. Explain Ostwald Theory of acid-base indicator

Best Answer

According to Ostwald Theory of acid-base indicator, an acid-base indicator is a weak organic acid or weak organic base which remains partially dissociated/protonated in aqueous solution and of which, ionised and unionised forms possess different colour. If HIn be an indicator (a weak acid) then its dissociation may be represented as



Upon decreasing the pH of solution containing the indicator, the dissociation equilibrium shifts towards left and the ratio   decreases and at certain ratio, the colour A(colour of the acidic form of the indicator dominates over the colour B (colour of the basic form of the indicator). Contrary to this at certain high pH the ratio   becomes high enough to push colour B to dominate over colour A. In between the two extreme ratios the indicator displays intermediate colour. If the minimum pH = x at which colour A predominates over colour B and the maximum pH = y at which colour B predominates over colour A, then x – y is known as pH range of the indicator.

During acid-base titration, the pH of acid solution increases very slowly in the beginning upon addition of alkali but as soon as and point is reached there is sudden jump in the pH value. Thus, a plot of pH of acid solution vs volume of NaOH solution consists of a vertical line corresponding to the equivalence (end) point as shown below.


In a titration between a strong acid with a strong base, the vertical line extends from pH = 4 to pH = 10. While due to salt hydrolysis, this line extends from pH = 4 to pH = 6 in the case of titration of a strong acid with a weak base and from pH = 8 to pH = 10 in the case of a weak acid with a strong base.

An indicator to be suitable, its pH range must lie in the pH jump range occurring at the end point and at least higher side of the pH-range must lie on the vertical line. 


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