Purification of Benzoic Acid by Crystallization Benzoic acid is a colourless crystalline solid
and it is an aromatic carboxylic acid. The pure crystals of benzoic acid can be prepared
from an impure sample by the process called crystallization. The crystallization process
involves: Preparation of solution of the impure sample
Filtration of the hot solution Cooling of the hot saturated solution
Separation and drying of crystals Our aim here is to prepare crystals of pure
benzoic acid from an impure sample. Materials required: Crude sample of benzoic acid, distilled water,
cold water, 250 ml beakers, funnel, filter paper, glass rod, spatula, fluted filter paper,
test tube, stand with clamp, trough, wire gauze, tripod stand and a Bunsen burner. Procedure: Preparation of Solution of the Impure Sample Take about 150 ml of distilled water in a
beaker. Boil the water in the beaker over the Bunsen
burner. In another beaker, take about 3 g of crude
sample of benzoic acid. To this, using a dropper, gradually add a
minimum quantity of boiling water, just sufficient to dissolve benzoic acid. Stir using the glass
rod. If required, heating can be done to dissolve
benzoic acid. Filtration of the Hot Solution Place a fluted filter paper in the funnel
and clamp the funnel to the stand. Place a beaker under the funnel.
Pour the solution into the funnel over the glass rod and collect the filtrate in the
beaker. The insoluble impurities are left on the filter
paper. Cooling the Hot Saturated Solution Let the filtrate come to room temperature
by itself. Cool it by placing the beaker in the trough
containing cold water. As the solution cools, white crystals of benzoic
acid begin to separate. Separation and Drying of Crystals Clamp the funnel in which a filter paper is
fixed to the stand. Place a beaker under the funnel and separate
the crystals by filtration. Wash the crystals with cold water.
Place the crystals on a filter paper which soaks up the solution.
Transfer the crystals to 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 benzoic acid are opaque white. Precautions: Cool the hot saturated solution without disturbing
it. Remove the insoluble impurities by filtration.