Potassium orthoarsenate

Potassium Arsenates.—Potassium Orthoarsenate, K3As04, is formed when excess of potassium hydroxide reacts with aqueous arsenic acid 6 or from a mixture of potassium carbonate and potassium monohydrogen arsenate.7 The salt crystallises in needles. The heat of formation is 396,200 calories.8... 

The potassium salt. K3As0 2S 2. H20, is unstable. It has been obtained in an impure form by treating the precipitate resulting from the action of hydrogen sulphide on aqueous potassium orthoarsenate with potassium hydroxide.6 On evaporation of the solution, the salt separates in yellow, hygroscopic crystals which cannot be recrystallised owing to decomposition to orthoarsenate and thioarsenate. 

Arsenates are oxidizing agents and are reduced by concentrated hydrochloric acid or sulfur dioxide. Treatment of a solution of orthoarsenate with silver nitrate in neutral solution results in the formation of a chocolate-brown precipitate of silver orthoarsenate, Ag AsO, which may be used as a test to distinguish arsenates from phosphates. With hydrofluoric acid, orthoarsenate solutions yield hexafluoroarsenates, eg, potassium hexafluoroarsenate [17029-22-0] (KAsFg)2 H2O. Arsenates of calcium or lead are used as insecticides sodium arsenate is used in printing inks and as a mordant. 

Chromium Arsenates.—Chromic Orthoarsenate, CrAs04, is formed as an apple-green precipitate when potassium dihydrogen orthoarsenate is added to a solution of a chromic salt.11... 

Colloidal cobalt arsenate has been obtained in the form of an opalescent jelly either by mixing in the cold a solution of a cobalt salt of a strong acid with potassium dihydrogen orthoarsenate,8 or by successively treating an aqueous solution of a cobaltous salt with ammonium sulphate, acetic acid and an excess of sodium orthoarsenate.9... 

Mercuric Orthoarsenate, Hg8(As04)2, is obtained by precipitating a solution of mercuric nitrate with a solution of sodium mono- or dihydrogen arsenate, or by dropping aqueous arsenic acid into excess of mercuric nitrate solution.4 It is a heavy citron-yellow powder. Hot water dissolves it slightly without decomposition, and shining crystals may separate from the cooled solution. Hydrochloric acid dissolves it freely, nitric acid less readily, and arsenic acid not at all. Brine solution converts it into red-brown mercury oxychloride. Potassium bromide solution colours it brown, and a yellow residue is ultimately left. Potassium iodide forms mercuric iodide. 

Potassium Monohydrogen Orthoarsenate, K2HAs04, is formed12 when potassium carbonate is added to aqueous arsenic acid until effervescence no longer occurs on evaporating the solution to dryness a white residue is obtained. It is soluble in water13 and may be crystallised in... 

The solubility of potassium dihydrogen orthoarsenate in water is 18-87 g. per 100 g. H20 at 6° C. and increases with temperature.14 The solution is acid to litmus.15 The salt is insoluble in alcohol. The dielectric constant16 is 31. 

Strontium Pyroarsenate, Sr2As207, is formed when the monohydrogen orthoarsenate is heated abo%re 360° C.1 or when potassium monohydrogen orthoarsenate is fused with strontium oxide or carbonate.2 It is slightly soluble in mineral acids. 

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