Thallous Borates

Thallous oxide dissolves in fused boron sesqui-oxide, and the liquid mass solidifies either to a clear glass or a crystalline solid. Various borates of thallium have been described by Buchtala.

Thallous tetraborate, Tl₂B₄O₇.2H₂O, is obtained by dissolving thallous carbonate (1 mol.) and boric acid (1-4 mols.) in water, and crystallising. It is also obtained, together with thallous hexaborate, Tl₂B₆O₁₀.3H₂O, by fusing the carbonate (1 mol.) with boric acid (6 mols.) and crystallising the product from water. If more boric acid (8 mols.) is used, a mixture of the hexaborate and thallous decaborate, Tl₂B₁₀O₁₆.8H₂O, is produced.

Thallous octaborate, Tl₂B₈O₁₃.4H₂O, and thallous dodecaborate, Tl₂B₁₂O₁₉.7H₂O, are prepared by dissolving thallous carbonate (1 mol.) in excess of aqueous boric acid (10 mols.). The dodecaborate also forms a pentahydrate, which is monoclinic (a:b:c = 1.583:1:1.955; β = 94°25').

Thallous metaborate, 2TlBO₂.H₂O, crystallises from water in colourless, wedge-shaped crystals which turn red and black on exposure to air.

Thallous perborate, Tl₂B₂O₇, i.e. is obtained as a white powder by adding 30 per cent, hydrogen peroxide to an aqueous solution of any of the preceding borates. It shows the usual reactions of a per-salt and explodes when rapidly heated. The aqueous solution decomposes on evaporation, thallous metaborate being produced.