Thallous selenate, Tl₂SeO₄

Thallous selenate, Tl₂SeO₄, may be prepared by neutralising selenic acid with thallous hydroxide or carbonate. It crystallises from water, in which it is not very soluble, in orthorhombic prisms (a:b:c = 0.5551:1:0.7243) of density 6.875 at 20°, and is therefore isomorphous with potassium selenate (a:b:c = 0.5731:1:0.7319). The mean refractive index, 1.9575 for the D-line, is very high, the value for potassium selenate being 1.5396. The salt is insoluble in alcohol and ether; it melts above 400°.

Of the double selenates that thallous selenate may be anticipated to form with the selenates of the bivalent metals, only the zinc salt, thallous zinc selenate, Tl₂SeO₄.ZnSeO₄.6H₂O, has been described. It forms monoclinic crystals (a:b:c = 0.7479:1: 0.5022; β = 105°54') isomorphous with the corresponding double sulphates and with potassium zinc selenate (a:b:c = 0.7458:1:0.5073; β = 104°12').