The solution is heated to ensure that everything, including the impurities, is completely dissolved. The solution is then carefully cooled to allow the pure drug to crystallize out of solution so that it can be isolated and washed. The choice of the solvent is critical to ensure that all of the crude material dissolves completely and that only the desired drug crystallizes in the final step. Otherwise, the loss of material in this step is too great for it to be economically feasible. Otherwise the impurity level is high enough to jeopardize label claims and efficacy.
Purity is twice as important as the Yield.
As such, Impurity is three times as important as the yield. The final temperature at which all of the material must dissolve into solution. This factor is continous and the experimental range is 70 to 90 degrees centigrade. The time required to heat the solvent and wait for complete dissolution. This factor is continuous and the experimental range is 1 to 2 hours.