Fractional Freezing - Purification of Solids

Purification of Solids

When a pure solid is desired, two possible situations can occur. If the contaminant is soluble in the desired solid, a multiple stage fractional freezing is required, analogous to multistage distillation. If, however, a eutectic system forms (analogous to an azeotrope in distillation), a very pure solid can be recovered, as long as the liquid is not at its eutectic composition (in which case a mixed solid forms, which can be hard to separate) or above its eutectic composition (in which case the undesired solid forms).