Question

Recrystillization Lab:

Suggest one chemical reason, specific to this experiment, why your percent recovery could be less than 100%.

Answer 1

Recrystallization is the most common technique used to purify solid products contaminated with other solids.

When an impure product is contaminated by a solid with different solubility characteristics, a solvent is chosen which will dissolve the desired compound but will not dissolve the impurity.

When this situation exists, the solvent is added, the mixture decanted or filtered to remove the solid impurity, and the product recovered by evaporation of the solvent

Many times in organic chemistry, the impurity has about the same solubility characteristics as the product itself, and the situation becomes a little more difficult. In this case, the mixture is dissolved in a minimum amount of hot solvent.

As the solvent cools down, the solution will become saturated in product, and fairly pure crystals of the product will begin to form.

The product will continue to crystallize and the contaminant remain in solution as the solution cools. Then the mixture is filtered and the crystals of product are washed to remove any traces of contaminant clinging to them.

The yield is not near 100% because a portion of the product remains dissolved in the liquid that contains the contaminant and this is usually discarded.