recrystallization chemistry lab

Question

A student performed successive recrystallizations on an impure mixture where there was a 10% by weight impurity. After one recrystallization, she obtained 80% of the original weight of crystals back. After a second recrystallization,
she obtained 60% of the original weight of crystals back. After a third recrystallization, she obtained 40% of the original weight of crystals back. If the original amount of contaminant was only 10%, why did she only obtain 40% of the final pure product?

Answer 1

There are several possible reasons for a low recovery upon recrystallization.  Some are 

(a) Choice of incorrect solvent. If the compound of interest has low solubility at all temperatures, or high solubility at all temperatures, the recovery will be low.

(b) Volume of solvent.  If too much solvent is used, it will be difficult to obtain sufficient yield of crystals and yield will be lwo.

(c) Time.  Not enough time was allowed for all of the crystals to form.  Crystallization is a time-dependent process.

(d) Technique.  Recrystallization is sort of an art, more than a science. There are certain techniques that improve yield, such as using ice-water bath to initiate the process and then changing the temperature, or seeding, etc.