Calcium Arsenates

In a basic form calcium arsenate is an important commercial product and is used in the form of a spray or dust as an insecticide for plants. The commercial arsenates are usually mixtures and exhibit a wide variation in chemical and physical characteristics. The system CaO-As₂O₅-H₂O has been investigated at various temperatures, and equilibrium data, microscopic and X-ray examination point to the existence of the following compounds: CaHAsO₄, Ca₅H₂(AsO₄)₄.5H₂O, Ca₃(AsO₄)₂.2H₂O and 4CaO.As₂O₅.xH₂O.

The normal salt, calcium orthoarsenate, Ca₃(AsO₄)₂, may be obtained by the action of lime-water or milk of lime on arsenic acid, or by addition of aqueous calcium chloride to aqueous sodium monohydrogen arsenate; in the latter case if the solutions are alkaline a product of a high degree of purity is obtained. Drying at 100° C. gives the dihydrate, Ca₃(AsO₄)₂. 2H₂O, which loses its water of crystallisation at 175° C. The anhydrous salt melts at 1455° C. It is reduced by hydrogen at 400° to 450° C. to calcium oxide, arsenic and water. The density of the dihydrate is 3.23, and of the anhydrous salt 3.62. The solubility of the latter at 25° C. is 0.0133 grm. per 100 grm. of water.

The commercial product is usually obtained either by adding arsenic acid to partly slaked lime containing sufficient quicklime to combine with the free water, thus yielding a dry product, or by heating a mixture of lime and white arsenic in the presence of an oxidising medium, such as oxygen, chlorine or nitrates. It is essential that the product should contain a minimum of soluble arsenic and that the presence of the more soluble acid arsenates should be avoided; the commercial arsenate is generally slightly basic. The results of analyses of 16 samples showed that the total As₂O₅ ranged only from 40.3 to 44.4 per cent. [As₂O₅ in Ca₃(AsO₄)₂ = 57.7 per cent.], and the average molecular quotient CaO/As₂O₅ was 3.4 The average commercial product contains 80 to 85 per cent, of what is probably a mixture of the normal arsenate and a basic arsenate, probably 4CaO.As₂O₅, together with about 6.5 per cent, each of calcium hydroxide and calcium carbonate, and small quantities of other impurities. If varying quantities of arsenic acid are added to a saturated solution of lime, the ratio of CaO:As₂O₅ in the precipitate shows a continuous variation according to the quantity of arsenic acid added and under these conditions there is no evidence of the real existence of calcium orthophosphate, Ca₃(AsO₄)₂.

An electrolytic method for the manufacture of calcium arsenate is as follows. A solution of arsenious oxide in caustic soda (As₂O₃: NaOH = 198:250) is electrolysed between iron electrodes. Hydrogen and a small quantity of metallic arsenic are liberated at the cathode and very little oxygen at the anode, the basic arsenite in solution being oxidised to arsenate. When this oxidation is complete, any arsenic is filtered off and the solution treated with milk of lime. A basic arsenate of extremely low solubility is precipitated and, after removal, is washed and dried.3 The reactions involved are the following:

As₂O₃ + 2NaOH = 2NaAsO₂ + H₂O
2NaAsO₂ + 4NaOH = 2Na₃AsO₄ + 2H₂
2Na₃AsO₄ + 4Ca(OH)₂ = Ca₄As₂O₉ + 6NaOH + H₂O

Less basic arsenates may be obtained by precipitation with calcium chloride or nitrate.

Calcium Monohydrogen Orthoarsenate, CaHAsO₄, may be prepared by treating calcium carbonate with arsenic acid, or by precipitation from solutions of calcium salts with sodium monohydrogen arsenate; both solutions should be acidified. When dried at 100° C. the monohydrate, CaHAsO₄.H₂O, is obtained, but the water of crystallisation is expelled before the temperature of 175° C. is reached. By placing a porous vessel containing sodium monohydrogen arsenate solution inside a vessel containing aqueous calcium nitrate, so that the two salts were able to mix by diffusion, well-defined monoclinic prisms of the dihydrate, CaHAsO₄.2H₂O, have been obtained. These two hydrates occur in Nature, the monohydrate as haidingerite and the dihydrate as pharmacolite, and calcium arsenate is a constituent of many other minerals. A third hydrate, 2CaHAsO₄.3H₂O, has been described, but its existence has not been confirmed.

A saturated solution of calcium monohydrogen arsenate contains at 25° C. 0.3108 grm. of the anhydrous salt per 100 grms. of solution. The dihydrogen arsenate is sparingly soluble in cold water and is decomposed by hot water, yielding the monohydrogen salt and arsenic acid.

The heat developed by the saturation of a normal solution of arsenic acid with calcium hydroxide in aqueous solution is as follows:

1st equivalent CaO, 14,500 calories
2nd equivalent CaO, 12,500 calories
3rd equivalent CaO, 2,520 calories
4th equivalent CaO, 280 calories
5th equivalent CaO, 250 calories

With the addition of three equivalents the precipitate is variable in composition but approximates to the normal salt; with 4 and 5 equivalents the precipitated arsenate is somewhat basic.

Calcium Dihydrogen Orthoarsenate has been obtained in the form of a monohydrate by adding nitric acid to a mixture containing equal proportions of calcium carbonate and arsenious oxide and allowing the solution to crystallise. It may also be prepared by adding excess of arsenic acid to calcium carbonate or to the normal or monohydrogen orthoarsenate. It forms colourless plates which lose their water at 180° C. and at 360° C. yield calcium metarsenate, Ca(NaO₃)₂, as a crystalline mass, insoluble in hydrochloric acid.

Calcium Pyroarsenate, Ca₂As₂O₇, in orthorhombic prisms, may be obtained by fusing calcium oxide with potassium monohydrogen arsenate, or with a mixture of the latter with less than 40 per cent, of calcium chloride. When treated with cold water the prisms break up into minute crystals of the hexahydrate, Ca₂As₂O₇.6H₂O. By leaving in contact with water the salt is gradually transformed into calcium monohydrogen orthoarsenate, CaHAsO₄.2H₂O. The pyroarsenate undergoes reduction when heated in hydrogen to above 400° C.

Several double orthoarsenates with the alkali metals are known: CaKAsO₄, CaNaAsO₄ and Ca₃Na₆(AsO₄)₄, Ca(NH₄)AsO₄.7H₂O, Ca(NH₄)AsO₄.6H₂O and Ca₃(NH₄)H₂(AsO₄)₃.3H₂O. Lefevre also isolated a calcium sodium pyroarsenate, Ca₄Na₄(As₂O₇)₃. Arsenoapatites similar to those of barium have been obtained.