Mother liquors

The filtrate obtained after collecting a recrystallised product is referred to as the 'mother liquor' and the isolated product as the 'first crop' of crystals. The mother liquor could contain a significant amount of the desired product along with the impurities from the crude compound. As a result is it sometimes desirable to use the mother liquor to obtain a 'second crop' of crystals. This can be acheived by concentrating the mother liquor, either partially so that a second crop forms, or entirely removing the solvent and carrying out a second recrystallisation.

Second crops of crystals are usually significantly less pure than first crops, or may not crystallise at all due to the higher levels of impurities present. Carrying out a second recrystallisation on the second crop improve the purity is not uncommon.