Purification of Impure Samples by Crystallization

Our Objective

Our Objective is to prepare the crystals of the following substances form their impure samples through crystallisation;

  1. Copper sulphate
  2. Potash alum
  3. Benzoic acid 

The Theory

What happens during crystallisation?

The chemicals used for various purposes should be pure, completely free from any type of impurities. Method of purification of a substance depends upon the nature of impurities present in it. There are various methods for the purification of substances, e.g., filtration, evaporation, decantation, distillation, and crystallization. Crystallisation is one of the very important purification techniques, purifying substances by removing unwanted by-products. Crystalline compounds are generally purified via this crystallisation process.

The principle behind the crystallisation is that the amount of solute that can be dissolved by a solvent increases with temperature.

In crystallisation, the impure substance is dissolved in a suitable solvent to reach its nearly saturated solution at a temperature higher than the room temperature. At this high temperature, the solute has very high solubility in that solvent, so a much smaller quantity of hot solvent is needed for dissolving the solute than the solvent at room temperature.  When the solution is cooled, the pure substance is crystallised. The solution left behind is called mother liquor. All the impurities are left behind in the mother liquor. The purification method depends on the differences in solubility between the compound and the impurity. 

Let us discuss the basic steps involved in the crystallisation process

Solvent and solute selection

Choosing an appropriate solvent is the important process of crystallisation, as crystallisation works only when a proper solvent is used. It is important to choose a solvent that will not dissolve the substance at room temperature. But as the temperature of the solvent increases, the solubility of the solute also increases. At the same time, the impurities that are present must either be soluble in the solvent at room temperature or must be insoluble in the solvent at a high temperature. If the solvent is not hot when the dissolution is carried out, too much solvent will be used, leading to diminished yield.

Dissolving the solute in the solvent

Add a small portion of the solvent to the beaker containing impure sample and boiling chips while the sample is heating.  Stir the contents gently. Add enough solvent to dissolve the solute to get a saturated solution at the boiling point of the solvent. If too much solvent is used, the recovery of the substance will be decreased.

Filtration of the hot solution

If the hot solution contains insoluble impurities, it can be removed by the process of filtration. For this process, Place a filter paper cone in a funnel and wet the filter paper with a spray of water to fix it inside the funnel properly. Place the funnel on a funnel stand and put a china dish below the funnel.

Note: The stem of the funnel should touch the wall of the china dish to avoid the solution splashing out.

Crystallisation of the filtrate

To concentrate the filtrate, heat the china dish containing filtrate gently with constant stirring. This is done to ensure uniform evaporation and to prevent formation of a solid crust. When the volume of the solution is reduced to one half, dip one end of a glass rod in the concentrated solution and cool the drop by blowing on it. The formation of a thin crust indicates that the crystallisation point has been obtained. 

Cooling the concentrated solution

Once it is determined that the solution is saturated with the compound, it is allowed to cool slowly at room temperature.

In order to cool the concentrated solution, pour the solution into a crystallising dish and keep it undisturbed. As the solution cools, crystals separate. Once the sample is cooled to room temperature, place it in an ice bath or in cold water to complete the crystallisation.

If the crystallisation does not start immediately, add a seed crystal or scratch inside the vessel containing the concentrated solution with a glass rod.

Separation and drying of crystals

The crystals formed are separated by either decanting the mother liquor or by the process of filtration.  Wash the crystals with cold water or alcohol. The crystals can be dried by pressing them gently between sheets of filter paper.  They can also be dried by spreading them on a porous plate or by placing the crystals in a vacuum desiccator. The crystals have a definite geometry, and therefore a definite shape.

Let us discuss the crystallisation of some samples

Crystallisation of copper sulphate (CuSO4.5H2O)

To crystallise copper sulphate, dissolve it in water and add a small quantity of dilute sulphuric acid to prevent the hydrolysis of copper sulphte. The impurities left behind in the solution are removed by filtration. The filtrate is concentrated to the crystallisation point and then cooled. On cooling, transparent blue crystals of copper sulphate separate. The copper sulphate crystal formed has triclinic shape

Crystallisation of Potash Alum

Potash alum is highly water soluble. To prepare pure crystals of potash alum, dissolve the sample in distilled water and remove the insoluble impurities by filtration. This solution is concentrated to the crystallisation point and cooled.  Colourless transparant crystals of potash alum separate. The soluble impurities are left behind in the mother liquor. Potash alum has an octahedral geometry.

Crystallisation of Benzoic Acid

Benzoic acid is a colourless crystalline solid. It is highly soluble in hot water, but poorly soluble in cold water. It can be recrystallised by dissolving it in hot water. The hot solution obtained is filtered and cooled. Upon cooling, opaque white crystals of benzoic acid crystallise.

Learning Outcomes

  1. Students understand the term ‘crystallisation’ from the experiment.
  2. Students understand different steps of a crystallisation process.
  3. Students acquire skill to perform the crystallisation using the following;
    • Copper sulphate
    • Potash alum
    • Benzoic acid
  4. Students may be able to do the experiment using other samples once they visualise the steps.