Which is good method to separate alum from impure samples?

Alum can be separated from impure samples by the method of Crystallisation.
This method is based on the difference in the solubility of the compound and the impurities in a suitable solvent.
The impure compound is dissolved in a suitable solvent in which it is sparingly soluble at room temperature but appreciably soluble at higher temperature. The solution is concentrated to get nearly a saturated solution. Filtered the solution while hot if there is some undissolved impurities in the solution. When this saturated solution is cooled , crystals of pure alum will be separated out.

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