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separating the lead either by passing sulphuretted hydrogen or evaporating with sulphuric acid.
SEPARATION FROM TUNGSTEN.
Tungsten may be completely precipitated from neutral solutions by uranyl nitrate or chloride (but not acetate, according to Hitchcock) in the form of uranyl tungstate. This fact, and the marked tendency of tungstic acid to precipitate spontaneously from acidified solutions, prevent uranium from being determined as phosphate in a mixture of the two metals. Evaporation to dryness on the water-bath with a good excess of hydrochloric acid is a very satisfactory means of making the separation, because, after boiling with dilute hydrochloric acid (1 in 5), all the tungsten is left in the residue, and requires to be merely ignited and weighed as tungstic oxide; and all the uranium is in the filtrate ready to be estimated in the usual manner.
From a synthetic mixture of Oʻ1992 gram tungsten and 0-2045 gram uranium there were obtained 0 1989 and 0-2040 gram respectively.
SEPARATION FROM CHROMIUM.
The separation from this element can be made most easily when it exists as chromic acid ; as chromic oxide it is completely precipitated along with the uranyl phosphate. The chromium can be conveniently oxidized with sodium peroxide, but it is almost impossible in the presence of uranium to entirely decompose the excess of peroxide by boiling. It is therefore necessary to proceed as follows:
Neutralize most of the free acid with sodium carbonate, and then add sodium peroxide until the solution becomes strongly alkaline. Both metals are precipitated and again dissolved, the chromium being entirely in the higher state of oxidation. Boil the solution well, then acidify, and add a small volume of decinormal permanganate (10 c.c. should be ample), and boil for about ten minutes. If the permanganate decomposes at all (thus showing that a portion of the chromic acid was reduced by the residual hydrogen peroxide in the acid solution) then filter off the manganic oxide through asbestos fibre, and destroy any unused permanganate with a few drops of hydrochloric acid. The acid liquor is now made slightly alkaline with ammonia, decidedly acid with acetic,
and when it boils briskly an excess of microcosmic salt is added, and the filtration proceeded with as in the separation from molybdenum. The chromium in the filtrate is estimated volumetrically.
Uranium could probably be precipitated from an acetic solution of chromic acid as uranyl vanadate; the converse of this is Klecki's method for separating vanadium from chromium.
The uranium may also be precipitated with caustic soda, or the chromium may be thrown down with mercurous nitrate. Other means of making the separation are those of electro-deposition, and that depending on ignition of the oxides in a current of hydrogen and extraction of the uranium with nitric acid.
SEPARATION FROM VANADIUM.
Vanadium is an essential constituent of Carnotite, and occurs also in small amounts in other uranium ores.
Artificial compounds of uranium and vanadium do not occur in commerce, and therefore the separations to be found in the literature of the subject refer almost exclusively to the analysis of particular ores, and may not be generally applied. For instance, such amounts of
vanadium as occur in pitchblende are separated (Lallemand) by pouring the filtered nitric acid solution of the mineral into sodium carbonate, which causes the vanadium to go down with the iron and the uranium to pass into solution. Fritchle, on the other hand, precipitates the dissolved carnotite with a mixture of sodic hydrate and carbonate, which throws out iron andkuranium, and leaves vanadium in solution. The separation with mercurous nitrate and mercuric oxide (Langmuir), and the extraction of uranium with ammonium nitrate after evaporating the mixture to dryness with nitric acid are both more widely useful.
The analysis of a complex ore may often be completed without any separation of uranium and vanadium being made. This is accomplished by determining vanadium volumetrically with ferrous sulphate and permanganate, or colorimetrically with hydrogen peroxide, according to its amount, and making the necessary correction in the weight of the precipitated compound used for the estimation of the uranium. The varied uses of devices of this kind cannot be explicitly stated, but it may generally be assumed that, when precipitated with ammonia, the uranium will carry down all the vanadium, and the weighed U,0, will be too
heavy by the V,0, equivalent of the amount of vanadium already determined. Also, when uranium is precipitated as phosphate in the ordinary manner, any vanadic acid carried down merely replaces its equivalent of phosphoric acid, and the error introduced
may be too trifling for consideration, although the exact amount of it can be arrived at by determining the vanadium in the ignited uranyl pyrophosphate.
An accurate separation can be made by pouring a faintly acid and heated solution of the two metals into a heated mixture of microcosmic salt and 5 or
10 c.c. ammonium acetate. The latter solution is kept violently agitated, while the former is added through a funnel with a stem so constricted that only a thin column passes out of it. The precipitate is washed by decantation and weighed as usual; the combined washings are evaporated to a convenient bulk, and titrated with ferrous sulphate and permanganate.
The following results were obtained :