Vanadates are precipitated by excess of silver nitrate solution in the presence of sodium acetate; after boiling, the precipitate consists of silver orthovanadate. The following reactions occur with a solution of a metavanadate:
2NaVO3 + 2CH3COONa + H2O ^± Na4V2O7 + 2CH3COOH Na4V207 + 4AgN03 ^ Ag4V2O7 + 4NaNO3
Ag4V2O7 + 2AgNO3 + 2CH3COONa + H2O ^±
2Ag3V04 + 2CH3COOH + 2NaNO3
Neutralise the solution (200mL), containing not more than 0.2 g of alkali vanadate, if acid by aqueous sodium hydroxide, or if alkaline by the addition of nitric acid to the boiling solution until it becomes yellow, followed by decolorisation with dilute ammonia solution. Add 3 g of ammonium acetate, 0.5 mL of concentrated ammonia solution, and then excess of silver nitrate solution, heat to boiling and then keep on a steam bath for 30 minutes. Test for complete precipitation with more silver nitrate solution; if a turbidity is produced, boil the liquid until it becomes clear. Allow the dense brown precipitate of silver vanadate to settle, and collect it on a weighed filtering crucible (sintered-glass, or porcelain), wash with hot water, and dry at 110 °C. Weigh as Ag3VO4.
It has been stated that the results obtained by precipitation of vanadate as silver orthovanadate Ag3VO4 are not altogether satisfactory. Better results are obtained by precipitation at pH 4.5 as silver metavanadate AgVO3; the precipitate is weighed after drying at 100-105 °C.