Samples Containing Thorium or Uranium

Oxides, Hydroxides, and Salts

Th: The principal oxide of thorium is ThO₂, which is insoluble in acids except hot, concentrated sulfuric acid. It is not affected by fusion with the alkalis. Thorium hydroxide, when freshly precipitated, is readily soluble in acids, but after drying is more resistant. It is soluble in the alkali carbonates. Thorium chloride and nitrate are soluble in water. The anhydrous sulfate is soluble in ice water, but upon heating, it separates as a hydrate. The solution chemistry of Th is centered on the chemistry of Th⁺⁴. Th⁺⁴ forms an insoluble hydroxide with NaOH or NH₄OH. The precipitate is not soluble in an excess of the reagent. Alkali carbonates will precipitate Th⁺⁴ as a basic carbonate, which is readily soluble in an excess of the concentrated reagent. Th⁺⁴ should not be mixed with oxalate, phosphate, sulfate, or fluoride where insoluble precipitates are formed.

U: UO₂ is soluble in nitric acid and aqua regia and will dissolve with difficulty in HCl, HBr, and sulfuric acid. U₃O₈ is insoluble in water, readily soluble in nitric acid, but only slightly soluble in HCl. In general, uranates (UO₄^-2) are insoluble in water and soluble in acids. Most uranyl (UO₂⁺²) salts are soluble in water and those insoluble are dissolved in HCl. The salts of uranium may be divided chiefly into two classes: (1) the uranous or uranium IV, U⁺² and (2) the uranyl UO₂⁺⁺. Fluoride will precipitate the IV, but not the uranyl ion. However, to play it safe avoid HF as well as phosphate, sulfate, and carbonate.

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