Detection of Uranium

Metallic uranium dissolves readily in hydrochloric or sulphuric acid, but is rather less soluble in nitric acid. It is insoluble in alkaline solutions. The colours of uranium compounds are distinctive, uranous salts being green or blue and forming green or bluish-green solutions, whilst the uranyl salts are yellow. The former act as powerful reducers; the latter are more stable and are in more general use. Both in solution exhibit characteristic absorption spectra.

When a uranium compound is heated in a borax bead, or in a bead of microcosmic salt, in an oxidising flame, an orange-yellow coloured bead results which may become greenish on cooling. In the reducing flame the bead assumes a green colour.

Solutions of uranous salts are rapidly oxidised in air, especially in sunlight or in ultra-violet light. The addition of alkali precipitates reddish-brown uranous hydroxide, insoluble in excess, but soluble in ammonium carbonate. Solutions of uranyl salts give with alkali hydroxides and carbonates a yellow amorphous precipitate of the alkali diuranate, which dissolves in ammonium carbonate, forming a soluble double carbonate. The following precipitation reactions also reveal the presence of uranyl salts in solution:

1. An alkali phosphate precipitates yellow uranyl hydrogen phosphate, UO₂HPO₄, or in presence of an ammonium salt, uranyl ammonium phosphate, UO₂NH₄PO₄; uranium may be qualitatively separated from chromium and vanadium by this means,
2. Potassium ferrocyanide produces a brown coloration in very dilute solutions; with more concentrated solutions a brownish-red precipitate of potassium uranyl ferrocyanide results.
3. Ammonium sulphide gives a brown precipitate of uranyl sulphide, which on prolonged heating decomposes with separation of sulphur and hydrated uranous oxide.

Uranyl salts may be reduced in solution to the green uranous condition by means of zinc and dilute acid, or more slowly by means of oxalic acid.

Solutions of uranyl salts are not precipitated by the addition of cupferron, the ammonium salt of nitrosophenyl-hydroxylamine, but if first reduced by means of zinc, all the uranium may be precipitated in acid solution by means of this reagent. The precipitate is soluble in chloroform. This reaction affords a means of separating vanadium and uranium by successive precipitation.

The presence of uranium may be detected in ores or slags by adding an excess of zinc to a solution in nitric acid. When the reaction subsides, a yellow deposit, apparently of the hydrated trioxide, UO₃.2H₂O, remains on the zinc. The test is not applicable in presence of hydrochloric or sulphuric acid, and it is not successful in presence of large quantities of iron or vanadium; other metals likely to be present, e.g. Au, Pt, Th, Pb, W, Ti, Cr, Hg, Cu, do not interfere.

For the microchemical detection of uranium, the formation of either thallium uranyl carbonate or sodium uranyl acetate may be employed.