Zinc Arsenites

When a mixture of zinc oxide and arsenious oxide is heated, combination occurs; at about 250° C. the metarsenite is formed, and at about 420° C. the orthoarsenite is the product. At higher temperatures oxidation occurs and at 500° C. As^III and As^V are present in approximately equal amounts, although the total arsenic content is reduced owing to volatilisation. At 600° C. the product contains only As^V, and with rising temperature the rate of combination rises to a maximum at 700° C., thereafter remaining constant.

The mineral barthite, occurring in the Otavi valley, contains zinc metarsenite associated with copper.

Zinc Orthoarsenite, Zn₃(NaO₃)₂, is obtained as a white crystalline precipitate when aqueous arsenious acid is added to a solution of zinc sulphate containing ammonia and ammonium chloride; precipitation occurs in the cold, an aggregate of crystalline needles being rapidly formed. A similar precipitate is thrown down by potassium orthoarsenite from a solution of zinc chloride in 50 per cent, alcohol, or from a solution of zinc sulphate by potassium tetrarsenite or sodium metarsenite. Zinc orthoarsenite dissolves readily in acids. It decomposes when heated.

Zinc Metarsenite, Zn(AsO₂)₂, is obtained as a granular white powder by dissolving arsenious acid in sodium hydroxide solution, making just acid to phenolphthalein with dilute sulphuric acid and adding hot zinc sulphate solution. It is also formed when aqueous arsenious acid is mixed with a solution of zinc acetate in dilute acetic acid.