Potassium Arsenites

At 25° C. the ternary system K₂O-As₂O₃-H₂O indicates the existence of two arsenites of potassium, soluble in water without decomposition; these are potassium tetrarsenite, K₂As₄O₇, and potassium metatetrarsenite, K₆As₄O₉.12H₂O.

Potassium Orthoarsenite, K₃NaO₃, has been obtained by the action of alcoholic potassium hydroxide on arsenious oxide; by the addition of potassium sulphate to a solution of barium orthoarsenite and evaporating the filtered solution to dryness over concentrated sulphuric acid in an atmosphere of hydrogen; and by the addition of potassium iodide to an aqueous solution of arsenious oxide and allowing the solution to evaporate in the air. It crystallises in transparent needles, which become turbid on exposure to air. It is readily soluble in water, and by adding excess of arsenious oxide and alcohol, potassium hydrogen diarsenite, KHAs₂O₄, may be obtained by slow crystallisation. The aqueous solution of the orthoarsenite reacts alkaline, and on exposure to air undergoes oxidation to arsenate.

Potassium Pyroarsenite, K₄As₂O₅.6H₂O, may be obtained 1 by addition of potassium sulphate to an aqueous solution of barium pyroarsenite; on evaporation of the filtered solution the potassium salt separates as white crystals.

Potassium Metarsenite, KAsO₂, is obtained in an impure form when potassium carbonate and the above diarsenite are boiled together in aqueous solution for several hours. A syrupy mass is obtained. That the metarsenite should exist has been shown by results obtained in measuring the effect of the progressive neutralisation of arsenious acid by potassium hydroxide on the freezing point of aqueous solutions.

Potassium Tetrarsenite, K₂As₄O₇.2H₂O, is formed when an excess of arsenious oxide is treated with cold aqueous potassium hydroxide, and alcohol added; it forms a syrupy liquid which crystallises in prisms on long standing. It also separates after boiling a concentrated solution containing arsenious oxide and potassium carbonate. If the crystals are dried over sulphuric acid, some water of crystallisation is lost. At 100° C. one molecule of water is lost, and at a slightly higher temperature the salt melts and undergoes partial decomposition; on cooling it sets to a glassy mass.

The effect of rapidly heating to 730° C. the various arsenites of potassium has been investigated. The diarsenite was undecomposed after several hours at that temperature; the others were gradually decomposed to arsenate and arsenic, the decomposition proceeding until an approximately fixed proportion of arsenious oxide was so converted. This proportion was about 14 per cent, for the meta-, 74 per cent, for the pyro- and 100 per cent, for the ortho-arsenite