Thorium Orthophosphate, Th₃(PO₄)₄

Thorium Orthophosphate is practically insoluble in water and only sparingly soluble in dilute acids; its formation therefore serves partially to separate thorium from other metals occurring in monazite sand, whose phosphates are more soluble in dilute sulphuric acid.

According to Cleve the normal phosphate, Th₃(PO₄)₄.4H₂O, is precipitated when disodium hydrogen phosphate is added to thorium nitrate solution; but Volck has shown that the precipitate contains sodium. Various complex phosphates of thorium and the alkali metals have been described, also the chlorophosphate ThCl₄.3ThO₂.2P₂O₅ and the bromophosphate ThBr₄.(3ThO₂.2P₂O₅)₃.