Calcium Arsenates

Tricalcium Orthoarsenate, Ca₃(AsO₄)₂, can be obtained by the action of lime-water on arsenic acid, or by the precipitation of calcium chloride with disodium hydrogen arsenate. A pure product may be prepared by pouring alkaline calcium chloride solution into alkaline sodium hydrogen arsenate. Drying at 100° C. gives the dihydrate Ca₃(AsO₄)₂.2H₂O. The water of crystallisation is lost at 175° C. The density of the dihydrate is 3.23, and of the anhydrous salt 3.31. The solubility at 25° C. is 0.0133 grm. per 100 grm. of water.

Calcium arsenate in the form of a spray may be used in place of lead arsenate as an insecticide for plants.

These are obtained in the same way as the analogous phosphate compounds.

Dicalcium Orihoarsenate, CaHAsO₄, is found in nature as the hydrates pharmacolite, CaHAsO₄.2H₂O, and haidingerite, CaHAsO₄.H₂O. The densities of the artificially prepared minerals are 2.754 and 2.967 respectively.

The salt may be prepared by treating calcium carbonate with arsenic acid, or by precipitating a soluble calcium salt with disodium hydrogen arsenate. Robinson recommends acidifying both solutions, and states that when dried at 100° C. the compound contains one molecule of water of crystallisation, which it loses before the temperature of 175° C. is reached. The dihydrate may be obtained at ordinary temperatures by slow diffusion. According to Debray and Kotschoubey, there is a hydrate, 2CaHAsO₄.3H₂O, but this is not confirmed by more recent investigators.

At 25° C. 0.3108 grm. of the anhydrous salt is contained in 100 grm. of the solution.

Monocalcium Orthoarsenate, CaH₄(AsO₄)₂, was obtained by Smith during investigations of the system, arsenic pentoxide: calcium oxide: water. At 35° C. it is stable in contact with solutions containing more than 27.5 per cent, of arsenic pentoxide. Below that concentration CaHAsO₄.H₂O is the stable salt.