GALLIUM (

See also:

• SYMBOL (Gr. o-uµ(3oXov, a sign)

symbol Ga; atomic

See also:

• WEIGHT

weight 69.9)  , one of the metallic chemical elements . It was discovered in 1875 through its spectrum, in a specimen of zinc blende by Lecoq de Boisbaudran (Comptes rendus, 1895, 81, p . 493, and following years) . The chief chemical and physical properties of gallium had been predicted many years before by D . Mendeleeff (c . 1869) from a consideration of the properties of aluminium, indium and zinc (see ELEMENT) . The metal is obtained from zinc blende (which only contains it in very small quantity) by dissolving the mineral in an acid, andprecipitating the gallium by metallic zinc . The precipitate is dissolved in hydrochloric acid and foreign metals are removed by sulphuretted hydrogen; the residual liquid being then fraction-ally precipitated by sodium carbonate, which throws out the gallium before the zinc . This precipitate is converted into gallium sulphate and finally into a pure specimen of the oxide, from which the metal is obtained by the electrolysis of an alkaline solution . Gallium crystallizes in greyish-white octahedra which melt at 30.1'5° C. to a silvery-white liquid . It is very hard and but slightly malleable and flexible, although in thin plates it may be bent several times without breaking . The specific gravity of the solid form is 5.956 (24.50 C.), of the liquid 6•o69, whilst the specific heats of the two varieties are, for the solid form o•079 (12-23° C.) and for the liquid 0•082 (106-119°) [M .

Berthelot, Comptes rendus, 1878, 86, p . 786] . It is not appreciably volatilized at a red heat . Chlorine acts on it readily in the cold, bromine not so easily, and iodine only when the mixture is heated . The atomic weight of gallium has been determined by Lecoq de Boisbaudran by ignition of gallium ammonium alum, and also by L . Meyer and K . Seubert . Gallium oxide Ga203 is obtained when the nitrate is heated, or by solution of the metal in nitric acid and ignition of the nitrate . It forms a white friable mass which after ignition is insoluble in acids . On heating to redness in a stream of hydrogen it forms a bluish mass which is probably a lower oxide of composition GaO . Gallium forms colourless salts, which in neutral dilute aqueous solutions are converted on heating into basic salts . The gallium salts are precipitated by alkaline carbonates and by barium carbonate, but not by sulphuretted hydrogen unless in acetic acid solution .

Potassium ferrocyanide gives a precipitate even in very dilute solution . In neutral solutions, zinc gives a precipitate of gallium oxide . By heating gallium in a regulated stream of chlorine the dichloride GaCl2 is obtained as a crystalline mass, which melts at 164° C. and readily decomposes on exposure to moist air . The tichloride GaCla is similarly formed when the metal is heated in a rapid stream of chlorine, and may be purified by distillation in an atmosphere of nitrogen . It forms very deliquescent long white needles melting at 75.5 C. and boiling at 215-220° C . The bromide, iodide and sulphate are known, as is also gallium ammonium alum . Gallium is best detected by means of its spark spectrum, which gives two violet lines of wave length 4171 and 4031 .