Science & Technology in Action

14th Edition

Modern separation methods

MSD

In the production of chemical substances, especially pharmaceuticals, it is important to ensure that the products are pure and uncontaminated. Separation techniques are the methods used to remove one substance from a mixture containing any number of other substances, for harvesting or identification.
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Lesson excerpt

Introduction

Separation techniques are the methods used to remove one substance from a mixture containing any number of other substances, for harvesting or identification.

In the production of chemical substances, especially pharmaceuticals, it is important to ensure that the products are pure and uncontaminated. If we want a particular reaction to occur we need to be sure that only the required reactants are present, otherwise unforeseen products may be formed and may change the properties of the substance we are trying to make. The un-purified raw materials are processed to the required purity so that they can be used as feedstock in a variety of pharmaceutical processes.

In almost all chemical reactions there will be more than one substance formed. The substance we want to produce is called the product and any other substances formed are called co-products or by-products. These co-products and any unreacted substances must be removed before the product can be used for commercial purposes, for both safety and quality reasons. The degree of purity required varies depending on the use. If the substance is used in food or in medicines, then purity is of the highest importance.

There are numerous ways to purify substances and the methods used depend on the physical and chemical properties of the substances involved. In most cases more than one method is needed to achieve the desired product purity. When a substance is separated, purified and collected for use, it is said to be harvested.

Filtration

Filtration is probably the simplest method. It involves passing the mixture of an insoluble substance in a fluid (liquid or gas) through a mesh (e.g. filter paper), which will catch the solid particles and allow the fluid to pass through. The size of the pores in the mesh used will depend on the size of the particles to be removed. For example, a colander (or sieve) has large holes to separate peas from the water in which they were boiled. Filter paper has very small holes to trap sand articles, while ceramic filters have pores small enough to prevent bacteria getting through. The substance trapped by the filter is called the residue while the fluid that passes through is called the filtrate. Either or both may be retained for further use.

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True or False?

  1. To obtain a pure substance it is normal to use more than one method of separation. true
  2. Eutrophication is a method of separating fertilisers from water. false
  3. In electrophoresis a potential (voltage) is used to separate the substances. true
  4. The liquid that passes through a filter is called the residue. false
  5. Fractional distillation depends on the differing solubility of the components. false
  6. Francis Aston invented the mass spectrometer. true
  7. Petrol does not dissolve in water because petrol is non-polar while water is polar. true
  8. Recrystallization could be used to remove salt which had been added to sugar. true
  9. A magnet could be used to purify a sample of sulphur contaminated by iron. true
  10. Raw materials are purified feedstock. false