Igloo writes Gravimetric analysis, by definition, includes all methods of analysis in which the final stage involves an accurate weighing. In the majority of cases gravimetric methods rely on the use of precipitation reactions. The ion whose concentration is being analysed needs to be converted into an insoluble salt and the procedure involves weighing the original sample, dissolving it in water, after which an excess of a precipitating agent is added. The precipitate which forms is filtered, washed thoroughly, dried and finally weighed to a high degree of precision. From the mass and known composition of the precipitate, the amount of the original ion can be deduced by calculation. Gravimetric analysis is used in the following cases: • Measurement of the essential elements in plant foods (phosphorus, for example, is converted into the insoluble salt, magnesium ammonium phosphate) • Estimation of pollutants in the air, such as sulphur dioxide (by conversion to insoluble barium sulphate