Chemical Properties of Indium

Indium is unaffected by dry air at ordinary temperatures, but at a red heat it burns with a blue flame, producing the sesqui-oxide with the liberation of 1044.6 cals. of heat per gram of metal. It unites directly with sulphur and the halogens. It is unaffected by boiling water or potassium hydroxide, but dissolves in mineral acids. The action of nitric acid is slow, and ammonia is found among the reduction products of the acid. The potential differences between indium and molar, tenth-molar, and hundredth-molar solutions of indium trichloride are 0.094, 0.108, and 0.119 volts respectively at 25°, the metal being negative to the solution. The electrolytic solution-pressure of indium, 10² to 10³ atmospheres, places it between iron and lead in the electromotive series.

Indium forms derivatives corresponding to the three types InX, InX₂, and InX₃ (X denoting a univalent acid radicle), but only the compounds of the last type are capable of existing in aqueous solution, in which they are appreciably hydrolysed. The solutions contain the colourless ion In^•••. The ions In^• and In^•• appear to be unstable and to undergo change as represented by the equations

Comparison of conductivities of indium chloride and bromide solutions with those of cadmium bromide and nitrate.

3In^• = In^••• + 2In;
3In^•• = 2In^••• + In.

Thus the lower halogen derivatives of indium are decomposed by water, metallic indium being deposited.

The trihalides of indium appear to resemble the corresponding compounds of cadmium in their ability to form complex anions in solution, a resemblance which is not surprising since indium and cadmium occupy adjacent positions in the periodic table. The variation of the equivalent conductivity (A) with the cube root of the concentration (m, in gram-equivalents per litre) is shown for the four salts InCl3, InBr3, CdBr2, and Cd(NO3)2 in fig. The abnormally low values for A in solutions of moderate concentration is shown in each case except that of cadmium nitrate, and this salt does not form complex anions in solution. The low values for indium chloride are all the more remarkable since the data plotted refer to a temperature of 25°, the other data holding good for 18° C.