Lead, copper(I), palladium(II), mercury (I), and thallium(I) ions interfere, as do cyanides and thiosulphates. If a mercury(I) [or copper(I) or thallium(I)] salt is present, it must be oxidised with concentrated nitric acid before the precipitation of silver; this process also destroys cyanides and thiosulphates. If lead is present, the solution must be diluted so that it contains not more than 0.25 g of the substance in 200 mL, and the hydrochloric acid must be added very slowly. Compounds of bismuth and antimony that hydrolyse in the dilute acid medium used for the complete precipitation of silver must be absent. For possible errors in the weight of silver chloride due to the action of light.
The solution (200mL) should
contain about 0.1 g of silver and about 1 per cent by volume of nitric acid.
Heat to about 70 °C, and add approximately 0.2M pure hydrochloric acid slowly
and with constant stirring until no further precipitation occurs; avoid a large
excess of the acid. Do not expose the precipitate to too much bright light.
Warm until the precipitate settles, allow to cool to about 25 °C, and test the
supernatant liquid with a few drops of the acid to be sure that precipitation
is complete. Allow the precipitate to settle in a dark place for several hours
or, preferably, overnight. Pour the supernatant liquid through a weighed
sintered-glass or porcelain filtering crucible, wash the precipitate by
decantation with 0.1M nitric acid, transfer the precipitate to the crucible,
and wash again with 0.01M nitric acid until free from chloride. Dry the
precipitate first at 100°C and then at 130-150°C, allow to cool in a desiccator
and weigh. Repeat the heating, etc., until constant weight is obtained. Weigh
as AgCl.