INDIUM, a soft malleable metal about as heavy as tin. Chemically it is an element (symbol In, atomic number 49, atomic weight 114.8) included in the periodic group containing aluminium, gallium and thallium (see PERIODIC LAW). It melts at 155° C and vaporizes above 1,450° C. Its specific gravity is 7.2 to 7.42 and its specific heat o•05695. Indium was discovered in the zinc blende of Freiburg in 1863 by F. Reich and Th.
Richter, who detected in the spectroscope the strong indigo-blue line (X 4,51I) in its spectrum (Latin, indices, Indian; cf. indigo). It is one of the very rare elements, occurring only in small quan tities in zinc blende. It is best obtained from commercial metallic zinc, which contains small amounts of indium, by treating the metal with insufficient hydrochloric acid for complete solution, whereby indium is precipitated on the undissolved zinc. The metallic residue is dissolved in nitric acid, the solution evaporated with excess of sulphuric acid and treated with ammonia, when indium and iron hydroxides are precipitated. These hydroxides are redissolved in hydrochloric acid and boiled with excess of sodium bisulphite, when basic indium sulphite is precipitated. These processes are repeated to obtain pure indium sulphite. Indium is also separated from a mixture of anhydrous chlorides by addition of alcoholic pyridine, when the complex salt is precipitated.
Indium oxide, a yellow powder formed by igniting the hy droxide, is readily reduced on being heated with carbon or hydro gen. The hydroxide, In a gelatinous precipitate thrown down by adding ammonia to any soluble indium salt, is readily soluble in caustic soda or potash but not in ammonia. Three chlorides are known : The trichloride, prepared by the action of chlorine on the metal or of chlorine and carbon or sulphur on the oxide, is a soluble deliquescent salt ; its vapour density corresponds with The dichloride, (vapour density normal at 1,300° C), prepared by the action of hydrogen chloride on the metal. (3) The dark red monochloride, InC1 (vapour density normal at I,IOO° C), obtained by distilling the dicloride over sodium. The di- and mono-chlorides are both decomposed by water to give the trichloride and indium.
The salts of indium correspond in type with the trichloride; among them are the indium alums, where R is NH,, Rb or Cs. Indium sulphide, is obtainable by direct combination of indium and sulphur as an infusible red mass, or as a yellow precipitate on passing sulphuretted hydrogen into a solution of an indium salt ; it dissolves in concentrated acids. Indium acetylacetone, colourless prismatic crystals, melts at 186° C and sublimes with partial decomposition. It exhibits isomorphous relationships with the acetylacetones of aluminium, gallium, iron and scandium (G. T. Morgan, H. D. K. Drew and T. V. Barker, 1921) and is obtained by dissolving freshly prepared indium hydroxide in acetylacetone.