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In anaqueous solution,precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution".[1][2] The solid formed is called theprecipitate.[3] In case of an inorganic chemical reaction leading to precipitation, the chemical reagent causing the solid to form is called theprecipitant.[4]
The clear liquid remaining above the precipitated or thecentrifuged solid phase is also called thesupernate orsupernatant.
The notion of precipitation can also be extended to other domains of chemistry (organic chemistry andbiochemistry) and even be applied to the solid phases (e.g.metallurgy andalloys) when solid impuritiessegregate from a solid phase.
The precipitation of a compound may occur when its concentration exceeds itssolubility. This can be due to temperature changes, solvent evaporation, or by mixing solvents. Precipitation occurs more rapidly from a strongly supersaturated solution.
The formation of a precipitate can be caused by a chemical reaction. When abarium chloride solution reacts withsulphuric acid, a white precipitate ofbarium sulphate is formed. When apotassium iodide solution reacts with alead(II) nitrate solution, a yellow precipitate oflead(II) iodide is formed.
Precipitate formation is useful in the detection of the type ofcation in asalt. To do this, analkali first reacts with the unknown salt to produce a precipitate that is thehydroxide of the unknown salt. To identify the cation, the color of the precipitate and its solubility in excess are noted. Similar processes are often used in sequence – for example, abarium nitrate solution will react withsulfate ions to form a solidbarium sulfate precipitate, indicating that it is likely that sulfate ions are present.
A common example of precipitation from aqueous solution is that ofsilver chloride. Whensilver nitrate (AgNO3) is added to a solution ofpotassium chloride (KCl) the precipitation of a white solid (AgCl) is observed.[5][6]
Theionic equation allows to write this reaction by detailing thedissociatedions present in aqueous solution.
TheWalden reductor is an illustration of areduction reaction directly accompanied by the precipitation of a less soluble compound because of its lower chemical valence:
The Walden reductor made of tinysilver crystals obtained by the immersion of acopper wire into a solution ofsilver nitrate is used to reduce to their lower valence any metallic ion located above the silver couple(Ag+ + 1 e– → Ag) in theredox potential scale.
Without sufficient attraction forces (e.g.,Van der Waals force) to aggregate the solid particles together and to remove them from solution by gravity (settling), they remain insuspension and formcolloids.Sedimentation can be accelerated by high speedcentrifugation. The compact mass thus obtained is sometimes referred to as a 'pellet'.
Digestion, orprecipitate ageing, happens when a freshly formed precipitate is left, usually at a highertemperature, in the solution from which it precipitates. It results in purer and larger recrystallized particles. The physico-chemical process underlying digestion is calledOstwald ripening.[7][8]
While precipitation reactions can be used for makingpigments, removing ions from solution inwater treatment, and inclassical qualitative inorganic analysis, precipitation is also commonly used to isolate the products of an organic reaction duringworkup and purification operations. Ideally, the product of the reaction is insoluble in the solvent used for the reaction. Thus, it precipitates as it is formed, preferablyforming pure crystals. An example of this would be the synthesis ofporphyrins in refluxingpropionic acid. By cooling the reaction mixture to room temperature, crystals of theporphyrin precipitate, and are collected by filtration on a Büchner filter as illustrated by the photograph here beside:[9]
Precipitation may also occur when anantisolvent (a solvent in which the product is insoluble) is added, drastically reducing the solubility of the desired product. Thereafter, the precipitate may be easily separated bydecanting,filtration, or bycentrifugation. An example would be the synthesis of Cr3+tetraphenylporphyrin chloride: water is added to thedimethylformamide (DMF) solution in which the reaction occurred, and the product precipitates.[10] Precipitation is useful in purifying many other products:e.g., crudebmim-Cl is taken up inacetonitrile, and dropped intoethyl acetate, where it precipitates.[11]
Proteins purification and separation can be performed by precipitation in changing the nature of the solvent or the value of itsrelative permittivity (e.g., by replacing water byethanol), or by increasing theionic strength of the solution. As proteins have complex tertiary and quaternary structures due to their specific folding and various weak intermolecular interactions (e.g., hydrogen bridges), these superstructures can be modified and proteins denaturated and precipitated. Another important application of an antisolvent is inethanol precipitation ofDNA.
In solid phases, precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, due to e.g. rapid quenching orion implantation, and the temperature is high enough that diffusion can lead tosegregation into precipitates. Precipitation in solids is routinely used to synthesizenanoclusters.[12]
Inmetallurgy, precipitation from asolid solution is also a way tostrengthen alloys.
Precipitation ofceramic phases inmetallic alloys such aszirconium hydrides inzircaloy cladding ofnuclear fuel pins can also render metallic alloys brittle and lead to their mechanical failure. Correctly mastering the precise temperature and pressure conditions when cooling downspent nuclear fuels is therefore essential to avoid damaging their cladding and to preserve the integrity of the spent fuel elements on the long term in dry storage casks and in geological disposal conditions.
Hydroxide precipitation is probably the most widely used industrial precipitation process in whichmetal hydroxides are formed by addingcalcium hydroxide (slaked lime) orsodium hydroxide (causticsoda) as precipitant.
Powders derived from different precipitation processes have alsohistorically been known as 'flowers'.
{{cite journal}}: CS1 maint: multiple names: authors list (link);Collected Volumes, vol. 10, p. 184.
