Trithallous orthophosphate, Tl₃PO₄

Trithallous orthophosphate, Tl₃PO₄, is obtained by adding excess of thallous carbonate to phosphoric acid, by mixing thallous sulphate and disodium hydrogen phosphate and adding a little ammonia, or by fusing thallium metaphosphate with two molecular proportions of sodium carbonate. It crystallises in long, colourless, anhydrous needles of density 6.89, and melts at a red heat. One litre of a saturated aqueous solution contains 4.97 grams of the phosphate at 15° and 6.71 at 100°. The phosphate is insoluble in alcohol, but dissolves readily in aqueous solutions of ammonium salts. The aqueous solution has an alkaline reaction.