Science & Technology in Action

2nd Edition

Recrystallisation of Benzoic Acid and Determination of its Melting Point


Jnr Science Cert
This lesson explains how recrystallisation is used to purify a substance, how to choose a suitable solvent, how the purity of a substance can be determined, what is meant by percentage yield and how crystallization is used in industry.
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Lesson excerpt

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. 

True or False?

  1. Filtration is used to separate two insoluble solids false
  2. Recrystallisation is a purifying technique. true
  3. An impure substance has a fixed melting point. false
  4. Organic substances tend to be less soluble in a hot solvent than in a cold solvent. false
  5. A range of 1 °C in melting point of a substance indicates a pure substance. true
  6. Benzoic acid is an ionic crystal due to its high melting point. false
  7. Polymorphs are substances with similar molecular composition but varying crystal structure. true
  8. A saturated solution is one which contains more solvent than solute. false
  9. The percentage yield is the mass of the pure sample divided by the mass of the initial sample. true
  10. Recrystallisation is used to produce crystals of specific size and shape. false