Tin Arsenates

The arsenates of tin are not well-defined. Stannous monohydrogen orthoarsenate, SnHAsO₄.0.5H₂O, separates as a white bulky precipitate when a solution of potassium orthoarsenate in concentrated acetic acid is gradually added to stannous chloride solution. On heating it decomposes into stannic oxide and arsenious oxide, and a trace of arsenic is liberated. Stannic orthoarsenate, Sn₃(AsO₄)₄.6H₂O, has been obtained 3 in the form of a gel by adding stannic chloride to moderately concentrated aqueous arsenic acid; the mixture gradually thickens and after some weeks becomes set. On dialysis it yields a transparent colourless gel of density 1.135. It is slowly soluble in water and is reprecipitated on addition of mineral acids or certain salts, such as chloride of ammonium, calcium or iron or silver nitrate, but not by mercuric chloride, alkali carbonate, acetic acid or alcohol. Concentrated acids and alkalies readily dissolve the gel. It is decomposed on heating, much water being expelled at 100° C. but a little being retained even above 200° C.; arsenious oxide is liberated below a dull red heat.

A basic stannic pyroarsenate, SnAs₂O₇.SnO₂.10H₂O, 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. The salt becomes anhydrous at 120° C.

By mixing sodium stannate and sodium orthoarsenate in alkaline solution, or by dissolving stannic orthoarsenate in sodium hydroxide and allowing to crystallise, the compound Na₂SnO₃.5Na₃AsO₄.60H₂O has been obtained as white needle-shaped crystals. The compound 6Na₂O.SnO₂.2As₂O₅.50H₂O has also been described.