Use is made of the homogeneous generation of chromate ion produced by the slow oxidation of chromium (III) by bromate at 90-95 °C in the presence of excess of lead nitrate solution and an acetate buffer. The crystals of lead chromate produced are relatively large and easily filtered; the volume of the precipitate is about half that produced by the standard method of precipitation.
2Cr3+ +BrO3-
+ 5H2O = 2CrO42- + Br- + 10H +
BrO3- + 5Br- +
6H + = 3Br2 + 3H2O
CrO42- + Pb2+
= PbCrO4
Cations forming insoluble chromates, such as those of silver, barium, mercury (I), mercury (II), and bismuth, do not interfere because the acidity is sufficiently high to prevent their precipitation. Bromide ion from the generation may be expected to form insoluble silver bromide, and so it is preferable to separate silver prior to the precipitation. Ammonium salts interfere, owing to competitive oxidation by bromate, and should be removed by treatment with sodium hydroxide.
BrO3- +2NH4+
= Br- + N2 + 2H+ + 3H2O
Use a sample solution containing about 50 mg of chromium (III). Neutralise the solution by the addition of sodium hydroxide solution until a precipitate just begins to form. Add 10 mL acetate buffer solution (6M in acetic acid and 0.6M in sodium acetate), 10 mL lead nitrate solution (3.5 g per 100 mL), and 10 mL potassium bromate solution (2.0 g per 100 mL). Heat to 90-95 °C: after generation (of chromate) and precipitation are complete (about 45 minutes), as shown by the clear supernatant liquid, cool, filter on a weighed sintered-glass or porcelain filtering crucible, wash with a little 0.1 per cent nitric acid, and dry to constant weight at 120 °C. Weigh as PbCrO4.