Question

What is a good solvent for crystallization of an impure solid compund, talk about solubility at different temperatures in general, not specific compounds.

Answer 1

1. The compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature. . If the compound is insoluble in the chosen solvent at high temperatures, then it will not dissolve. If the compound is very soluble in the solvent at room temperature, then getting the compound to crystallize in pure form from solution is difficult.

2. The unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent. This way, after the impure solid is dissolved in the hot solvent, any undissolved impurities can be removed by filtration. After the solution cools and the desired compound crystallizes out, any remaining soluble impurities will remain dissolved in the solvent.

3. The solvent should not react with the compound being purified. The desired compound may be lost during recrystallization if the solvent reacts with the compound.

4. `The solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized. This allows for easy and rapid drying of the solid compound after it has been isolated from the solution.

In general, if in a nearly saturated solution,the dissolution process is endothermic,the solublity should increase with rise in temperature,if it is exothermic, the solublity should decrease with rise in temprature.