Purification of Copper Sulphate by Crystallization The chemicals used for various purposes should
be completely free from any type of impurities. Crystallization is an important purification
process in which solid crystals are formed from a uniform solution. The different steps
involved in the crystallization process are: Preparation of solution of the impure sample.
Filtration of the solution. Concentration of the filtrate.
Cooling of the concentrated solution. Separation and drying of crystals. Our aim is to prepare crystals of pure copper
sulphate from an impure sample. Materials required: Crude sample of copper sulphate, dilute sulphuric
acid, distilled water, ethyl alcohol-cold water mixture, 250 ml beakers, china dish,
glass rod, spatula, funnel, filter papers, measuring jar, test tube, watch glass, evaporating
dish, trough, stand with clamps and sand bath. Procedure: Preparation of Solution of the Impure Sample
Take about 30 ml of distilled water in a beaker. To this, add powdered crude sample of copper
sulphate using a spatula. Stir the solution gently with the glass rod
so that the powder dissolves. Add the powdered sample several times till
a little of it remains undissolved in the beaker, even if it is stirred for some time.
Now, add about 3 ml of dilute sulphuric acid to the solution using the measuring jar and
gently stir the solution to make it clear. This prevents the hydrolysis of copper sulphate. Filtration of the Solution Place a filter paper in the funnel and clamp
the funnel to the stand. Place the china dish under the funnel. The
stem of the funnel should touch the wall of the china dish to avoid the solution splashing
out. Pour the solution into the funnel over the
glass rod and collect the filtrate in the china dish.
The insoluble impurities are left on the filter paper. Concentration of the Filtrate Heat the china dish in a sand bath till the
solution is reduced to about one-third of its original volume.
To avoid crust formation on the side of the dish, stir the solution continuously with
a glass rod. To know if the solution has reached its crystallization
point, remove a drop of the solution at the end of the glass rod and cool it by blowing.
The appearance of a crust or tiny crystals on the glass rod shows that the crystallization
point has been reached. Now, transfer the hot saturated solution into
the evaporating dish. Cooling of the Concentrated Solution Place the evaporating dish containing the
hot saturated solution in a trough containing water.
Cover the dish with the watch glass and allow it to cool slowly for some time.
As the solution cools, deep blue crystals of copper sulphate separate and the crystallization
will complete after half an hour. Separation and Drying of Crystals Decant the mother liquor carefully into a
beaker. Wash the crystals 3 or 4 times with a little
ethyl alcohol containing a small amount of cold water.
Using the spatula, place the crystals on a filter paper which soaks up the solution.
Transfer the crystals on another filter paper and dry them by pressing gently between the
folds of the filter paper. Now, transfer the crystals into a dry test
tube. The crystals of pure copper sulphate are transparent and blue. Precautions: The filtrate should be evaporated slowly by
gentle heating during concentration. The filtrate should never be heated to dryness.
The solution should be cooled slowly without disturbing it.
Overheating results in the formation of tiny crystals.