The chemical reaction involved in acid-base titration is known as neutralisation reaction. It involves the combination of H3O+ions with OH-ions to form water.In acid-base titrations, solutions of alkali are titrated against standard acid solutions. The estimation of an alkali solution using a standard acid solution is calledacidimetry. Similarly, the estimation of an acid solution using a standard alkali solution is calledalkalimetry.

a)Ostwald, developed a theory of acid base indicators which gives an explanation for the colour change with change in pH. According to this theory, a hydrogen ion indicator is a weak organic acid or base. The undissociated molecule will have one colour and the ion formed by its dissociation will have a different colour.

Let the indicator be a weak organic acid of formulae HIn. It has dissociated into H+and In-. The unionized molecule has one colour, say colour , while the ion, In-has a different colour, say colour. Since HIn and In-have different colours, the actual colour of the indicator will dependent upon the hydrogen ion concentration [H+]. When the solution is acidic, that is the H+ions present in excess, the indicator will show predominantly colour. On other hand, when the solution is alkaline, that is, when OH-ions present in excess, the H+ions furnished by the indicator will be taken out to form undissociated water. Therefore there will be larger concentration of the ions, In-. thus the indicator will show predominantly colour .

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