Precipitation (chemistry)

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This article is about the chemical phenomenon. For other uses, see Precipitation (disambiguation).
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Principle of chemical precipitation in aqueous solution

In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a supersaturated solution.[1][2] The solid formed is called the precipitate.[3] In case of an inorganic chemical reaction leading to precipitation, the chemical reagent causing the solid to form is called the precipitant.[4]

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

The notion of precipitation can also be extended to other domains of chemistry (organic chemistry and biochemistry) and even be applied to the solid phases (e.g. metallurgy and alloys) when solid impurities segregate from a solid phase.

Supersaturation

Main article: Supersaturation

The precipitation of a compound may occur when its concentration exceeds its solubility. 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 a barium chloride solution reacts with sulphuric acid, a white precipitate of barium sulfate is formed. When a potassium iodide solution reacts with a lead(II) nitrate solution, a yellow precipitate of lead(II) iodide is formed.

Inorganic chemistry

See also: Qualitative inorganic analysis

Precipitate formation is useful in the detection of the type of

cation in a salt. To do this, an alkali first reacts with the unknown salt to produce a precipitate that is the hydroxide 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, a barium nitrate solution will react with sulfate ions to form a solid barium sulfate
precipitate, indicating that it is likely that sulfate ions are present.

A common example of precipitation from aqueous solution is that of silver chloride. 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

ions
present in aqueous solution.

Reductive precipitation

Illustration of the Walden reductor. Copper from a wire is displaced by silver from a silver nitrate solution it is dipped into, and metallic silver crystals precipitate onto the copper wire.

The

reduction reaction
directly accompanied by the precipitation of a less soluble compound because of its lower chemical valence:

The Walden reductor made of tiny

redox potential
scale.

Colloidal suspensions

Without sufficient attraction forces (e.g.,

colloids. Sedimentation can be accelerated by high speed centrifugation
. The compact mass thus obtained is sometimes referred to as a 'pellet'.

Digestion and precipitates ageing

Digestion, or precipitate ageing, happens when a freshly formed precipitate is left, usually at a higher temperature, in the solution from which it precipitates. It results in purer and larger recrystallized particles. The physico-chemical process underlying digestion is called Ostwald ripening.[7][8]

Organic chemistry

Crystals of meso-tetratolylporphyrin from a reflux of propionic acid precipitate on cooling. Photograph of the Büchner funnel on top of a Büchner flask.

While precipitation reactions can be used for making

workup 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, preferably forming pure crystals. An example of this would be the synthesis of porphyrins in refluxing propionic acid. 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]

Porphyrin synthesis

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. Thereafter, the precipitate may be easily separated by

bmim-Cl is taken up in acetonitrile, and dropped into ethyl acetate, where it precipitates.[11]

Biochemistry

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. 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 in ethanol precipitation of DNA.

Metallurgy and alloys

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 or

nanoclusters.[12]

In

strengthen alloys
.

Precipitation of

zircaloy cladding of nuclear fuel pins can also render metallic alloys brittle and lead to their mechanical failure. 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.

Industrial processes

Hydroxide precipitation is probably the most widely used industrial precipitation process in which metal hydroxides are formed by adding calcium hydroxide (slaked lime) or sodium hydroxide (caustic soda) as precipitant.

History

Powders derived from different precipitation processes have also historically been known as 'flowers'.

See also

References

  1. ^ "Precipitation (Chemical) - an overview". ScienceDirect. Retrieved 2020-11-28.
  2. Encyclopedia Britannica
    . Retrieved 2020-11-28.
  3. ^ "precipitate". Merriam-Webster.com Dictionary. Retrieved 2020-11-28.
  4. ^ "precipitant". Merriam-Webster.com Dictionary. Retrieved 2020-11-28.
  5. ISBN 978-1-133-71013-4
    .
  6. ISBN 978-1-337-67132-3
    .
  7. ISSN 0001-6160
    .
  8. S2CID 14865117
    .
  9. doi:10.1021/jo01288a053
    .
  10. doi:10.1016/0022-1902(70)80535-8
    .
  11. ^ Dupont, J., Consorti, C., Suarez, P., de Souza, R. (2004). "Preparation of 1-Butyl-3-methyl imidazolium-based room temperature ionic liquids". Organic Syntheses{{cite journal}}: CS1 maint: multiple names: authors list (link); Collected Volumes, vol. 10, p. 184.
  12. S2CID 98639891
    .

Further reading

External links

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