Cupferron does not react with uranium(VI), but uranium(IV) is quantitatively precipitated. These facts are utilised in the separation of iron, vanadium, titanium, and zirconium from uranium (VI). After precipitation of these elements in acid solution with cupferron, the uranium in the filtrate is reduced to uranium(IV) by means of a Jones reductor and then precipitated with cupferron (thus separating it from aluminium, chromium, manganese, zinc, and phosphate). Ignition of the uranium(IV) cupferron complex affords U3O8.
If uranium is to be determined in the filtrate from the precipitation of the iron group by cupferron, concentrate the solution to 50 mL, add 20 mL of concentrated nitric acid and 10 mL of concentrated sulphuric acid (if not already present) and evaporate until fumes of sulphur trioxide appear. If organic matter still remains (as shown by the appearance of a dark colour upon evaporation), repeat the treatment with nitric acid. Finally, expel the nitric acid by evaporating to strong fuming, after the addition of a little water. Dilute the solution so that it contains about 6 mL of concentrated sulphuric acid per 100 mL. Cool to room temperature and pass the solution through a Jones reductor (Section 10.138); wash the reductor with 5 per cent sulphuric acid, cool the combined reduced solution and washings to 5-10°C, and add excess of a freshly prepared 6 per cent solution of cupferron. The precipitate does not usually form until about 5 mL cupferron solution has been added. Introduce a Whatman 'accelerator' or one-quarter of an 'ashless tablet', allow to settle for a few minutes, and filter through a quantitative filter paper. Wash with cold 4 per cent sulphuric acid containing 1.5 g L~l of cupferron. Dry the precipitate
at 100 °C, ignite cautiously in a platinum crucible, first at a low temperature and then at 1000°C, to constant weight. Weigh as U3O8.