Double Indicator Titration Method Experiment [PDF]

Double Indicator Titration Method Experiment

Here,in this experiment we will determine and compare the alkalinity of water samples 'A' and 'B' by double indicator titration method.

Alkalinity is a measure of the ability of water to neutralize acids. The alkalinity of water is mainly due to the presence of

• Soluble hydroxide only (OH ions)

• Soluble bicarbonate only (HCO3 ions)

• Soluble carbonate only ( CO3^ - 2 ions)

• A mixture of soluble carbonate and   bicarbonate.

• A mixture of soluble hydroxide and carbonate 

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The presence of hydroxide (OH) and bicarbonate ( HC O 3 ^ sim ) ions is not possible because they combine together to form carbonate (C * O_{3} ^ - 2) ion.

The types and extent of alkalinity present in water sample can be determined by titrating it against standard acid using phenolphthalein and methyl orange as indicators.Hence this titration is known as double indicator titration. The chemical reactions involved can be represented by the following equations.

A known volume of water sample is titrated against standard acid. The volume of acid used up to phenolphthalein end point (P) corresponds to reactions 1 and 2 onlyi.e. complete neutralization of OH ions and half neutralization of C*O_{3} ^ - 2 ions. The volume of acid used up to methyl orange end point (M) corresponds to the reactions 1, 2 and 3 i.e. complete neutralization of OH, C * O_{3} ^ - 2 and HC*O_{3} ^ - ions. The total amount of acid used from the beginning of experiment to methyl orange end point(M) corresponds to total alkalinity present in water sample.

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