Bismuth Orthoarsenate is obtained as a white crystalline precipitate by the action of arsenic acid or an alkali arsenate on a nitric acid solution of bismuth nitrate. When dried at 100° to 120° C. the product is the hemihydrate, BiAsO4.½H2O, which loses water only at red heat. By evaporating the mixture on a water-bath, de Schulten obtained a product which he described as the anhydrous salt; on adding water and keeping for a few days he obtained microscopic monoclinic prisms of density 7.142 at 15° C. The hemihydrate is usually obtained as microscopic spherical, stellate or octahedral crystals. It is tasteless, sparingly soluble in water, soluble in dilute hydrochloric acid; it melts only with difficulty.
A basic arsenate, 2BiAsO4.3Bi2O3, has been prepared by the action of sodium arsenate upon an ammoniacal solution of bismuth citrate. It forms a gelatinous precipitate which resembles the corresponding phosphate in its chemical properties.
Basic arsenates of bismuth occur naturally in a number of minerals, especially in Saxony and Utah, the most noteworthy being atelestite, rhagite, mixite (containing copper) and walpurgite (containing uranium). |