Thallous dihydrogen phosphate, TlH₂PO₄

Thallous dihydrogen phosphate, TlH₂PO₄, is obtained by adding phosphoric acid to a solution of the foregoing salt and crystallising the solution. It crystallises in pearly plates or long needles, which are monoclinic (a:b:c = 3.175:1:1.458; β = 91°44') and isomorphous with di-ammonium hydrogen phosphate, (NH₄)₂HPO₄. The salt melts at 190°; its density is 4.723. The aqueous solution has an acid reaction. According to Lamy and Descloizeaux, there exists a double salt of the formula Tl₂HPO₄.2TlH₂PO₄.