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Calculations and titrations

Carrying out a titration

Measuring purity

For some synthesised chemical compounds - pharmaceuticals, for example - it is very important that they have a high level of purity. Just a tiny amount of an impurity in a drug could cause a great deal of harm to a patient.

Samples of chemicals that are synthesised must be checked for purity. This is often done by carrying out a titration. A titration is used to measure the volume of one solution that exactly reacts with another solution.

Steps of titration

A titration is carried out using a number of steps:

  1. If the sample is a solid, it is weighed using an accurate balance, and then dissolved to make up a known volume of solution (usually 100cm3).

  2. A pipette is used to measure accurately a volume of this solution - for example, 10cm3. A safety pipette filler is used to draw solution into the pipette. This is emptied into a conical flask.

  3. A few drops of an indicator may be added to the conical flask. This will show a change of colour when the titration is complete.

  4. A second chemical is placed in a burette. This other solution is of a chemical that will react with the synthesised chemical sample in the conical flask. Often the solution in the burette is an acid or alkali, and it must be of a precise, known concentration.

  5. The solution from the burette is run into the conical flask. The solution is added one drop at a time, with swirling to mix the solutions as the end-point is approached. Eventually, a colour change shows that the correct amount has been added to react completely with the synthesised chemical in the sample.

  6. The volume of solution added from the burette is noted. The titration results can then be used to calculate the amount of the synthesised chemical in the sample, and therefore find its purity.

pipette, conical flask, burette, safety pipette filler

Apparatus required for titration (from left to right): burette, conical flask, safety pipette filler and pipette

Read on if you are taking the Higher paper.

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