5.22 : Washing, Drying, and Ignition of Precipitates

After filtration, the precipitate is washed to remove coprecipitated impurities and any remaining mother liquor. Colloidal precipitates, such as silver chloride, are washed with an electrolyte (such as dilute nitric acid) to prevent the peptization of the precipitate. In the case of slightly soluble precipitates, the wash solution contains a common ion to reduce solubility. Lead sulfate, which is slightly soluble in water, is washed with dilute sulfuric acid. Similarly, wash solutions may be basic or acidic to prevent the hydrolysis of salts of weak acids and weak bases, respectively. For example, magnesium ammonium phosphate hexahydrate is washed with a dilute ammonia solution. Following washing, the precipitate is dried to remove adsorbed water and electrolyte. In some cases, ignition is required to convert the precipitate to a suitable weighing form. For example, heating at 900 °C converts magnesium ammonium phosphate hexahydrate to magnesium pyrophosphate for weighing. Here, the excess water that may or may not be in the crystals depending on the levels of heating or drying prevents stoichiometric analysis. Finally, the precipitate is cooled in a desiccator and weighed.