Recrystallization Definition / Meaning

Table of Contents

What is recrystallization?

Recrystallization definition: Recrystallization is a technique of purification in which crystalline product is separated from impurities by dissolving in a suitable solvent, and crystallized again to produce pure crystals. It is the most important and delicate method of purification of solid organic compounds.

Principle of Recrystallization

It consists of dissolving the substance in a minimum amount of a solvent in which the substance is more soluble in hot than in cold. The impurities are removed by filtering the hot solution through a hot water funnel, if necessary, and the filtrate is allowed to cool when crystals of pure substances are obtained.

These are filtered and if necessary, again dissolved for recrystallisation. It should be noted that rapid filtration cooling produces tiny but purer crystals, while slow cooling produces larger but impure crystals. The liquid left after the separation of the recrystallisation product is known as the mother-liquor. It is concentrated by evaporation and cooled to obtain a fresh crop of crystals of pure substance which are further subjected to recrystallization.

Recystallization process/procedure

  1. Choice of suitable solvents
  2. Drying of recrystallized substance
  3. Decolourization of undesired colours

1. Choice of suitable solvent

The choice of a suitable solvent is of great importance in the purification of substances by crystallization. The solvent is selected on the ‘hit and trial basis and it is necessary to try a number of solvents before arriving at a conclusion. If the solvent is dissolved the substances in cold or a large excess of hot solvent is required to dissolve the substance, it is obviously unsuitable for recrystallization.

Only such a solvent is to be selected which does not dissolve or dissolves a very small amount of substances in cold but dissolves its appreciable amount on boiling or heating. If neither solvent seems good enough for recrystallization, a combination of two or more solvents can be used. Nonpolar substances are generally soluble in nonpolar solvents, e.g., benzene, petroleum ether, etc. The substances whose molecules are polar are more soluble in polar solvents. The selected solvent should dissolve the impurities completely or the impurities should be completely insoluble in it.

2. Drying of recrystallized substance

The recrystallized substance is dried either by pressing it between several layers of filter paper and occasionally transferring it onto fresh filter paper or by heating over a water bath or in an electric oven depending on the type of substance.

The safe and reliable method for drying, over much time, is the use of desiccator with a vacuum arrangement known as vacuum desiccator. The recrystallized substances are spread over a watch glass and kept in vacuum desiccator and vacuum is applied, The substance is allowed to stay under these conditions for several hours and then vacuum is released very slowly when the dry recrystallized substance is obtained.

3. Decolurisation of undesirable colour

Often, the crude substance may have traces of resinous colouring matter that affect the properties and particularly the appearance of the substance. These impurities generally dissolve in the hot solvent during recrystallization but are again absorbed by the crystals as they are formed thus yielding a coloured appearance to the substance.

Such impurities are removed by boiling the substance in solution with a sufficient amount of finely powdered animal charcoal for about 15-30 minutes and then filtering the hot solution. The coloured impurities are adsorbed by animal charcoal and the pure decolourized substance crystallises out from the filtrate on cooling.

Recrystallization of benzoic acid

Take 1 gram of crude benzoic acid and add about 50 ml water to it in a 100 ml beaker. Heat the mixture to boiling when benzoic acid dissolves completely. Filter the solution while hot through small pleated filter paper. Allow the filtrate to cool immediately by immersing the beaker in cold water. Benzoic acid crystallizes in the form of fine needles. Filter the crystals, dry them and determine the melting point.

Recrystallization of benzoic acid, crystallization, benzoic acid crystallization

Recrystallization temperature

Recrystallization occurs when the temperature rises to one-third to one-half of the melting point (in degrees Kelvin), increasing atomic mobility and causing recrystallization.

FAQs

What is the purpose of recrystallization?

Recrystallization is most important for the purification of solid chemical substances.

What is recrystallization temperature?

Recrystallization occurs when the temperature rises to one-third to one-half of the melting point (in degrees Kelvin), increasing atomic mobility and causing recrystallization.

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