Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Producing pure substances is a very important process, especially in the pharmaceutical industry. Separating insoluble solids from solvents is easily achieved using a simple technique called filtration. Soluble solids again are easily separated from solvents by evaporation, andchromatography is used to separate a mixture of liquids. But what happens when we have two soluble solids that we want to separate? In this lesson we will look atrecrystallisation, a commonly-used process in the pharmaceutical industry.
What is recrystallisation?
Recrystallisation is a very important purification technique, purifying substances by removing unwanted by-products. It is also used to manufacture the correct crystal size and shape of a material. These factors can have a very significant impact on how a medicine acts when taken by a patient. The same principles and techniques of recrystallisation can be applied both on a laboratory and industry scale.
What are the principles behind recrystallisation?
The process depends on two principles; the fact that substances tend to be more soluble in a hot solvent than in cold solvent, and that each solute tends to behave as though it were alone in the solvent.
How do I know what solvent to use?
Using the correct solvent is a very important part of the process. The solute must be insoluble in the solvent at room temperature, and as the temperature of the solvent increases, the solubility of the solute also increases. It is also important that the impurities present are soluble in the solvent at room temperature and insoluble at higher temperatures.
An excellent substance for showing this process is benzoic acid (C6H5COOH). A molecular crystal rather than an ionic crystal, shown by its low melting point of 122 °C, benzoic acid is used as an anti-microbial agent and is found in toothpastes, mouthwashes, cosmetics and deodorants.
Impure benzoic acid contains the impurities phthalic acid and benzylbenzoate. If the impure sample is dissolved in a minimal volume of hot solvent – in this case boiling water – and filtered to remove insoluble impurities, the resulting solution will contain dissolved benzoic acid as well as dissolved impurities.