Estimation of Uranium

Many methods for the quantitative determination of uranium have been described, but the following appear to be the most trustworthy: (1) precipitation with ammonia as ammonium diuranate, and ignition to urano-uranic oxide; (2) precipitation of uranyl ammonium phosphate by means of ammonium phosphate in presence of ammonium acetate, and ignition of the precipitate to uranyl pyrophosphate; and (3) precipitation as uranyl sulphide by means of ammonium sulphide, and ignition to urano-uranic oxide.

In the first process the solution containing the uranyl salt is made slightly acid with hydrochloric or sulphuric acid and treated with a slight excess of ammonium hydroxide (free from carbonate) at the boiling-point. It is best to avoid the use of glass vessels as the ammonium uranate tends to carry down silica. The ignition of the precipitate should be carried out in a porcelain crucible allowing access to air, as platinum permits diffusion of gases from the flame which cause slight reduction. It has been considered to be more accurate to ignite the precipitate in a stream of hydrogen and weigh as uranous oxide, but this is not the case, owing to the formation of nitride.

The phosphate method is difficult owing to the fineness of the precipitate, and a complete separation from the alkali metals is almost impossible. After boiling for about fifteen minutes, the precipitate becomes crystalline and is more easily filtered. Precautions must be taken in cooling and weighing the ignited uranyl pyrophosphate since it rapidly absorbs moisture.

Precipitation with ammonium sulphide is best performed at 80° C. in presence of ammonium chloride. Some ammonium uranate is always formed, and complete separation from the alkali metals is only obtained by repeating the precipitation. The results obtained by this method are liable to be high owing to some sulphide being oxidised to sulphate during ignition.

Other methods, such as precipitation with hydrogenperoxide, or precipitation with hydrogen fluoride after reduction to the uranous condition, are less satisfactory.

Uranium minerals may be obtained in solution, in a suitable condition for estimation, by the following process. The ore is dissolved in aqua regia, or, if necessary, fused with alkali bisulphate and extracted with hot hydrochloric acid. After evaporation to dryness, the residue is taken up with dilute hydrochloric acid, and the solution saturated with hydrogen sulphide in order to remove any copper, lead, bismuth, arsenic, antimony, or any other metal yielding an insoluble sulphide. The filtrate is concentrated and treated with ammonium carbonate, which precipitates the carbonates of the alkaline earths, iron, and most of the rare earths. The filtrate is neutralised by hydrochloric acid, evaporated to dryness, and the residue ignited to drive off ammonium salts, and then redissolved in dilute acid. The remaining rare earths, and particularly thorium, are next precipitated by the addition of oxalic acid. The filtrate, which contains the uranium in the uranyl condition, may now be precipitated by any of the methods described above.

Uranium salts in solution in acetic acid may be quantitatively separated by electrolytic means. The uranium is completely precipitated at the anode as hydrated oxide, and this may be converted by ignition to urano-uranic oxide, and weighed as such. A rotating anode should be used.

Volumetric Methods for estimating uranium are less satisfactory than those described above. The usual course is to reduce the uranium to the uranous condition by means of zinc and sulphuric acid, and then to titrate the solution with a standard solution of permanganate. The uranium is re-oxidised according to the equation:

5U(SO₄)₂ + 2KMnO₄ + 2H₂O = 2KHSO₄ + 2MnSO₄ + H₂SO₄ + 5UO₂SO₄.

According to Pierle the method is inaccurate owing to the uranium being reduced beyond the uranous condition, and it does not appear possible to eliminate this error, or to take the reduction to any further definite point. The reduction is less satisfactory in hydrochloric acid solution, or by means of stannous chloride. Rapid reduction may be obtained by shaking with zinc amalgam. The end point in the titration may be observed directly, as the green solution gradually becomes yellow and finally assumes a pink tinge; or the process may be followed electrometrically. Methods of titration involving the use of dichromate and of potassium iodide have been described.

A colorimetric method of estimating small quantities of uranium in solution depends upon the red colour obtained when a uranyl salt is treated with sodium salicylate. Free mineral acid, iron, acetic acid, alcohol, or acetone must be absent, but neutral alkali salts do not interfere.

The frequent occurrence of vanadium in uranium minerals renders the separation of these two metals of importance. One method in use is based on the solubility of uranyl nitrate in ether, whilst vanadie and also molybdic and tungstic acids are insoluble. A solution containing these substances may therefore be evaporated to dryness, and the uranyl salt extracted from the residue with ether. Another method depends upon the fact that uranyl nitrate is readily soluble, whilst vanadium compounds are insoluble, in acetic acid of 95 per cent, strength to which nitric acid has been added in the proportion 1:20.