5.17 : Precipitation Gravimetry

Precipitation gravimetry is based on converting an analyte into a sparingly soluble precipitate, which is separated by filtration and weighed. An ideal precipitate should be pure, insoluble, of known composition, and easily filtered from the reaction mixture.

In determining nickel by gravimetric analysis, a precipitant of ethanolic dimethylglyoxime is added to a hot nickel salt solution. This is quickly followed by the dropwise addition of dilute ammonia solution until precipitation occurs. A slight excess of ammonia is added to ensure complete precipitation, followed by digestion for a few hours over a hot water bath to remove any impurities that may have occluded into the precipitate by dissolving and slowly reforming it multiple times. The resultant solution is allowed to stand for some time and cooled. The cold solution is filtered using a sintered glass funnel to obtain the red precipitate. This precipitate is washed with cold water, and the sintered glass funnel is dried in a hot air oven for an hour. The dried precipitate is cooled in a desiccator and weighed. From the mass of nickel dimethylglyoximate, the nickel content is calculated using the stoichiometric mole relationship.