Calcium Arsenites

The ternary system CaO-As₂O₃-H₂O, investigated at 0°, 25° and 99° C. by the method indicated under barium arsenites, gives evidence of the formation of two arsenites,

Ca(OH)AsO₂ or CaHNaO₃, which may be considered to be either a basic metarsenite or calcium hydrogen orthoarsenite, and Ca(AsO₂)₂, calcium metarsenite. When mixtures of arsenious oxide and quicklime are heated, reaction commences at about 300° C. and at 465° C. considerable heat is evolved and calcium orthoarsenite, Ca₃(NaO₃)₂, is formed. An industrial process for the manufacture of this compound consists in treating a dry mixture of quicklime (3 moles) and arsenious oxide (1 mole) with dry steam. It may also be obtained by precipitation of dilute calcium chloride solution with potassium orthoarsenite, or of lime-water with boiling aqueous arsenious acid. It is a white amorphous powder, which may be dried at 100° C. It is only slightly soluble in water, but dissolves in acids. At red heat it decomposes with volatilisation of arsenic, leaving calcium arsenate. The latter is also formed when the arsenite is heated in oxygen. Calcium dihydrogen arsenile, Ca(H₂NaO₃)₂.aq., is formed when excess of lime-water is added to an ammoniacal solution of arsenious oxide, or when lime-water, or a solution of calcium chloride, is added to aqueous ammonium metarsenite, keeping the latter in excess. It is obtained as a white powder or as a gelatinous mass, and contains 5 to 11H₂O. It is fairly soluble in water, the solution having an alkaline reaction. It is insoluble in absolute alcohol. Calcium monohydrogen arsenite is mentioned above.

Calcium Pyroarsenite, Ca₂As₂O₅, is a white powder obtained by slowly precipitating a solution of arsenious acid with excess of lime-water, or by adding calcium chloride or sulphate to aqueous ammonium arsenite, and heating the precipitate to 105° C. If the product is dried in the air at the ordinary temperature, the monohydrate is obtained. At red heat calcium arsenate is formed. The pyroarsenite is only slightly soluble in water, 100 parts dissolving 0.025 to 0.030 part of the salt. It is more soluble in the presence of alkali chlorides and some ammonium salts, such as the nitrate, sulphate, acetate and succinate. It also dissolves in dilute acids.

Calcium Metarsenite, Ca(AsO₂)₂, is an amorphous white powder, anhydrous at 100° C., obtained by precipitation of calcium chloride with ammonium arsenite in ammoniacal solution. It is also produced as a commercial preparation by the interaction of milk of lime and arsenious oxide with vigorous stirring at 100° C., or by heating together at 60° to 70° C. arsenious oxide and slaked lime. This product is somewhat impure, containing calcium arsenate and a trace of free lime. Its solubility in water at 15° C. is about 0.04 to 0.05 per cent. When strongly heated in an inert gas it decomposes, forming calcium and arsenious oxides and a little arsenic.

A salt of composition Ca₃As₄O₉.3H₂O, which may be regarded as consisting of 1 molecule of pyro-combined with 1 molecule of metarsenite, Ca₂As₂O₅.Ca(AsO₂)₂, or as a derivative of a hypothetical tetrarsenious acid derived thus -

2As₂O₃ + 3H₂O = H₆As₄O₉

has been prepared by adding a solution of the corresponding potassium salt to a concentrated solution of calcium chloride. The white precipitate, after washing with alcohol, may be dried in the air. At 100° C. it loses 1 molecule of water. It is somewhat soluble in water.