Thorium Arsenates

When solutions of thorium salts are treated with aqueous arsenic acid, the precipitates vary in composition according to conditions. The normal orthoarsenate has not been prepared. Thorium monohydrogen orthoarsenate, Th(HAsO₄)₂.6H₂O, is obtained as a crystalline precipitate by adding aqueous arsenic acid (40 per cent.) to a boiling solution of thorium nitrate ( = 2 per cent. ThO₂). If the solutions are more dilute, amorphous or gelatinous precipitates of variable composition are formed. The crystals readily lose water in air. They are gradually decomposed in contact with water, owing to hydrolysis. The salt is insoluble in dilute nitric acid.

Thorium Dihydrogen Orthoarsenate, Th(H₂AsO₄)₄.4H₂O, is obtained in a manner similar to the above, using 50 per cent, aqueous arsenic acid and thorium nitrate solution containing the equivalent of 5 per cent, thoria. It separates as colourless crystals. The salt is converted to the monohydrogen salt by the action of water.