Precipitation is the formation of a
solid in a
solution or inside another
solid during a
chemical reaction or by
diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the
precipitate, or when compacted by a
centrifuge, a
pellet. The liquid remaining above the solid is in either case called the
supernate or
supernatant. Powders derived from precipitation have also
historically been known as
flowers.
Natural methods of precipitate include settling or sedimentation, where a solid forms over a period of time due to ambient forces like gravity or centrifugation. During chemical reactions, precipitation may also occur particularly if an insoluble substance is introduced into a solution and the density happens to be greater (otherwise the precipitate would float or form a suspension). With soluble substances, precipitation is accelerated once the solution becomes supersaturated.
In solids, 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
.
An important stage of the precipitation process is the onset of nucleation. The creation of a hypothetical solid includes the formation of an interface, which requires some energy based on the relative surface energy of the solid and the solution. If this energy is not available, and no suitable nucleation surface is available, supersaturation occurs.
Applications
precipitate on cooling]]
Precipitation reactions can be used for making
pigments, removing
salts from water in
water treatment, and in
classical qualitative inorganic analysis.
Precipitation is also useful to isolate the products of a reaction during workup. Ideally, the product of the reaction is insoluble in the reaction solvent. 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:
:
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 easily be separated by filtration, decanting, or centrifugation). An example would be the synthesis of chromic tetraphenylporphyrin chloride: water is added to the DMF reaction solution, and the product precipitates. Precipitation is also useful in purifying products: crude bmim-Cl is taken up in acetonitrile, and dropped into ethyl acetate, where it precipitates.
In metallurgy, precipitation from a solid solution is also a useful way to strengthen alloys; this process is known as solid solution strengthening.
Representation using chemical equations
An example of a precipitation reaction: Aqueous
silver nitrate (AgNO
3) is added to a solution containing
potassium chloride (KCl) and the precipitation of a white solid,
silver chloride is observed. (Zumdahl, 2005)
The silver chloride (AgCl) has formed a solid, which is observed as a precipitate.
This reaction can be written emphasizing the dissociated ions in a combined solution. This is known as the ionic equation.
:Ag+ (aq) + NO3− (aq) + K+ (aq) + Cl− (aq) → AgCl (s) + K+ (aq) + NO3− (aq)
A final way to represent a precipitate reaction is known as a net ionic reaction. In this case, any spectator ions (those that do not contribute to the reaction) are left out of the formula completely. This simplifies the above equations to the following:
:Ag+ (aq) + Cl− (aq) → AgCl (s)
Cation sensitivity
Precipitate formation is useful in the detection of the type of
cation in
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 to separate chemically similarly.
Digestion
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 cleaner and bigger particles. The physico-chemical process underlying digestion is called
Ostwald ripening.
References
Zumdahl, Steven S. Chemical Principles. 4th ed. New York: Houghton Mifflin Company, 2005.
Mark Smith Principles of Science 1993
External links
http://www.answers.com/supernatant
Precipitation reactions of certain cations
Digestion Instruments
Category:Separation processes
Category:Chemical processes
