Treatment of a concentrated solution of a sodium salt with a large excess of zinc uranyl acetate reagent results in the precipitation of sodium zinc uranyl acetate. This substance is moderately soluble in water (5.88 g in 100 mL of water at 20 °C) so that a special washing technique must be used. The solubility in a solution containing excess of the reagent is less. About 10 volumes of the reagent are added for each volume of the sample solution, which should not contain more than 8 mg of sodium per mL; precipitation of the triple acetate is usually complete in 1 hour. One milligram of sodium yields 66.88 mg of the triple salt; the latter is relatively bulky, so that the amount of sodium that can be handled in a single determination is limited.
Lithium interferes, since it forms a sparingly soluble triple acetate. Potassium has no effect provided not more than 50mg mL-1 are present. Sulphate must be absent when potassium is present, for potassium sulphate is sparingly soluble in the reagent. Moderate amounts of ammonium salts, calcium, barium, and magnesium may be tolerated; for larger amounts, a double precipitation is necessary. Phosphates, arsenates, molybdates, oxalates, tartrates, sulphates (in the presence of potassium), and strontium interfere.
The neutral or feebly acid sample solution, free from the interfering substances mentioned above, should contain not more than 8 mg of sodium per mL, preferably as chloride. Treat the sample solution (say, 1.5 mL) (Note 1) with 15 mL of zinc uranyl acetate reagent (Note 2), and stir vigorously, preferably mechanically for at least 30 minutes. Allow to stand for 1 hour, and filter through a weighed sintered-glass or porcelain filtering crucible (porosity No. 4). Wash the precipitate four times with 2 mL portions of the precipitating reagent (allow the wash liquid to drain completely before adding the next portion), then ten times with 95 per cent ethanol saturated with sodium zinc uranyl acetate at room temperature (2mL portions), and finally with a little dry diethyl ether or acetone. Dry for 30 minutes only at 55-60 °C. Weigh as NaZn(UO2)3(C2H3O2)9,6H2O.
Notes. (1) A suitable solution for practice may be prepared by evaporating 20.0 mL of 0.1N sodium chloride, prepared from the salt, to about 1.5 mL on a water bath.
(2) The reagent is prepared by mixing equal volumes of Solutions A and B and filtering after standing overnight.
Solution A: dissolve 20g crystallised uranyl acetate UO2(C2H3O2)2,2H2O in 4mL glacial acetic acid and 100 mL water (warming may be necessary).
SolutionB:dissolve60g crystallised zinc acetate Zn(C2H3O2)2,3H 2 O in 3 mL glacial acetic acid and 100mL of water.