5.18 : Precipitate Formation and Particle Size Control

In precipitation gravimetry, the precipitating agent should react specifically or selectively with the analyte. While a specific reagent reacts with the analyte alone, a selective reagent can react with a limited number of chemical species.

The obtained precipitate should be either a pure substance of known composition or easily converted to one by a simple process, such as ignition or drying. In addition, the precipitate should be insoluble and easily filterable. In general, filterability increases with the size of the precipitate particles. Colloidal suspensions contain minuscule particles with diameters varying from 10⁻⁹ to 10⁻⁶ m, which are invisible to the naked eye and not easily filtered. However, crystalline suspensions have larger particles that settle quickly and are readily filtered.

The temperature, precipitate solubility, reactant concentrations, and speed of mixing of reactants can affect the particle size. The overall effect of these attributes is called relative supersaturation, RSS, which can be expressed in terms of the concentration of the solute (Q) and its equilibrium solubility (S). The size of the obtained particles is inversely proportional to the average relative supersaturation when the reagent is added. As a result, when the relative supersaturation ratio is high, colloidal precipitates are favored, while crystalline precipitates with large particle sizes are obtained at low relative supersaturation ratios.