Barium Arsenates

Barium orthoarsenate, Ba₃(AsO₄)₂, is formed as large colourless plates when a mixture of barium oxide, alkali chloride and alkali hydrogen orthoarsenate is fused and allowed to cool. It is also said to be formed by the action of ammonia on an aqueous solution of barium hydrogen arsenate; the product, however, varies in composition, barium ammonium arsenate sometimes being formed and barium orthoarsenate rarely. The latter salt retains water, which may be expelled at 150° C.; definite hydrates have not been obtained. It is only slightly soluble in cold water. The anhydrous orthoarsenate has density 5.095 and melts at 1605° C. It is reduced by hydrogen at 400° to 450° C. to barium oxide, arsenic and water.

A study of the acid section of the system As₂O₅-BaO-H₂O at 30° C. shows that the stable solid phases are barium monohydrogen orthoarsenate, BaHAsO₄. H₂O, barium dihydrogen orthoarsenate, Ba(H₂AsO₄)₂.2H₂O, and the (3, 5)-hydrate of arsenic pentoxide, 3As₂O₅.5H₂O.

Barium Monohydrogen Orthoarsenate may be obtained as a gelatinous precipitate by adding baryta-water drop by drop to aqueous arsenic acid until the solution is alkaline to phenolphthalein. On shaking, the precipitate becomes crystalline. Berzelius obtained this salt in the form of small crystalline scales by adding sodium hydrogen arsenate drop by drop to excess of barium chloride solution. The precipitate may be dried at 100° C. The crystals are transparent rhombic plates of density 3.926 at 15° C.; from acetic acid it may be recrystallised in the form of octahedra. When heated at 120° C. the salt loses its water of crystallisation, and at 320° C. it loses constitutional water and forms barium pyroarsenate.

Barium Dihydrogen Orthoarsenate crystallises as the dihydrate from a solution of barium orthoarsenate or barium monohydrogen orthoarsenate in aqueous arsenic acid, or it may be precipitated by the addition of baryta-water or barium carbonate to aqueous arsenic acid. The crystals are monoclinic, with axial ratios a:b:c = 1.160:1:0.625 and β = 108°34'. The salt is only slightly soluble in water and in the presence of excess of water changes to the monohydrogen arsenate. It is readily soluble in hydrochloric acid; less soluble in acetic acid. When heated, it loses water, and above 230° C. barium metarsenate, Ba(NaO₃)₂, remains. The latter may also be obtained by evaporation of a solution of the pyroarsenate and calcination of the residue.

Barium Pyroarsenate, Ba₂As₂O₇, is formed when barium monohydrogen orthoarsenate is ignited, or when barium oxide is fused with an alkali hydrogen arsenate. In the latter case it may be extracted with anhydrous glycerol. In contact with cold water the heccahydrate, Ba₂As₂O₇.6H₂O, is formed; boiling water converts it to the orthoarsenate.

Mixed orthoarsenates of composition BaKAsO₄, BaNaAsO₄.9H₂O, Ba(NH₄)AsO₄.½H₂O and Ba(NH₄)₂H₂(AsO₄)₂ have been prepared, as also have complex alumini- and ferri-arsenates of composition BaH₄[Al(AsO₄)₃].H₂O and Ba₃H₆[Fe(AsO₄)₃]₂. Barium chlorarsenoapatite, 3Ba₃(AsO₄)₂.BaCl₂, can be obtained by fusing a mixture of barium oxide, an alkali chloride and potassium or sodium metarsenate containing more than 60 per cent, of the arsenate. Bromo- and iodo-arsenoapatites are also known.