Calcium Cyanide, Ca(CN)₂

Calcium Cyanide, Ca(CN)₂, is obtained, in solution only, by treating calcium hydroxide with a solution of hydrocyanic acid, or by fusing calcium ferrocyanide out of contact with air and extracting with water. Very dilute solutions are stable, but if concentrated above 15 per cent, they decompose rapidly, evolving ammonia and hydrocyanic acid and precipitating lime. Owing to the hydrolysis of the cyanide, the solution has an alkaline reaction and a large partial pressure of hydrocyanic acid. Even so weak an acid as carbonic acid decomposes the cyanide, liberating hydrocyanic acid.

A double calcium cuprous cyanide, CaCu(CN)₃.4H₂O, in prismatic needles, and two calcium mercury salts, CaHg₂(CN)₆.3H₂O (and 8H₂O), in colourless hygroscopic needles, and Ca₂Hg₃(CN)₁₀.6H₂O, in leaflets, have been described. There are also two calcium manganocyanides, Ca₂Mn(CN)₆ and CaMn₂(CN)₆, a manganicyanide, Ca₃Mn₂(CN)₁₂, a crystalline double calcium silver cyanide, and an uncrystallisable calcium zinc cyanide. A compound, Hg(CN)₂.Ca(CN)₂. HgI₂.7H₂O, is apparently immediately formed in solution on mixing mercuric cyanide and calcium iodide. The heat of formation is 49.5 Cal.