Thallous pyrophosphate, Tl₄P₂O₇

Thallous pyrophosphate, Tl₄P₂O₇, is prepared by heating dithallium hydrogen phosphate to dull redness. It dissolves in water (with the separation of some insoluble white powder), and on evaporating the solution, beautiful, lustrous monoclinic crystals (a:b:c = 1.427:1:1.2921; β = 114°0') of the anhydrous pyrophosphate separate. On standing, the mother liquor deposits monoclinic crystals (a:b:c = 2.1022:1:1.9217; β = 114°57') of the dihydrate, Tl₄P₂O₇.2H₂O. The density of the anhydrous salt is 6.786.

An acid pyrophosphate, Tl₂H₂P₂O₇, is obtained by heating dihydrogen thallium phosphate to 240°. It is readily soluble in water, from which it crystallises in small, short prisms, melts at 270°, and is converted into the metaphosphate at a red heat.