« AnteriorContinuar »
undesirable one. It is much better to precipitate the uranium from acetic solutions with ammonium phosphate, which, according to Fresenius, effects a complete separation from alkalis and alkaline earths. Potash is the most difficult element to separate, and if present in large amounts a second precipitation may
be necessary. The separation of calcium as oxalate from ammonium carbonate solutions has been strongly disparaged by Alibigoff
. He effects the separation- also from alkalis and strontium, but not barium — by boiling the solution, containing ammonium chloride or nitrate, with an emulsion of mercuric oxide.
The separation of barium with sulphuric acid is accurate and convenient. The separation of calcium and strontium, in the same way, from alcoholic solutions is practised, but for small amounts is much less satisfactory.
The precipitation of uranium as phosphate by boiling acetic acid solutions with thiosulphate effects a complete separation from calcium. If no acetic acid is present a portion of the calcium is precipitated as phosphate.
The following tests were made on solutions (300 c.c.) containing o'lo gram of calcium and 0-2045 gram uranium :
The lime in the filtrate can be determined simply by adding an excess of ammonium oxalate, allowing the solution to cool, filtering, igniting the residual oxide, transforming it to sulphate, and weighing. The weight of the oxide cannot be relied upon, as it may contain sulphuric acid.
Magnesium does not interfere with the precipitation of uranium.
The presence of metals whose sulphides are insoluble in very dilute acid solutions would interfere with the chosen method of precipitating uranium. When any single member of this group—which includes lead, cadmium, mercury, bismuth, copper, silver, tin, antimony, and arsenic
-has to be separated, its precipitation with sulphuretted hydrogen, from a suitably acidified solution, can be arranged. But if it is desired to detect and separate any (unknown) one of them, the acidity must be slight enough to permit the complete precipitation of the one whose sulphide is most easily dissociated, viz. lead. A complete precipitation of lead as sulphide can be made from solutions containing 2 per cent. of concentrated hydrochloric acid. When 2) per cent. of concentrated acid is present the precipitation is incomplete. This is the conclusion arrived at by Kern, after making a series of quantitative separations under varying conditions of acidity and temperature.
SEPARATION FROM SILICA.
The ignited pyrophosphate always contains silica when the precipitation has been made from a solution containing a soluble silicate. Thus, from a solution containing sodium silicate equal to Oʻ184 gram silica, 0-2045 gram uranium carried down in the two cases O'010 and 0.006 gram silica respectively.
If the silica is not removed by evaporation with acids prior to the precipitation, it remains insoluble on digesting the pyrophosphate with
nitric acid, and can be filtered off, weighed, and approximately allowed for, or the uranium may be reprecipitated from the filtrate.
Uranyl pyrophosphate loses weight when treated with hydrofluoric acid alone.
With a mixture of hydrofluoric and nitric acid no loss occurs ; but it is necessary to evaporate a second time with nitric acid only, in order to secure the yellow pyrophosphate free from fluorides. A mixture of hydrofluoric and sulphuric acids may also be used if the weight of the green
residue is being depended upon; but in that case a very strong ignition is needed in order to drive off all the sulphuric acid.
SEPARATION FROM MOLYBDENUM.
Molybdenum is partly precipitated as sulphide when an acid solution containing it is boiled with thiosulphate. The means of precipitating uranium previously adhered to cannot therefore be used in the presence of this element; but a very satisfactory determination can be made by raising the clear acid solution of the two metals to boiling point, and adding at once the required amount of phosphate mixed with as much hot ammonium acetate as will destroy all the free mineral acid.
The precipitate is then washed well by decantation, and collected on the filter. The washing is somewhat facilitated by adding a small quantity of floured sulphur during the decantation.
The estimation of the molybdenum in the filtrate, at least so far as its precipitation as lead molybdate is concerned, is complicated by the presence of phosphoric acid ; it may therefore be remarked, that molybdenum can be estimated on a separate portion in the following manner, without previously removing the uranium :
To the hot solution containing a few cubic centimetres hydrochloric acid in excess, add enough lead acetate to combine with all the molybdenum present, and then sufficient ammonium acetate to destroy the free hydrochloric acid. The precipitated lead molybdate, which is very granular and easily washed, is ignited and weighed. The traces of uranium which the precipitate may contain can be recovered, if desired, by pouring the hydrochloric acid solution of it into caustic soda ; the error which they introduce is, however, quite insignificant. The uranium in the original filtrate may be estimated as phosphate only after