Potassium metarsenite

Barium Metarsenite, Ba(As02)3, 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.2 It is a white powder, easily soluble in water, but it can also be obtained as a gelatinous mass 3 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.4... 

Potassium Metarsenite, KAs02, is obtained in an impure form when potassium carbonate and the above diarsenite are boiled together in aqueous solution for several hours.1 A syrupy mass is obtained. That the metarsenite should exist has been shown 2 by results obtained in measuring the effect of the progressive neutralisation of arsenious acid by potassium hydroxide on the freezing point of aqueous solutions. 

Mercurous Metarsenite, HgAs02, separates as a yellowish precipitate when potassium tetrarsenite is gradually added to mercurous nitrate solution.6 The salt is dried at 125° C. It is unstable and is decomposed by excess of the alkali arsenite, by alkali hydroxides and by aqueous ammonia. 

Tin Arsenites.—Stannous Orthoarsenite, Sn3(As03)2.H20, was obtained by Stavenhagen 3 by adding a solution of arsenious oxide to a solution containing potassium ehlorostannite and potassium chloride. It may also be obtained 4 by the action of sodium metarsenite or potassium tetrarsenite on a solution of stannous chloride. It is a white amorphous powder, sparingly soluble in water, but readily dissolved by dilute acids and alkalis with separation of metallic arsenic. It is also soluble in excess of arsenious acid. 

Uranyl Metarsenite, U02(As02)2, is formed as a yellow precipitate when a solution of sodium metarsenite 2 or of potassium tetrarsenite 3 is added to one of uranyl nitrate. It is insoluble in ammonia, but dissolves readily in acids. On heating, it darkens in colour and decomposes. 

Zinc Orthoarsenite, Zn3(As03)2, is obtained7 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,8 or from a solution of zinc sulphate by potassium tetrarsenite9 or sodium metarsenite.10 Zinc orthoarsenite dissolves readily in acids. It decomposes when heated. 

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