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In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble
solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structura ...
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 In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...
. 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.


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, fro ...
implies the formation of an interface with the solution. This involves
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
changes depending on the dissolution reaction free energy ( endothermic or exothermic process accompanied by an
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
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 energ ...
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. When silver nitrate (AgNO3) 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
ions 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 conve ...
present in aqueous solution. :Ag+ + NO3^- + K+ + Cl^- -> AgCl (v) + K+ + NO3^-


Reductive precipitation

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: :Cu + 2 Ag+ -> Cu^2+ + 2 Ag The Walden reductor made of tiny
silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
crystals obtained by the immersion of a
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...
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 typi ...
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 conve ...
in a
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...
. 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 ...
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 ion, 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 salt (chemistry), ...
ions to form a solid barium sulfate precipitate, indicating that it is likely that sulfate ions are present.


Colloidal suspensions

Without sufficient attraction forces (''e.g.'',
Van der Waals force In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and t ...
) to aggregate the solid particles together and to remove them from solution by gravity ( settling), they remain in suspension and form colloids.
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 t ...
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 Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...
, 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
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic comp ...
s, removing ions from solution in
water treatment Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, inclu ...
, 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. 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 the ...
,
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 filte ...
, 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 Cr3+ tetraphenylporphyrin 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-Cl is taken up in acetonitrile, and dropped into ethyl acetate, where it precipitates.


Biochemistry

Protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...
s purification and separation can be performed by precipitation in changing the nature of the solvent or the value of its
dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...
(''e.g.'', by replacing water by
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a h ...
), 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 Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the target. Ion implantation is used in semiconductor device fa ...
, and the temperature is high enough that diffusion can lead to segregation into precipitates. Precipitation in solids is routinely used to synthesize nanoclusters. In
metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the sci ...
, 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 Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoi ...
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 Metal hydroxides are hydroxides of metals. They are often strong bases. They consist of hydroxide anions and metallic cations. Some metal hydroxides, such as alkali metal hydroxides, ionize completely when dissolved. Certain metal hydroxides are ...
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 Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkal ...
(''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 * Effervescence, the "up-arrow" * Salting in * Salting out


References


Additional reading

*


External links


Precipitation reactions of certain cations

Digestion Instruments

A Thesis on pattern formation in precipitation reactions
{{DEFAULTSORT:Precipitation (Chemistry) Liquid-solid separation