon it. Prod. 15 to 18%. See Indigo, &c.
INDIUM. In. = 113·4. This very rare metal was discovered by means of the spectroscope by Messrs Reich and Richter in a specimen of zinc-blende from Freiberg, in 1863. Since then it has been found in the flue dust of some zinc furnaces worked in the Hartz mountains, also in a black blende, known as christophite, occurring in Saxony; in the Wolfranc of Zinnwald, associated with zinc, as well as in Wolfranc alone; and also in the blende met within steatite, near Schlaggenwald. In all of these substances the indium is present in very minute quantity,[354] and is more or less associated with lead, arsenic, cadmium, iron, and copper; its separation from which is matter of no inconsiderable labour and difficulty.
[354] In the flue dust of the zinc furnaces it is present to the amount of about 0·1 per cent.; in christophite in the proportion of 0·0062 per cent.
The following process for the detection of indium in zinc-blende, and its extraction from the same source, is given by Winkler. Precipitate the hydrochloric acid solution of the roasted ore with metallic zinc at the boiling heat; dissolve the precipitate in nitro-hydrochloric acid; remove the arsenic, cadmium, &c., by sulphuretted hydrogen, and precipitate the indium as oxide by barium carbonate. Should this precipitate contain any iron, it must be removed by resolution, heating with sodium sulphate, and digestion with barium carbonate in a closed vessel. The indium may also be precipitated from the original solution, either directly by barium carbonate, or from a solution containing sulphuric acid, by neutralisation with sodium carbonate, till a precipitate begins to form, and