Thursday, May 6, 2010


It is the process of separation or formation of a solid or an insoluble compound formerly held in suspension or solution in the liquid.

Process of precipitation:
Chemical precipitation is completed in a number of manners. One of the most important methods for precipitation is by displacement reaction taking place in the solution in which more active metal displaces inactive or less active metal.

Precipitation is the process of separation of the solid substance from a solution:

1. Either by altering the substance to an insoluble form
2. Or by altering the solvent composition to lessen the solubility of the substance in it.

The insoluble solid which has been separated from solution or suspension, due to chemical reaction or some other process, is referred to as precipitate.

Precipitating agent which is the cause of precipitation from a suspension or solution is referred to as precipitant.

Supernatant liquid:
It represents the liquid above the insoluble solid obtained after settling by precipitation, sedimentation or centrifugation.

Application of the precipitation:
In pharmacy, Precipitation is used for many purposes. It is primarily used for the separation and analysis of metal ions in aqueous solutions. It is also used for the preparation and purification of some of the substances



1. Silver ions, found in the solution of silver nitrate, are precipitated by the additional chloride ions for example by a solution of sodium chloride. In this process, after the addition of sodium chloride, silver ions and chloride ions are combined forming silver chloride which is insoluble in water and separated.

2. Calcium ions are precipitated by the addition of oxalate ions.

3. Barium ions are precipitated by sulphate ions.

Qualitative chemical analysis:
In some of the processes such as in qualitative chemical analysis, assay of the mixtures of metal ions is done by sequential usage of reagents that causes precipitation of specific ions or collection of related ions.

Quantitative chemical analysis:

Gravimetric analysis:

In many of the cases, it is also possible to choose the factors in which the substance is precipitated in favourably pure and readily separable form. Separation of such type of precipitates and measuring their weights leads to accurate methods for finding the quantity of different compounds as in gravimetric analysis.

White lotion is prepared by precipitation.

Purification of solids can also be done by the process of precipitation. The process of recrystallization is to be done in purification.

In this process, impure solid is absorbed in a suitable solvent at increased temperature. After cooling, much of the impurities remain absorbed in the solvent while the purified solid is precipitated.

This procedure is done again and again as necessary by using different solvents.

Removal of undesired constituents during precipitation:

Sometimes in the process of precipitation of a single substance from a solution composed of several different components, unwanted ingredients are often added to the crystals resulting in the reduction of the purity leading to impaired accuracy of the analysis. This addition of unwanted ingredients can be decreased by performing the processes with dilute solutions and by the addition of precipitating agent at a slow rate.

On the other hand, in some of the difficult processes, it may be essential to separate an impure precipitate, dissolving the precipitate again and again precipitating it. In this way, most of the meddlesome substances are separated in the original solution and the second precipitation is done in their absence.

Multiple Choice Questions (MCQs) of Precipitation from Pharmaceutics

1. _______ is the formation of a solid in solution or inside another solid during a chemical reaction or by diffusion in a solid.

a. Suspension
b. Emulsion
c. Precipitation
d. Collodions
e. None of the above
2. Natural methods of precipitation include ________ or _________

a. trituration or moulding
b. dry gum method or wet gum method
c. settling or sedimentation
d. All of the above
e. None of the above
3. With __________, precipitation is accelerated once the solution becomes supersaturated.

a. Soluble substances
b. Insoluble substances
c. More soluble substances
d. Sparingly soluble substances
e. None of the above
4. In solids, precipitation occurs if the concentration of one solid is ______ the solubility.

a. Above
b. Below
c. Equal
d. Parallel
e. None of the above
5. An important stage of the precipitation process is the onset of nucleation.

a. hydrolysis
b. liquefaction
c. acidosis
d. nucleation
e. none of the above
Answers to Multiple Choice Questions (MCQs) of Precipitation from Pharmaceutics

1. c
2. d
3. a
4. a
5. d

(These MCQs are helpful for the preparation of Pharmacy Exams)
Further Studies:

Remington: The Science and Practice of Pharmacy (Remington the Science and Practice of Pharmacy)
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