How does one obtain an appropriate mixture of methanol and w

How does one obtain an appropriate mixture of methanol and water for recrystallizing a solid sample?

Solution

The choice (ratio) of solvent really depends on the compound you want to crystallize. Following are few important points which will help you in choosing the ratio of methanol:water as your solvent system

1) The compound you want to recrystallize should be very soluble near the boiling point of the solvent, and only sparingly soluble at lower temperatures. 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) To prevent recrystallization of unwanted impurities alongwith your compound, you have to make sure that the impurities are either very soluble at room temp., or insoluble at the boiling point of the solvent.

3) There should be no interaction of the solvent with the compound being crystallized. The interaction of solvent will decrease the yield of your compound.

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