coprecipitate

Coprecipitation is the process by which a precipitate formed from a solution carries along or incorporates other species that were not the intended precipitating component. The result is a solid material, the coprecipitate, that contains traces of dissolved substances accompanying the primary precipitate.

The mechanisms of coprecipitation fall into three main categories. True coprecipitation occurs when impurities are incorporated into the crystal lattice during precipitation, forming a solid solution or undergoing isomorphous substitution. Occlusion coprecipitation involves impurities being physically trapped or occluded within the growing crystal. Adsorption or surface coprecipitation happens when impurities are drawn to and adhere to the surface during precipitation and may be entrained in the solid when filtration or aging occurs.

Coprecipitates are commonly classified as either incidental or deliberate. Incidental coprecipitation arises unintentionally during routine precipitation and can bias quantitative analyses. Deliberate coprecipitation is used in separation and preconcentration techniques to recover or concentrate trace elements by co-precipitating them with a chosen principal solid.

Factors influencing the extent of coprecipitation include pH, temperature, ionic strength, the presence of complexing or stabilizing agents, precipitation rate, and competing ions in solution. Slow, controlled precipitation with stabilizing ligands tends to reduce unwanted coprecipitation, whereas rapid or strongly binding conditions can increase it.

Applications and implications vary by field. In analytical chemistry, coprecipitation can bias gravimetric and spectrometric measurements but can also be exploited to preconcentrate trace elements. In environmental and materials contexts, coprecipitation affects the mobility of contaminants and the properties of synthesized solids.