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Test kits for water analysis
Test kits for water analysis

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5 Titrimetric methods

A titration is a procedure whereby an unknown quantity of a particular substance is measured by adding it to a standard reagent with which it reacts in a definite and known proportion. If you refer to a balanced chemical equation for the reaction, the point when the reactants have reacted together stoichiometrically is called the equivalence point, and this is what we determine by titration.

Whereas it is possible to measure an equivalence point instrumentally, observing a colour change using a chemical indicator is more convenient. The volume of standard added when the colour change occurs is called the end-point. Ideally the end-point should be the same as the equivalence point but these do not always match exactly so there is generally an in-built error when titrating using an indicator. In practice though, this is extremely small and tends to be ignored.

Now watch Video 5 Doing titrations in which a titration is demonstrated.

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Video 5 Doing titrations.
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The challenge for test kit developers is to take the apparatus shown in Video 5 and convert it into a form that can be used in the field.

Activity 3  Titrations in the field

Timing: Time allowed: 2 hours

A number of kits incorporating portable titration apparatus have been developed to allow titrations to be carried out on-site: hand-held titrators and drop-count bottles to name but two.

Look again at the websites of some of the major manufacturers listed in Activity 1, or any other appropriate resource you have available, and write a brief summary of two marketed kits for field titration. As far as possible include such information as detection limits, the accuracy claimed and how reagents are disposed of.