Coprecipitation is the contamination of a precipitate by otherwise soluble species and occurs via different processes. In colloidal precipitates, coprecipitation occurs via surface adsorption. For instance, barium sulfate has a primary layer of adsorbed barium ions and a secondary layer of nitrate counterions. This results in contamination of the precipitate by barium nitrate.
Sometimes, ions in a crystal lattice can undergo isomorphous replacement by inclusions of similar charge and size. For example, during the precipitation of cadmium sulfide, manganese in the solution can replace cadmium to form a mixed crystal. Similarly, magnesium ammonium phosphate can be contaminated by potassium ions in the solution, resulting in mixed crystals of magnesium potassium phosphate. The formation of mixed crystals can be prevented by removing the interfering ion or using a different precipitant.
Coprecipitation can also occur via occlusion, where foreign ions are trapped within the growing lattice. Similarly, pockets of solution are trapped between adjacent crystals in mechanical entrapment. Slow precipitation can minimize occlusion, whereas rapid dissolution and reprecipitation in a clean, fresh solvent can remove it.
