INDIUM (symbol In, atomic weight 114.8), a metallic chemical element, included in the sub-group of the periodic classification of the elements containing aluminium, gallium and thallium. It was first discovered in 1863 by F. Reich and Th. Richter (Journ. für prak. Chem., 1863, 89, p. 444) by means of its spectrum. It occurs naturally in very small quantities in zinc blende, and is best obtained from metallic zinc (which contains a small quantity of indium) by treating it with such an amount of hydrochloric acid that a little of the zinc remains undissolved; when on standing for some time the indium is precipitated on the undissolved zinc. The crude product is freed from basic zinc salts, dissolved in nitric acid and the nitric acid removed by evaporation with sulphuric acid, after which it is precipitated by addition of ammonia. The precipitated indium hydroxide is converted into a basic sulphite by boiling with excess of sodium bisulphite, and then into the normal sulphite by dissolving in hot sulphurous acid. This salt on strong ignition leaves a residue of the trioxide, which can be converted into the metal by heating in a current of hydrogen, or by fusion with sodium (C. Winkler, Journ. für prak. Chem., 1867, 102, p. 273). Indium is a soft malleable metal, melting at 155° C. Its specific gravity is 7.421 and its specific heat 0.05695 (R. Bunsen).
Indium oxide, In2O3, is a yellow powder which is formed on ignition of the hydroxide. It is readily reduced on heating with carbon or hydrogen, and does not pass into an insoluble form when ignited. The hydroxide, In(OH)3, is prepared, as a gelatinous precipitate, by adding ammonia to any soluble indium salt. It is readily soluble in caustic potash, but insoluble in ammonia.
Three chlorides of indium are known: the trichloride, InCl3, a deliquescent salt, formed by heating a mixture of the oxide and carbon in a current of chlorine; the dichloride, InCl2, obtained by heating the metal in hydrochloric acid gas; and the monochloride, InCl, which is prepared by distilling the vapour of the dichloride over metallic indium. The mono- and dichlorides are decomposed by water with the formation of the trichloride, and separation of metallic indium. Indium Sulphate, In2(SO4)3, is obtained as a white powder very soluble in water by evaporating the trioxide with sulphuric acid. Concentration of the aqueous solution in a desiccator gives a deposit of crystals of a very deliquescent salt, H2In2(SO4)4·8H2O. An indium ammonium alum, In2(SO4)3·(NH4)2SO4·24H2O is known.
The atomic weight of indium has been determined by C. Winkler and by R. Bunsen by converting the metal into its oxide. Thiel (Ber., 1904, 37, p. 1135) obtained the values 115.08 and 114.81 from analyses of the chloride and bromide, whilst F. C. Mathers (Abst. J.C.S., 1907, ii. 352) obtained 114.88 and 114.86. Indium salts can be recognized by the dark blue colour they give in the flame of the Bunsen burner; and by the white beads of metal and the yellow incrustation formed when heated on charcoal with sodium carbonate.