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chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, coprecipitation (CPT) or co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed. Analogously, in
medicine Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pr ...
, coprecipitation is specifically the precipitation of an unbound "antigen along with an antigen-antibody complex". Coprecipitation is an important topic in
chemical analysis Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
, where it can be undesirable, but can also be usefully exploited. In gravimetric analysis, which consists on precipitating the analyte and measuring its mass to determine its concentration or purity, coprecipitation is a problem because undesired impurities often coprecipitate with the analyte, resulting in excess mass. This problem can often be mitigated by "digestion" (waiting for the precipitate to equilibrate and form larger and purer particles) or by redissolving the sample and precipitating it again.Harvey, D. (2000). ''Modern Analytical Chemistry''. McGraw-Hill. On the other hand, in the analysis of trace elements, as is often the case in
radiochemistry Radiochemistry is the chemistry of radioactive materials, where radioactive isotopes of elements are used to study the properties and chemical reactions of non-radioactive isotopes (often within radiochemistry the absence of radioactivity leads ...
, coprecipitation is often the only way of separating an element. Since the trace element is too dilute (sometimes less than a part per trillion) to precipitate by conventional means, it is typically coprecipitated with a ''carrier'', a substance that has a similar crystalline structure that can incorporate the desired element. An example is the separation of
francium Francium is a chemical element with the symbol Fr and atomic number 87. It is extremely radioactive; its most stable isotope, francium-223 (originally called actinium K after the natural decay chain it appears in), has a half-life of only 22&n ...
from other radioactive elements by coprecipitating it with
caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
salts such as caesium perchlorate.
Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist who was a pioneer in the fields of radioactivity and radiochemistry. He is referred to as the father of nuclear chemistry and father of nuclear fission. Hahn and Lise Meitner ...
is credited for promoting the use of coprecipitation in radiochemistry. There are three main mechanisms of coprecipitation: inclusion, occlusion, and adsorption. An inclusion (incorporation in the crystal lattice) occurs when the impurity occupies a lattice site in the
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...
of the carrier, resulting in a
crystallographic defect A crystallographic defect is an interruption of the regular patterns of arrangement of atoms or molecules in crystalline solids. The positions and orientations of particles, which are repeating at fixed distances determined by the unit cell para ...
; this can happen when the ionic radius and charge of the impurity are similar to those of the carrier. An adsorbate is an impurity that is weakly, or strongly, bound ( adsorbed) to the surface of the precipitate. An occlusion occurs when an adsorbed impurity gets physically trapped inside the crystal as it grows. Besides its applications in chemical analysis and in radiochemistry, coprecipitation is also important to many environmental issues related to water resources, including
acid mine drainage Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD) is the outflow of acidic water from metal mines or coal mines. Acid rock drainage occurs naturally within some environments as part of the rock weathering ...
,
radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transfer ...
migration around waste repositories,
toxic heavy metal A toxic heavy metal is any relatively dense metal or metalloid that is noted for its potential toxicity, especially in environmental contexts. The term has particular application to cadmium, mercury and lead, all of which appear in the World ...
transport at industrial and defense sites, metal concentrations in aquatic systems, and
wastewater treatment Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environm ...
technology. Coprecipitation is also used as a method of magnetic nanoparticle synthesis.Lu, A.-H., Salabas, E. L. and Schüth, F. (2007). Angew. Chem. Int. Ed., 46,1222–1244.


Distribution between precipitate and solution

There are two models describing of the distribution of the tracer compound between the two phases (the precipitate and the solution):ALAN TOWNSHEND and EWALD JACKWERTH, "PRECIPITATION OF MAJOR CONSTITUENTS FOR TRACE PRECONCENTRATION : POTENTIAL AND PROBLEMS", Pure & App. Chem., Vol.61, No.9, pp. 1643-1656, 1989.
(pdf)
/ref> * Doerner-Hoskins law (logarithmic): :\ln = \lambda \ln * Berthelot-Nernst law: : = D where: : ''a'' and ''b'' are the initial concentrations of the tracer and carrier, respectively; : ''a'' − ''x'' and ''b'' − ''y'' are the concentrations of tracer and carrier after separation; : ''x'' and ''y'' are the amounts of the tracer and carrier on the precipitate; : ''D'' and λ are the distribution coefficients. For ''D'' and λ greater than 1, the precipitate is enriched in the tracer. Depending on the co-precipitation system and conditions either λ or ''D'' may be constant. The derivation of the Doerner-Hoskins law assumes that there in no mass exchange between the interior of the precipitating crystals and the solution. When this assumption is fulfilled, then the content of the tracer in the crystal is non-uniform (the crystals are said to be heterogeneous). When the Berthelot-Nernst law applies, then the concentration of the tracer in the interior of the crystal is uniform (and the crystals are said to be homogeneous). This is the case when diffusion in the interior is possible (like in the liquids) or when the initial small crystals are allowed to recrystallize. Kinetic effects (like speed of crystallization and presence of mixing) play a role.


See also

* Fajans–Paneth–Hahn Law


References

{{Reflist, 2 Chemical processes Analytical chemistry Radiochemistry