In an aqueous solution, precipitation is the process of transforming a dissolved
substance into an insoluble
solid from a
super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemical reagent causing the solid to form is called the ''precipitant''.
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
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
In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a li ...
solution.
The formation of a precipitate can be caused by a chemical reaction. When a
barium chloride
Barium chloride is an inorganic compound with the formula Ba Cl2. It is one of the most common water-soluble salts of barium. Like most other water-soluble barium salts, it is white, highly toxic, and imparts a yellow-green coloration to a flame. ...
solution reacts with
sulphuric acid, a white precipitate of
barium sulfate
Barium sulfate (or sulphate) is the inorganic compound with the chemical formula Ba SO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and ...
is formed. When a
potassium iodide solution reacts with a
lead(II) nitrate
Lead(II) nitrate is an inorganic compound with the chemical formula Pb( NO3)2. It commonly occurs as a colourless crystal or white powder and, unlike most other lead(II) salts, is soluble in water.
Known since the Middle Ages by the name plumbum ...
solution, a yellow precipitate of
lead(II) iodide
Lead(II) iodide or lead iodide is a chemical compound with the formula . At room temperature, it is a bright yellow odorless crystalline solid, that becomes orange and red when heated. It was formerly called plumbous iodide.
The compound current ...
is formed.
Nucleation
An important stage of the precipitation process is the onset of
nucleation. The creation of a solid
particle
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from s ...
implies the formation of an
interface
Interface or interfacing may refer to:
Academic journals
* ''Interface'' (journal), by the Electrochemical Society
* '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics''
* '' Int ...
with the solution. This involves
energy changes depending on the dissolution reaction
free energy (
endothermic
In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. p. ...
or
exothermic process accompanied by an
entropy increase) and the relative
surface energy
In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...
developed between the solid and the solution. If energy changes are not favorable, or without suitable nucleation sites, no precipitation occurs and the solution remain supersaturated.
Inorganic chemistry
Precipitation in aqueous solution
A common example of precipitation reaction in aqueous solution is that of
silver chloride
Silver chloride is a chemical compound with the chemical formula Ag Cl. This white crystalline solid is well known for its low solubility in water (this behavior being reminiscent of the chlorides of Tl+ and Pb2+). Upon illumination or heat ...
. When
silver nitrate (AgNO
3) is added to a solution of
potassium chloride
Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a sa ...
(KCl) the precipitation of a white solid (AgCl) is observed.
:
AgNO3 + KCl -> AgCl (v) + KNO3
The
ionic equation
A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and chemical formulas. The reactant entities are given on the left-hand side and the product entities on the right-hand side with a plus sign between ...
allows to write this reaction by detailing the
dissociated
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid ...
ions present in aqueous solution.
:
Ag+ + NO3^- + K+ + Cl^- -> AgCl (v) + K+ + NO3^-
Reductive precipitation
The
Walden reductor The Walden reductor is a reduction column filled with metallic silver
which can be used to reduce a metal ion in aqueous solution to a lower oxidation state. It can be used ''e.g.'' to reduce UO22+ to U4+. The method is named after George H. Walden ...
is an illustration of a
reduction reaction
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
directly accompanied by the precipitation of a less soluble compound because of its lower chemical valence:
:
Cu + 2 Ag+ -> Cu^2+ + 2 Ag
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 in the
redox potential scale.
Precipitate colors
Many compounds containing
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...
ions produce precipitates with distinctive colors. The following are some typical colors for various metals. However, many of these compounds can produce colors very different from those listed.
Many compounds often form white precipitates.
Anion/cation qualitative analysis
Precipitate formation is useful in the detection of the type of
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
in a
salt. To do this, an
alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of ...
first reacts with the unknown salt to produce a precipitate that is the
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
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
Barium nitrate is the inorganic compound with the chemical formula Ba( NO3)2. It, like most barium salts, is colorless, toxic, and water-soluble. It burns with a green flame and is an oxidizer; the compound is commonly used in pyrotechnics.
