The two theories have been proposed to explain change of colour of acid-base indicators with the change in pH.

(i) Ostwald's Theory  (ii) Quinonoid theory

(1) Selection of suitable indicator or choice of indicator : In order to choose a suitable indicator, it is important to understand the pH changes in the titrations. The change in the pH value in the vicinity of the equivalence point is most important for this purpose. The curve attained by plotting pH value as ordinate against the volume of alkali added as abscissa is known as neutralisation or titration curve. The appropriate indicators for the following titrations are,

(i) Strong acid Vs strong base : Phenolphthalein (pH range from 8.3 to 10.5), methyl red (pH range from 4.4 - 6.5) and methyl orange (pH range from 3.2 to 4.5).

(ii) The weak acid versus strong base : Phenolphthalein.

(iii) Strong acid versus weak base : Methyl red and methyl orange.

(iv) Weak acid versus weak base : No suitable indicator can be used for such a titration.

Reason for use of different indicators for different systems : Indicators are either weak acids or weak bases and when dissolved in water their dissociated form acquires a colour different from that of the undissociated form. Take a weak acid indicator of the general formula HIn, wherein represents indicator. The equilibrium in aqueous solution will be

Let  be the equilibrium constant

The human eye can detect change in colour if the ratio of two forms of indicator ranges from 0.1 to 10.

If,  , the colour visible is yellow

, the colour visible is red.

, the colour visible is green.

Or we can say that

The colour visible to us will be red, when the 

The colour visible to us will be yellow, when the 

The colour visible to us will be green, when the 

Therefore, our imaginary indicator will be red at any pH which just falls below   and green at any pH which just exceeds  . The indicator changes its colour in the narrow pH range to   from red to (yellow, yellow-green, red-yellow) green. We can hence use this indicator to locate this narrow pH range. In other words, in order to use the indicator effectively in this range, we should have a solution for which pH is very near to   of the indicator. The colour change of an indicator can, thus, be summarised in the following tabular form,

It is for this reason that we make use different indicators for different systems.

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