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Gravimetrically, the determination of uranium depends on its precipitation as alkaline uranate, which is weighed as such or ignited to oxide; as protoxide by means of ammonium sulphide ; or as phosphate. The yellow precipitate formed when caustic soda is added to a solution of uranyl nitrate is a hydrated diuranate, which loses water merely on ignition, and may be weighed as Na, U.O. When ammonia is used instead of soda, the analogous precipitate is decomposed on ignition, and yields a compound approximating more or less to UAO, according to circumstances. If only moderately heated the ammonium uranate does not ignite uniformly to the dark green colour of U,O, ; it contains visible streaks and patches of a yellow or brown colour." At higher temperatures, however, the oxide becomes practically black (though
1 These are said to be formed when traces only of alkaline earths are present.
it gives a green streak), and loses slightly in weight. According to Zimmermann, pure UAO, is formed only when heated and cooled in a current of oxygen; if heated in air and cooled quickly a small amount of oxygen is lost, and in an indifferent gas the loss is still greater: nevertheless, the elaboration of a current of oxygen may be omitted in technical assays, as the error is comparatively trifling. The precipitation of uranium by means of ammonium sulphide is accurate, and lends itself to a number of important separations; but great care must be taken to exclude carbonates from the solution and the reagent, as they prevent a complete precipitation. The precipitate thus formed may be ignited in hydrogen and weighed as UO, or ignited in air to USO. Kern found that (NH), U.O., could be readily converted to UO, by ignition in hydrogen, but that a previously ignited precipitate could not be so reduced and reweighed for purposes of control, particularly if the operation was carried on in a porcelain crucible. The precipitate of UO, HPO, which is formed when sodium phosphate is added to an acetic acid Solution containing uranium, is gelatinous, and washes badly—worse than ammonium uranate
even,-and is to be desired only on account of the lower factor needed to convert the weight of the ignited precipitate to metallic uranium. This difficulty can be partly met, as Kern points out, by the careful use of dihydrogen ammonium phosphate instead of the sodium salt; the most satisfactory procedure, however, is that noticed in the next section.
Attempts to weigh the dried precipitate as ammonium uranyl phosphate (UO,NH, PO,) have not met with much success; it is customary, therefore, to ignite it apart from the paper, and weigh as pyrophosphate (UO), P.O. If ignited at low redness the precipitate has a pure yellow colour, but at higher temperatures it is partly reduced and becomes green. It is feasible, of course, to ignite precipitate and paper together, and then treat the green residue with nitric acid, evaporate, and ignite to low redness, so as to restore the characteristic yellow colour.
Uranium may be electro-deposited from acetate solutions, and ignited to UAOs. Separations from barium, calcium, magnesium, and zinc have been based on this reaction, but it is interfered with by many of the common metals, particularly those of the iron group.
Volumetric processes for the determination of uranium have attracted little attention, and failed to inspire much confidence. The titration with a standard solution of microcosmic salt, using ferrocyanide as a spot indicator, is known rather as the inverse of a common way of titrating phosphoric acid than as a widely usable means of estimating uranium. Guyard's process depends on the precipitation of a triple ammonium-uraniummanganese phosphate when a solution of manganic metaphosphate is added to an acetate solution of uranium. The process is only suitable for the estimation of large amounts of uranium. The volumetric method most in vogue depends on the reduction of UOs to UO, in acid solutions by means of zinc, aluminium, or magnesium, followed by titration with permanganate in the usual manner, contact with air being prevented as much as possible. There are no decisive colour changes to indicate complete reduction, so that an abundance of the metallic reducing agent must be used, and a time limit, depending on the amount of uranium dealt with, must be adhered to. Under the most favourable circumstances this limit exceeds the time needed to perform a gravimetric estimation, so that the usefulness of volumetric processes generally are confined to
THE DETERMINATION OF URANIUM AS PHOSPHATE.
ABOUT ten times as much microcosmic salt as there is judged to be uranium present is added to the boiling solution; then dilute ammonia, until a small precipitate is formed ; then just as much nitric acid as clears the solution, but no more; and, finally, sodium thiosulphate equivalent to about Io grams of the crystallized salt. The solution becomes intensely yellow, and almost immediately deposits a voluminous precipitate. After boiling for ten or fifteen minutes, this precipitate contains all the uranium, and is so dense that it settles immediately, and may be easily washed by decantation and collected on the filter. The precipitate is transferred to a porcelain crucible, ignited, the green residue weighed, just moistened with nitric acid and gently heated. It dissolves to a yellow solution with slight effervescence, and is then dried, ignited at low redness, and the yellow