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, and the liquid remaining above the solid is called the supernate. Powders derived from precipitation have also historically been known as flowers.
Natural methods of precipitation 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 particle 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.
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.
An example of a precipitation reaction: Aqueous silver nitrate (AgNO3) 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.
A final way to represent a precipitate reaction is known as a net ionic reaction. In this case, any spectator ions (those which do not contribute to the reaction) are left out of the formula completely. This simplifies the above equations to the following:
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 which 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 similar elements, such as the Alkali earth metals.
Digestion, or precipitate ageing, happens when a freshly-formed precipitate is left, usually at a higher temperature, in the solution from which it is precipitated. It results in cleaner and bigger particles. The physico-chemical process underlying digestion is called Ostwald ripening.