Purification of the product is an integral part of any synthesis experiment. The process of rerecrystallisation is often used to obtain the product in pure form. In industry, one of the important steps is obtaining pure chemicals for various purposes where the final purity may depend on the end application of the product.

The current activity aims to acquaint the reader with rerecrystallisation as a learning activity and its process.

During rerecrystallisation, particles of solute deposited from a saturated solution and according to their shapes, fit onto growing crystal lattices. Crystallization from solution can be thought of as a two-step process. The first step is the phase separation or “birth” of new crystals. The second step is the growth of these crystals to larger sizes. These two processes are known as nucleation and crystal growth, respectively. The solubility of a particular solute in a solvent depends upon its polarities and the interactions with one another and temperature. The difference in solubilities of a solute in a given solvent at different temperatures is the key feature that is used for recrystallisation of the solute.

Before starting the activity let us get some idea about the solvents that are generally used for recrystallization (Table I). The information regarding the miscibility of these solvents in one another (Table II) is equally important as often we may have to select a mixed solvent system for rerecrystallisation. This data is available from standard chemical data books such as CRC, etc. This will also give you practice in the handling of data books about chemical compounds.  

Table I: General list of solvents used for recrystallisation

Solvent Boiling points (C)(760 torr) Dielectric constants Flammable
Water 100 78.54 No
Ethanol 78 25.30 Yes
Methanol 64 33.00 Yes
Acetone 56 21.01 Yes
Petroleum ether 1.90 Yes
Hexane 68 1.89 Yes
Dichloromethane* 41 9.08 No
Toluene 111 2.38 Yes
Chloroform* 61 4.81 Yes
Cyclohexane 81 6.00 Yes
Acetic acid 118 6.15 No
Diethyl ether 35 4.34 Yes
Ethyl acetate 77 6.02 Yes
*-generally use of chlorocarbon solvents is avoided