Recrystallisation
Introduction to Recrystallisation
Solid products recovered from reactions are often described as crude, meaning that they are impure to some extent. Crude products may appear crystalline, but likely contain a range of different impurities, such as solvents, left-over reactants, by-products etc. Recrystallisation is a useful purification technique for solid compounds. It involves dissolving the solid in a hot solvent (or mixture of solvents), then cooling the solution, and inducing the compound to crystallise from the solvent, in such a way that impurities can be separated, leaving crystals of pure compound.
Rate of cooling
The rate of cooling in a recrystallisation has a large effect on crystal growth. Generally, the slower a solution cools the larger and more regular the crystals formed.
Precipitation
Precipitation is not really a form of recrystallisation, but the result of a compound being forced out of solution either by crash cooling or addition of antisolvent. Precipitated compounds are not usually regular in appearance and are usually classified as powders rather than crystals.
Crystal descriptions
There are many different terms used to describe the physical appearance of crystals. For recrystallisation, crystal habit terms are usually used which are empirical observations of the crystal form. Some of the common terms used are given below along with images of example crystals.
Needles
One dimensional growth. Needles are long, fine crystals which resemble needles in appearance. They are usually many times longer than they are wide or deep.
Benzil recrystallised from water/ethanol (solvent/antisolvent)
4-Methoxybenzoic acid recrystallised from 7:3 water/ethanol
Plates
Two dimensional growth. The shape may vary substantially, and may include for example: cicular, triangular, square or hexagonal form.
Acetanilide recrystallised from water
Blocks
Three dimensional growth. There is much variation in shape and in the three dimensions. May include descriptions such as cubic, prismic or tetrahedral to better describe their form.
Copper(II) sulfate recrystallised from water
Microcrystalline
This is a catch all term for any crystals which are too small to visually assign a crystal habit term. They are distinct from a powder in that they do have defined edges resulting in a shiny surface from their regularity which powders do not exhibit. If larger crystals were grown it is likely their habit could be identified.
Potassium dichromate recrystallised from water
Powder
This is where a compound does not exhibit regularity in its form (and is therefore not crystalline) The compounds appear dulled in appearance. Powders when recrystallised usually exhibit a crystal habit.
Magnesium sulfate (anhydrous)