Calcium Arsenide, Ca₃As₂

Calcium Arsenide, Ca₃As₂, is formed by the direct combination of calcium with arsenic vapour at dull red heat. When a mixture of hydrogen and arsenic vapour is passed over heated quicklime the product is mainly arsenite, only a little arsenide being formed. It is best prepared by the reduction of calcium arsenate by carbon in an electric furnace. Lebeau used a mixture of 100 parts of the arsenate with 31 parts of petroleum coke and employed a current of 950 to 1000 amperes at 45 volts, the heating being continued for 2 to 3 minutes. Guerin has shown that calcium arsenate is reduced by carbon slowly at 800° C. and rapidly at 850° C., the arsenite being first produced and finally calcium oxide and arsenic; in order to obtain the arsenide, rapid heating to 1500° to 1600° C. is necessary. If the heating is slow, loss of arsenic occurs.

The arsenide is obtained as a crystalline mass, small fragments of which are transparent and reddish-brown in colour. Its density is 2.5, and it is harder than calcite. It is readily attacked by the halogens, but the higher the atomic number of the latter, the higher the temperature necessary for reaction to occur. It remains unchanged in dry air or oxygen at the ordinary temperature, but when heated in the latter it burns brilliantly to form arsenate; if the oxygen supply is limited, however, arsenious oxide and arsenic may be produced. With oxidising agents there is vigorous heat evolution. Hydrogen does not attack the arsenide, even at 700° to 800° C. Water decomposes it in the cold with formation of calcium hydroxide and evolution of arsine; it is consequently unstable in moist air. It is attacked by sulphur or hydrogen sulphide at dull red heat. Cold concentrated sulphuric acid is reduced to sulphurous acid. The gaseous halogen acids react at red heat giving arsenic and a calcium salt. Fuming nitric acid does not attack it in the cold, but there is a rapid reaction on heating. Carbon and boron are without action at 1000° C., but the former can decompose it completely at the temperature of the electric furnace. Many metallic salts are decomposed when heated with the arsenide.

The presence of small amounts of arsenic in calcium retards the absorption of nitrogen by the latter.