Technique of Recrystallization in Crystallography

by Ritika

Recrystallization is the technique of purifying solids based on their different solubility. That is, recrystallization deals with the cleaning method or the transformation of a crystal structure, as a result of the change in external factors, for example temperature and pressure.

By the process of recrystallization in crystallography a satisfactory solution of a substance is manufactured and the substance is crystallized by cooling the solution. The developing crystals are definitely in a purer form in all respects, especially if the supplementary impurities display some other solution behavior. The entire process may be done over and over again until the preferred degree of purity is attained. The crystallization process by itself assists the purification process since, as the crystals get formed, they choose the correct molecules that match the crystal lattice and disregard the wrong molecules. This is not a flawless procedure, but it definitely increases the purity level of the final creation.

Technique Used for Recrystallization in Crystallography

Selection and Addition of Solvent

First, careful selection of appropriate solvent is made and a little quantity of this solvent is added to the container containing impure solid. Choosing the solvent is very vital and essential care must be taken to select a solvent such that the impure compound has a low solubility at lower temperatures, however, it may be totally soluble at high temperatures. That is, the solvent must be selected in such a way such that it fully dissolves the impure substance when heated and also move out of the solution on cooling. Ensure you add little quality of solvent rather than in larger amounts such that it fully dissolves the sample. This is because it is possible to add more solvent as needed when heating as described in the next step. The solubility of the compound in the solvent used for the recrystallization process is essential. In an ideal situation, the solvent will dissolve the compound fully to be purified at high temperatures, which is mostly the boiling point of the solvent, and the compound will be totally insoluble in that solvent at room temperature or at 0oC. Moreover, the impurity will either be totally insoluble in that solvent at high temperatures, or will be very soluble in the solvent at low temperature. So, the process of recrystallization relies on the property that for most compounds, as the temperature of a solvent increases, the solubility of the compound in that solvent also increases.

Process of Heating

To allow the solid to dissolve, the process of heating is carried out until the solid gets dissolved in the solvent. For carrying out the heating process, one can use either a hot water bath or steam bath. These would help in carrying out the heating process in a gentle, controlled, efficient way. Some people prefer to use even a hot plate or gas burner for carrying out the heating process. The next process is the process of cooling which is described below.

Process of Cooling

The above heated solution is cooled. This enables crystallization of the product. This also allows it to get more pure solid precipitation thereby leaving the impurities to be dissolved in the solvent. The secret in achieving efficient higher purity product is slower cooling. So, ensure that you always allow the solution to cool to room temperature before setting the flask in an ice bath or refrigerator.

The process of forming crystals normally starts at the bottom of the container. One can also aid crystallization by scratching the container with a glass rod particularly at the air-solvent junction. Though this is not an essential step, by doing this scratching process, the glass surface area gets increased giving a roughened surface on which the solid can crystallize. Once can also follow the seeding technique for aiding the crystallizing processes. This is done by adding a small crystal of the desired pure solid to the cooled solution. But while doing this, care must be taken to add the crystal only after the solution is cooled which otherwise would cause the crystal to dissolve. Sometimes, one might encounter that no crystals has resulted from the solution. Then the possible reasons for this could be usage of too much solvent. So, for correcting this, allow the solvent to evaporate. Then follow the process of reheating and cool the solution as explained above for pure crystals to be formed.

Vacuum Filtration Process

After the formation of crystals now we have to separate the crystal from the solution for which vacuum filtration process is carried out. The process of vacuum filtration is applied which isolates the crystals. For isolating the crystals of purified solid, one can also try washing the purified solid with chilled solvent. But while washing, ensure that you use only cold solvent, because otherwise it would result in dissolving the crystals.

Process of Drying

Allow the solid to dry which results in getting a purified solid. The waste solution could be discarded. The previous process of vacuum filtration itself would have removed almost all the solvents. This step namely the open-air drying would enable removing more solvent, if anything further is present.

One can repeat the process of recrystallization if they wish to further purify the sample. Crystallization of a solid is not the same as precipitation of a solid. In crystallization, there is a slow, selective formation of the crystal framework resulting in a pure compound. In precipitation, there is a rapid formation of a solid from a solution that usually produces an amorphous solid containing many trapped impurities within the solid’s crystal framework. Solid-state recrystallization is what takes place under circumstances of extreme pressure and temperature where grains, atoms or even molecules of a rock / mineral are crammed even closer to each other, to create a whole new crystal structure. It is possible to check the level of purity by taking a melting point range of the solid. It is then compared with an accepted melting point range. In general, the thumb rule is solids show higher melting points if they are pure. That is, for pure solids, the melting points get raised. It is also possible to use two or more solvents in the technique of recrystallization in which case, the technique is termed as multi-solvent recrystallization.