Question

Consider a crystallization of sulfanilamide in which 10 mL of hot 95% ethyl alcohol is added to 0.10 g of impure sulfanilamide. After the solid has dissolved, the solution is cooled to room temperature and then placed in an ice water bath. No crystals form, even afterscratching with a glass rod. Explain why this crystallization failed. What would you have to do at this point to make the crystallization work? Assume that starting over with a new sample is not an option. (You may need to refer to Tech 11, figure 11.2.)

Answer 1

The sulfanilamide has both the polar (amine and sulfonamide) and the nonpolar (benzene) moieties in its structure. Hence, it needs a solvent of intermediate polarity to dissolve it completely. 95% ethanol containing 5% highly polar water is a good solvent to dissolve the sulfanilamide.

The possible reason for the lack of crystal formation lies with the quantities used in this situation. The ratio of solvent to solute is far disproportionate to be able to give the desired results, i.e. crystal. The solute needs to be dissolved in a minimal and sufficient volume of solvent. The lack of crystal formation is obvious by using 10mL (large amount) of the alcohol. Hence, small drops of the 95% alcohol should be introduced into the flask of solute (approximately 1mL), instead of the large volume initially added (10 mL). Then you will definitely get the desired crystal.