In chemistry, coprecipitation (CPT) or coprecipitation is the transport by a precipitate of normally soluble substances under the conditions used. Similarly, in medicine, coprecipitation is specifically the precipitation of an "unbound antigen together with an antigen-antibody complex".
Coprecipitation is an important topic in chemical analysis, where it is often undesirable, but in some cases it can be exploited. In gravimetric analysis, which involves precipitating the analyte and measuring its mass to determine its concentration or purity, coprecipitation is a problem because unwanted impurities often coprecipitate with the analyte, resulting in excess mass. This problem can often be mitigated by "digestion" (expecting the precipitate to equilibrate and form larger, purer particles) or by redisolating the sample and precipitating it again.
On the other hand, in the analysis of trace elements, as is often the case in radiochemistry, coprecipitation is often the only way to separate an element. Since the trace element is too dilute (sometimes less than one part per trillion) to precipitate by conventional means, a substance having a similar crystal structure which typically incorporates the desired element is typically coprecipitated with a carrier. An example is the separation of francium from other radioactive elements by coprecipitating it with cesium salts such as cesium perchlorate. It is attributed to Otto Hahn for promoting the use of coprecipitation in radiochemistry.
There are three main coprecipitation mechanisms: inclusion, occlusion and adsorption. Inclusion occurs when the impurity occupies a lattice site in the crystal structure of the conveyor, resulting in a crystallographic defect; this can happen when the ionic radius and the charge of the impurity are similar to those of the carrier. An adsorbate is a weakly bound (adsorbed) impurity to the surface of the precipitate. Occlusion occurs when an adsorbed impurity is physically trapped within the crystal as it grows.
In addition to its applications in chemical analysis and radio-chemistry, coprecipitation is also "potentially important for many environmental problems closely related to water resources, including acid mine drainage, migration of radio-nuclides into contaminated waste sites, transport of metal contaminants in industrial and defense sites, aquatic systems and wastewater treatment technology. Coprecipitation is also used as a method for the synthesis of magnetic nanoparticles.