Precipitation (chemistry)

[4] The clear liquid remaining above the precipitated or the centrifuged solid phase is also called the supernate or supernatant.

When silver nitrate (AgNO3) is added to a solution of potassium chloride (KCl) the precipitation of a white solid (AgCl) is observed.

[5][6] The ionic equation allows to write this reaction by detailing the dissociated ions present in aqueous solution.

The Walden reductor is an illustration of a reduction reaction directly accompanied by the precipitation of a less soluble compound because of its lower chemical valence: The Walden reductor made of tiny silver crystals obtained by the immersion of a copper wire into a solution of silver nitrate is used to reduce to their lower valence any metallic ion located above the silver couple (Ag+ + 1 e– → Ag) in the redox potential scale.

By cooling the reaction mixture to room temperature, crystals of the porphyrin precipitate, and are collected by filtration on a Büchner filter as illustrated by the photograph here beside:[9] Precipitation may also occur when an antisolvent (a solvent in which the product is insoluble) is added, drastically reducing the solubility of the desired product.

An example would be the synthesis of Cr3+tetraphenylporphyrin chloride: water is added to the dimethylformamide (DMF) solution in which the reaction occurred, and the product precipitates.

[11] Proteins purification and separation can be performed by precipitation in changing the nature of the solvent or the value of its relative permittivity (e.g., by replacing water by ethanol), or by increasing the ionic strength of the solution.

Correctly mastering the precise temperature and pressure conditions when cooling down spent 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.