Pyroarsenic acid

X 10 , and A3 = 3.0 x 10 . Hydrated arsenic acid loses water when heated at about 100°C with the formation of pyroarsenic acid [13453-15-1/, H4AS2O2. At higher temperatures additional water is lost to yield metaarsenic acid [10102-53-1/, HAsO. Arsenates derived from each of these acids are known. Treatment of a meta- or pyroarsenate with cold water gives the orthoarsenate. 

Arsenic acid is prepared by treating arsenic trioxide with concentrated nitric acid or by combination of arsenic pentoxide with water. The latter reaction is very slow. It is also formed when meta- or pyroarsenic acid is treated with cold water. 

Arsenic acid reacts with metal salts forming their orthoarsenates, e.g., calcium orthoarsenate. Reaction with silver nitrate in neutral solution produces a chocolate-brown precipitate of silver orthoarsenate. It forms pyroarsenic acid (or pyroarsenate) on heating over 100°C. It is reduced to arsenous acid (or arsenites) when treated with reducing agents. 

Hydrates of Arsenic Pentoxide.—The existence of hydrates of arsenic pentoxide corresponding to the ortho-, meta- and pyro-phos-phorie acids has not been established. Many such products have been described in the literature, the formation of which could not be confirmed by subsequent workers.1 It appears certain that a tetrahydrate, As,05.4H20, and a (3, J)-hydrate, 3As205.5H20, exist there is also evidence of the formation of a heptahydrate, As,03. H,0. and of a dihydrate (pyroarsenic acid), As203.2H20. 

The tetrahydrate loses water even at -10° C., and dehydration over sulphuric acid, phosphorus pentoxide or potassium hydroxide proceeds regularly at the ordinary temperature until the (3, 5 )-hydrate remainsunder these conditions there is no indication that pyroarsenic acid, As205.2H20, is formed as an intermediate product. Complete dehydration occurs when either of the above hydrates is heated to 180° to 200° C., and the isobaric decomposition curve 8 of the tetrahydrate gives no indication of the formation of any hydrate other than 3As2Os. 5H20 neither solid solutions nor mixed crystals appear to be formed. 

The dihydrate, As205.2H20, or pyroarsenic acid, H4As207, was described by Kopp10 as a hard mass formed when aqueous arsenic acid was evaporated at a temperature between 140° and 180° C., but the compound could not be obtained by Auger11 and other workers.12 Rosenheim and Antelmann,13 however, maintain that pyroarsenic acid does exist and is obtained in the form of hard microscopic prismatic crystals by evaporating a concentrated aqueous solution of pure arsenic... 

Pyroaxsenic acid—H As,0,—266.—Arsenic acid when heated to 160° (320° F.) is converted into compact mosses of pyroarsenic acid 2H, AbO, = H,As,0, -h H,0. It is very prone to revert to ortboarsenic acid by taking up water. 

All the condensed arsenates resulting in this way are hydrolyzed to orthoarsenate not only in aqueous solution but also on exposure to moist air. The acid triarsenate is an intermediate but not the pyroarsenate (310). Clearly the triarsenate is marked by special stability, for triarsenic acid, H5AS3O10, also occupies a special place in the series of hydration products form arsenic(V) oxide (269, 310). 

Aluminium Pyroarsenate, Al4(As207)3, is obtained7 by fusing at as low a temperature as possible a mixture of alumina and 15 to 16 times its weight of sodium or potassium dihydrogen arsenate colourless transparent crystals are formed on cooling. These dissolve only very sparingly in hot water, but are readily soluble in dilute acids. 

Complex pyroarsenates of composition BaCuAs207 and BaHgAs207. H20 have been obtained 6 by heating aqueous solutions of arsenic acid with mixtures of barium hydroxide and the oxide or carbonate of the other metal at 180° to 200° C. Using copper carbonate and solutions of arsenic acid of 5 to 25 per cent, concentration, the barium-copper salt is formed quantitatively and the precipitate is of constant composition,6 but outside the above limits of acid concentration the precipitate consists mainly of a mixture of orthoarsenates. 

Cadmium Metarsenate, Cd(As03)2, has been obtained by dissolving cadimum chloride in arsenic acid at 200° C. and drying the precipitate at 100° C.2 When fused with cadmium oxide or carbonate, cadmium pyroarsenate, Cd2As207, is obtained as colourless crystals.3... 

Manganic Orthoarsenate, Mna(As04)2.2H20, may be prepared by treating a hot aqueous solution of arsenic acid with a concentrated solution of manganese nitrate.10 It forms a dark grey powder which dissolves slowly in hydrochloric acid but is insoluble in nitric acid. When heated to redness it forms manganous pyroarsenate. 

The salt may also be obtained by the action of hot water on strontium dihydrogen orthoarsenate 8 or on the pyroarsenate 9 or by the action of ammonia on a hot solution in hydrochloric acid of the precipitate obtained by mixing solutions of strontium chloride and sodium monohydrogen orthoarsenate.10 In the last case the anhydrous salt crystallises out, but if the mother liquor is kept in an atmosphere of ammonia, the monohydrate, SrHAs04.H20, crystallises out. The density of the latter is 3-606 at 15° C. and of the anhydrous salt 4-035. The monohydrate becomes anhydrous at 130° C. and at a higher temperature yields the pyroarsenate.11... 

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. 

A basic stannic pyroarsenate, SnAs2O7.SnO3.10H2O, is obtained when the white gelatinous precipitate formed by boiling a solution containing sodium stannate, excess of sodium orthoarsenate and nitric acid, is dried at the ordinary temperature.4 The salt becomes anhydrous at 120Q C. 

Zireonyl Pyroarsenate, (Zr0)2As207.nH20, is obtained as a white precipitate when sodium monohydrogen orthoarsenate is added to a solution of zirconium sulphate.7 When dried at 110° C. the monohydrate, (Zr0)2As207.H20, remains. It is insoluble in water, but readily dissolves in hydrochloric acid. 

If the test is made with the sulphide precipitated in acid solution, only mercury will interfere by converting the arsenic into magnesium ammonium arsenate and heating to redness, the pyroarsenate Mg2As207 remains and any mercury salts present are volatilized. This forms the basis of a delicate test for arsenic. 

SYNS DIARSENIC ACID, TETRASODIUM SALT SODIUM ARSENATE (DOT) SODIUM DIARSENATE SODIUM PYROARSENATE TETRASODIUM... 

Chromium Pyroarsenate, Cr4(As20-)3, is produced when a small quantity of chromium sesquioxide is fused at a low temperature with sodium or potassium meta-arsenate. It jdelds green transparent prisms, sometimes elongated, sometimes flattened, wnth oblique extinctions insoluble in dilute acids. 

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