Zinc Arsenite

A crystalline powder of normal zinc orthoarsenite, Zn₃(AsO₃)₂, precipitates at 100° C. when aqueous arsenious acid is added to a solution of zinc sulphate containing ammonia and ammonium chloride. When precipitation occurs in the cold the precipitate rapidly crystallises into an aggregate of needles. A similar precipitate is thrown down by potassium arsenite from a solution of zinc chloride in 50 per cent, alcohol, or from a solution of zinc sulphate by acid potassium arsenite, K₂O.2As₂O₃. Zinc arsenite dissolves readily in ammonia solution, acids, and solutions of ammonia salts. It is also precipitated from zinc sulphate solutions by sodium metarsenite.

A granular white powder of zinc metarsenite, Zn(AsO₂)₂, is obtained by dissolving arsenious acid in sodium hydroxide solution, making just acid to phenolphthalein with dilute sulphuric acid and adding hot zinc sulphate solution.