Precipitation (chemistry)

Precipitation is the formation of a suspension).

Uses

Precipitation reactions can be used for making pigments, removing salts from water in water treatment, and for qualitative chemical analysis.

This effect is useful in many industrial and scientific applications whereby a chemical reaction may produce a solid that can be collected from the solution by various methods (e.g. filtration, decanting, centrifuging). Precipitation from a solid solution strengthening.

Mechanism

An important stage of the precipitation process is the onset of 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

Ag⁺(aq) + NO₃^-(aq) + K⁺(aq) + Cl^-(aq) → AgCl(solid) + 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 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 rare 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.^[1] The physico-chemical process underlying digestion is called Ostwald ripening.

Coprecipitation

Main article: 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 radiochemistry, coprecipitation is often the only way of separating an element.

References

Zumdahl, Steven S. Chemical Principles. 4^th ed. New York: Houghton Mifflin Company, 2005.
Mark Smith Principles of Science 1993

^ Chemical dictionary definition.