Precipitation (chemistry)
Precipitation defined as the creation of a solid from a solution. When the reaction occurs in a liquid solution, the solid formed is called the 'precipitate'. The chemical that causes the solid to form is called the 'precipitant'. Without sufficient force of gravity to bring the solid particles together, the precipitate remains in suspension. After sedimentation, especially when using a centrifuge to press it into a compact mass, the precipitate may be referred to as a 'pellet'. Precipitation can be used as a medium. The precipitate-free liquid remaining above the solid is called the 'supernate' or 'supernatant'. Powders derived from precipitation have also historically been known as 'flowers'. When the solid appears in the form of cellulose fibers which have been through chemical processing, the process is often referred to as regeneration.
Sometimes the formation of a precipitate indicates the occurrence of a chemical reaction. If silver nitrate solution is poured into a solution of sodium chloride, a chemical reaction occurs forming a white precipitate of silver chloride. When potassium iodide solution reacts with lead nitrate solution, a yellow precipitate of lead iodide is formed.
The precipitation may occur if concentration of a compound exceeds its solubility. Precipitation may also occur rapidly from a supersaturated solution.
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
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 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. Another important application of an antisolvent is in ethanol precipitation of DNA.
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 is added to a solution containing potassium chloride, the precipitation of awhite solid, silver chloride, is observed.
The silver chloride 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.
A final way to represent a precipitate reaction is known as a net ionic reaction.
Precipitate colors
Many compounds containing metal ions produce precipitates with distinctive colors. The following are typical colors for various metals. However, many of these compounds can produce colors very different from those listed.| Gold | black |
| Chromium | Deep green, murky green, orange, purple, yellow, brown |
| Cobalt | Pink |
| Copper | Blue |
| Iron | Green |
| Iron | Reddish brown |
| Manganese | Pale pink |
| Nickel | Green |
| Lead | Yellow |
Other compounds generally form white precipitates.