Lead Arsenides

An investigation of the system Pb-As reveals no evidence of the existence of any definite compounds. At its melting point lead does not dissolve any arsenic, but at higher temperatures two layers are formed; 1 the lower layer consists of lead containing a small quantity of arsenic dissolved in it, and the upper layer is arsenic containing a small quantity of lead. At atmospheric pressure only up to 35 per cent, of arsenic can be alloyed with lead. There is a eutectic at 290° C. with 2.6 per cent, of arsenic. At this temperature the lead retains in solution 0.045 to 0.05 per cent. As. The solubility of arsenic in lead diminishes rapidly as the temperature is lowered and at room temperature it is probably less than 0.01 per cent. As. With arsenic- rich alloys the primary crystals are almost pure arsenic, so that the solubility of lead in arsenic is practically nil.

The addition of small quantities of arsenic reduces the shrinkage of lead during solidification and increases the hardness and, in small degree, the compressive strength. It has no effect on the rate of corrosion in water.

The following arsenides have been described in the literature: Pb₉As, a malleable crystalline alloy obtained by heating arsenic with molten lead for 20 hours; Pb₆As, crystalline plates found associated with tin in Cornwall; Pb₂As, obtained by melting excess of arsenic with lead under fused boric acid and heating to a moderate temperature; if the temperature in the latter case is kept as low as possible the product has the composition Pb₃As₄; if the mixture is heated strongly, Pb₃As₂ is formed; finally PbAs, obtained by reduction of lead arsenate under fused potassium cyanide. The existence of none of these as a true compound has been confirmed, however.