Man ...
solution will react with
sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
ions to form a solid
barium sulfate
Barium sulfate (or sulphate) is the inorganic compound with the chemical formula Ba SO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and ...
precipitate, indicating that it is likely that sulfate ions are present.
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
Settling is the process by which particulates move towards the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction ...
), they remain in
suspension and form
colloids
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
.
Sedimentation
Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...
can be accelerated by high speed
centrifugation
Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...
. The compact mass obtained so 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.
Organic chemistry
While precipitation reactions can be used for making
pigments, removing ions from solution in
water treatment, and in
classical qualitative inorganic analysis, precipitation is also commonly used to isolate the products of an organic reaction during
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
Propionic acid (, from the Greek words πρῶτος : ''prōtos'', meaning "first", and πίων : ''píōn'', meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2CO2H. It is a liqu ...
. 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:
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
decanting
Decantation is a process for the separation of mixtures of immiscible liquids or of a liquid and a solid mixture such as a suspension. The layer closer to the top of the container—the less dense of the two liquids, or the liquid from which th ...
,
filtration
Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter m ...
, or by
centrifugation
Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...
. An example would be the synthesis of Cr
3+tetraphenylporphyrin
Tetraphenylporphyrin, abbreviated TPP or H2TPP, is a synthetic heterocyclic compound that resembles naturally occurring porphyrins. Porphyrins are dyes and cofactors found in hemoglobin and cytochromes and are related to chlorophyll and vitamin ...
chloride: water is added to the
dimethylformamide (DMF) solution in which the reaction occurred, and the product precipitates. Precipitation is useful in purifying many other products: ''e.g.'', crude
bmim
1-Butyl-3-methylimidazolium hexafluorophosphate, also known as BMIM-PF6, is a viscous, colourless, hydrophobic and non-water-soluble ionic liquid with a melting point of -8 °C. Together with 1-butyl-3-methylimidazolium tetrafluoroborate, BMI ...
-Cl is taken up in
acetonitrile
Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not classed ...
, and dropped into
ethyl acetate, where it precipitates.
Biochemistry
Proteins purification and separation can be performed by precipitation in changing the nature of the solvent or the value of its
dielectric constant (''e.g.'', by replacing water by
ethanol), or by increasing the
ionic strength
The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such a ...
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 Ethanol precipitation is a method used to purify and/or concentrate RNA, DNA, and polysaccharides such as pectin and xyloglucan from aqueous solutions by adding ethanol as an antisolvent.
DNA precipitation Theory
DNA is polar due to its h ...
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 Segregation may refer to:
Separation of people
* Geographical segregation, rates of two or more populations which are not homogenous throughout a defined space
* School segregation
* Housing segregation
* Racial segregation, separation of humans ...
into precipitates. Precipitation in solids is routinely used to synthesize
nanoclusters.
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
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of the ...
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 fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor an ...
s 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
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime ( calcium oxide) is mixed or slaked with water. It has m ...
(''slaked lime'') or
sodium hydroxide (''caustic'' ''soda'') as precipitant.
History
Powders derived from different precipitation processes have also
historically
History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the invention of writing systems is considered prehistory. "History" is an umbrella term comprising past events as well ...
been known as 'flowers'.
See also
*
Coprecipitation
In chemistry, coprecipitation (CPT) or co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed. Analogously, in medicine, coprecipitation is specifically the precipitation of an unbound "a ...
*
Effervescence
Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from that release. The word effervescence is derived from the Latin verb ''fervere'' (to boil), preceded by the adverb ''ex''. It has the same li ...
, the "up-arrow"
*
Salting in
*
Salting out
Salting out (also known as salt-induced precipitation, salt fractionation, anti-solvent crystallization, precipitation crystallization, or drowning out) is a purification technique that utilizes the reduced solubility of certain molecules in a s ...
References
Additional reading
*
External links
Precipitation reactions of certain cationsDigestion InstrumentsA Thesis on pattern formation in precipitation reactions
{{DEFAULTSORT:Precipitation (Chemistry)
Liquid-solid separation