Precipitation (chemistry)

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Principle of chemical precipitation in aqueous solution Chemical precipitation diagram multilang.svg
Principle of chemical precipitation in aqueous solution

In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid 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]

Contents

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

Concepts and complications

Compounds precipitate from a solution when its concentration exceeds its solubility, i.e. the solutoin is supersaturated. Supersaturation can arise from temperature changes, solvent evaporation, or by mixing solvents. Precipitation occurs more rapidly from a strongly supersaturated solution.

Colloidal suspensions

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

Digestion and 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. [5] [6]

Applications

Analytical chemistry

Potassium hexachloroplatinate, the result of gravimetric analysis for potassium Potassium hexachloroplatinate(IV).jpg
Potassium hexachloroplatinate, the result of gravimetric analysis for potassium

Precipitate formation is a core step in gravimetric analysis, which is used to identify and quantify ions. 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. [7] [8]

AgNO3 + KCl → AgCl↓ + KNO3

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

Ag+ + NO−3 + K+ + Cl− → AgCl↓ + K+ + NO−3

Potassium is quantified using hexachloroplatinic acid as the precipitating agent. [9] Treatment of a solution containing K+ ions with an excess of this platinic acid quantitatively affords of potassium hexachloroplatinate, which is easily weighed and is non-hygroscopic:

2 K+ + H2[PtCl6] → K2[PtCl6] + 2 H+

Homogeneous precipitation involves formation of the precipitate from a single homogeneous solution, as in the case of barium sulfate. [10] A sample solution containing barium ions is treated with an excess of sulfamic acid. This solution is heated to induce hydrolysis of sulfamic acid to bisulfate:

2 H2NSO3H + 2 H2O → 2 NH+4 + 2 HSO−4

The bisulfate readily reacts with barium ions to give the sulfate:

2 HSO−4 + Ba2+ → BaSO4 + 2 H+

Inorganic chemistry

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 (causticsoda) as precipitant.

Organic chemistry

Precipitation may also occur when an antisolvent (a solvent in which the product is insoluble) is added, lowering the solubility of the desired product. Thereafter, the precipitate may be easily separated by decanting, filtration, or by centrifugation. 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] Precipitation is useful in purifying many other products: e.g., crude bmim-Cl is taken up in acetonitrile, and dropped into ethyl acetate, where it precipitates. [12]

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 ion implantation, and the temperature is high enough that diffusion can lead to segregation into precipitates. Precipitation in solids is routinely used to synthesize nanoclusters. [13]

In metallurgy, precipitation from a solid solution is also a way to strengthen alloys.

Precipitation of ceramic phases in metallic alloys such as zirconium hydrides in 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.

History

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

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. Precipitation of Silver on Copper 1.jpg
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.

Electroplating is a kind of precipitation whereby a metal is deposited on a surface. The precipitating agent is a reducing agent. The preparation of a Walden reductor illustrative. It is made of tiny silver crystals obtained by the immersion of a copper wire into a solution of silver nitrate:

Cu + 2 Ag+ → Cu2+ + 2 Ag

See also

References

  1. ↑ "precipitation" . IUPAC Gold Book. 2014. doi: 10.1351/goldbook.P04795 .
  2. ↑ "Chemical precipitation". Encyclopedia Britannica . Retrieved 2020-11-28.
  3. ↑ "precipitate". Merriam-Webster.com Dictionary . Merriam-Webster. Retrieved 2020-11-28.
  4. ↑ "precipitant". Merriam-Webster.com Dictionary . Merriam-Webster. Retrieved 2020-11-28.
  5. ↑ Vengrenovitch, R.D. (1982). "On the Ostwald ripening theory". Acta Metallurgica. 30 (6): 1079–1086. doi:10.1016/0001-6160(82)90004-9. ISSN   0001-6160.
  6. ↑ Voorhees, P.W. (1985). "The theory of Ostwald ripening". Journal of Statistical Physics. 38 (1–2): 231–252. Bibcode:1985JSP....38..231V. doi:10.1007/BF01017860. ISSN   0022-4715. S2CID   14865117.
  7. ↑ Zumdahl, Steven S.; DeCoste, Donald J. (2012). Chemical Principles. Cengage Learning. ISBN   978-1-133-71013-4.
  8. ↑ Zumdahl, Steven S.; DeCoste, Donald J. (2018). Introductory Chemistry: A Foundation. Cengage Learning. ISBN   978-1-337-67132-3.
  9. ↑ Burkhardt, Elizabeth R. (2006). "Potassium and Potassium Alloys". Ullmann's Encyclopedia of Industrial Chemistry. Vol. A22. pp. 31–38. doi:10.1002/14356007.a22_031.pub2. ISBN   978-3-527-30673-2.
  10. ↑ Rattenbury, Evelyn M. (1966). "Introduction and General Principles". Introductory Titrimetric and Gravimetric Analysis. pp. 143–153. doi:10.1016/B978-0-08-011950-2.50011-7. ISBN   978-0-08-011950-2.
  11. ↑ Alan D. Adler; Frederick R. Longo; Frank Kampas; Jean Kim (1970). "On the preparation of metalloporphyrins". Journal of Inorganic and Nuclear Chemistry. 32 (7): 2443. doi:10.1016/0022-1902(70)80535-8.
  12. ↑ 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.
  13. ↑ Dhara, S. (2007). "Formation, Dynamics, and Characterization of Nanostructures by Ion Beam Irradiation". Critical Reviews in Solid State and Materials Sciences. 32 (1): 1–50. Bibcode:2007CRSSM..32....1D. doi:10.1080/10408430601187624. S2CID   98639891.

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