Precipitate Formation and Particle Size Control

In precipitation gravimetry, ideally, the precipitant reacts specifically or selectively with the analyte to yield a precipitate with certain attributes.

The obtained precipitate should be either a pure substance of a known composition or easily converted to one by a simple process.

It should also be highly insoluble and completely precipitated. This can be achieved by adding or removing a common ion, and optimizing the reaction pH and temperature.

The particle size of the precipitate is also important.

Colloidal suspensions have very small particles that pass through the filter paper, whereas crystalline suspensions have larger particles that settle quickly and are easily filtered.

Particle size is impacted by multiple factors and can be expressed in terms of the relative supersaturation. Here, Q is the concentration of the solute and S denotes its equilibrium solubility.

Colloidal suspensions form when the relative supersaturation is high, while crystalline suspensions form when the ratio is low.