Question

Explain briefly why recrystallization can never result in 100% mass recovery even if you used pure material. Also explain why mass recovery is greater if you cool to 0 degrees C rather than room temperature.

Explain very briefly why mass recovery is reduced if you use an excess amount of boiling solvent and why washing product crystals with excessive amounts of solvent especially warm solvent can result in reduced mass recovery.

Answer 1

When performing a Recrystallization, when the crystals are collected in hirsch funnel, it should be washed with cold solvent which does not dissolve the solute while separating the remaining impurities.

If it is washed in room temperature solvent, then some part of solute may get dissolved in solvent, therefore the purity of product will remain same while the recovery amounnt will decrease.

If product crystals are washed with excessive amounts of solvent especially warm solvent then crystallisation will occur slowly resulting in trapped impurities in the crystals resulting in reduced mass recovery.