Barium Arsenites

In an investigation of the ternary system BaO-As₂O₃-H₂O in which the two oxides in varying proportions were shaken with water at the temperatures 25° and 50° C., the solution and solid phase subsequently being analysed, evidence of the formation of only two arsenites was obtained: a basic metarsenite, Ba(OH)AsO₂.2H₂O, and an arsenite of composition 2BaO.3As₂O₃.4H₂O.

Barium Orthoarsenite, Ba₃(NaO₃)₂, has been prepared by the addition of potassium orthoarsenite to an aqueous solution of barium chloride and drying the precipitate at 100° C. in a current of hydrogen. It is obtained as a white amorphous powder, readily soluble in hot water and in dilute acids. When heated rapidly to about 730° C. it is decomposed into arsenic and the arsenate; the mechanism of the reaction is not understood. Slow heating below 300° C. causes loss of arsenious oxide.

Barium Dihydrogen Orthoarsenite, Ba(H₂NaO₃)₂.aq., is obtained, according to Perper, by adding an excess of baryta water to aqueous arsenious acid, by treating an ammoniacal solution of arsenious oxide with excess of barium chloride, or by adding potassium pyroarsenite to barium chloride solution. According to Stavenhagen and others, this salt is to be regarded as barium pyroarsenite, Ba₂As₂O₅.aq. If the precipitate is air-dried, it contains 8 molecules of water of crystallisation according to Stavenhagen, and 4 molecules according to Stein; if dried at 100° C. it retains 2 molecules. Perper formulated the dihydrogen orthoarsenite with 34H₂O, but stated that it lost water on exposure to air. When heated to a high temperature it yields arsenic and barium arsenate. It is slightly soluble in cold water, more readily in hot water, the solution having a slightly alkaline reaction. Whether the products obtained by the various methods described above are identical, and what their exact constitution is, are problems which have not yet been clarified. Further, Story and Anderson's basic metarsenite may equally well be formulated as barium monohydrogen orthoarsenite, BaHAsO₃.2H₂O.

Barium Metarsenite, Ba(AsO₂)₂, may be obtained by warming barium chloride with a solution of ammonium arsenite to which acetic acid has been added until arsenious acid is on the point of precipitation. The precipitate is then dried at 100° C. It is a white powder, easily soluble in water, but it can also be obtained as a gelatinous mass when a mixture of barium chloride and potassium metarsenite in solution is left to stand for a few hours. On strongly heating it decomposes to form arsenate and free arsenic, but to a much less extent than is the case with the orthoarsenite.