Potassium orthoarsenite

Barium Orthoarsenite, Ba3(As03)2, has been prepared8 by the addition of potassium orthoarsenite to an aqueous solution of barium chloride and drying the precipitate at 100° C. in a current of hydrogen. It is obtained as a white amorphous powder, readily soluble in hot water and in dilute acids. When heated rapidly to about 730° C. it is decomposed into arsenic and the arsenate 9 the mechanism of the reaction is not understood. Slow heating below 300° C. causes loss of arsenious oxide. 

Cobalt Arsenites.—Several compounds have been prepared. Cobalt ortho arsenite, Co3(As03)2.4H20, is obtained3 as a pink precipitate when a solution of cobaltous chloride in 50 per cent, alcohol is treated with a solution of potassium orthoarsenite which has been just neutralised with acetic acid. The precipitate is soluble in dilute acids. On heating it turns black. A basic salt, of composition 7Co0.As203, is obtained when a solution of cobalt nitrate is treated with sodium orthoarsenite,6 the former being in excess the amethyst-coloured precipitate first formed contains combined water, which may be completely removed at 150° C. 

The monohydrate, Pb3(As03)2.H20, was described by Stavenhagen,6 who obtained it by drying in air the precipitate resulting from the addition of potassium orthoarsenite to a dilute aqueous solution of lead nitrate. 

Magnesium Orthoarsenite, Mg3(As03)2, is obtained as a white precipitate, easily soluble in water and in dilute acids, by adding to a solution of magnesium chloride in 50 per cent, alcohol a solution of potassium orthoarsenite made just neutral with acetic acid.7 It is also formed when a solution of potassium tetrarsenite is added slow ly to an aqueous solution of a magnesium salt.8 When heated in an inert atmosphere the salt decomposes forming arsenious and magnesium oxides.9... 

Strontium Arsenites.—An impure strontium orthoarsenite, containing more strontium than corresponds to the formula Sr3(As03)2, has been obtained10 as a white floeculent precipitate by treating strontium chloride solution with potassium orthoarsenite in the cold. If the precipitate is dissolved in a dilute solution of acetic acid, on evaporation strontium hydrogen orthoarsenite, SrHAsOs.H20, is obtained. 

Thallous Orthoarsenite, Tl3As03, is a yellowish-red crystalline solid obtained 3 by boiling a solution of thallous sulphate with potassium orthoarsenite and gradually adding potassium hydroxide solution or the thallous salt may be boiled with alkali and then treated with aqueous arsenious oxide or potassium orthoarsenite. The crystals are sparingly soluble in water or alcohol, but readily dissolve in acids. 

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. 

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... 

Ferric Orthoarsenite, FeAs03, was described by Reichard 9 as a rust-yellow powder obtained by adding potassium tetrarsenite to a dilute aqueous solution of ferric chloride. A substance of similar chemical composition has been found in a crystalline deposit formed during the Deacon process of making chlorine. The crystals of the pure salt are monoclinic, their crystallographic elements being 10... 

Lead Orthoarsenite, Pb3(As03)2, is obtained as a white precipitate on adding a solution of basic lead acetate to a boiling aqueous solution of arsenious oxide,9 or of potassium tetrarsenite,10 or by the action of an alkali plumbite on an alkali arsenite.11 When dried in a dark air oven the arsenite remains white, but when exposed to light some specimens turn brown, a change which has been attributed to reduction of the lead to suboxide,12 but some arsenate and free arsenic are formed. All specimens... 

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. 

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