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Precipitation (chemistry)

Precipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate.

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 (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.

Mechanism

Precipitation can occur when an insoluble substance is formed in the solution due to a chemical reaction or when the solution has been supersaturated by a compound. The formation of a precipitate is a sign of a chemical change. In most situations, the solid forms ("falls") out of the solute phase, and sinks to the bottom of the solution (though it will float if it is less dense than the solvent, or form a suspension).

The solid may reach the bottom of a container by means of settling, sedimentation, or centrifugation.

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.

Representation using chemical equations

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)

AgNO₃ (aq) + KCl (aq) → AgCl (s) + KNO₃ (aq)

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 which 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 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

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.

Coprecipitation

Coprecipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed. It is an important issue in chemical analysis, where it is often undesirable, but in some cases it can be exploited. In gravimetric analysis, it is a problem because undesired impurities often coprecipitate with the analyte, resulting in excess mass. On the other hand, in the analysis of trace elements, as is often the case in radiochemistry, coprecipitation is often the only way of separating an element.