5.18:
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.
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.