299.

Describe Ostwald’s theory of acid-base indicators.
Or
How does Ostwald’s theory explain the colour change of:
(i) Phenolphthalein
(ii) Methyl orange in acid-base titrations?

(i) Ostwald’s theory: According to this theory, an acid-base indicator is either a weak organic acid or a weak organic base whose undissociated molecule has a. colour different from the ions furnished by it. These different coloured ions are produced in the solution under the influence of a strong acid or a strong base.
To understand this theory, consider the case of phenolphthalein which is written as HPh. It is a weak acid. Its undissociated molecules are colourless. When its molecules are dissociated, they give colourless H⁺ ions and deep pink coloured Ph⁺ ions.



When phenolphthalein is added to the acidic solution, the dissociation of phenolphthalein is practically nil due to increasing the concentration of hydrogen ions. It means that the solution remains colourless.

When strong alkali is added to the phenolphthalein, OH^– ions furnished by alkali combine with H⁺ ions furnished by phenolphthalein to form feebly ionised water and sodium salt NaPh. The NaPh remains in ionic state and imparts a pink colour to it. This is explained as follows:



Similarly, this theory also explains the action of methyl orange. It is a weak base and may be, represented as MeOH. Its undissociated molecules are yellow while its ions (Me⁺) are red. thus,



When a base is added to MeOH. the OH^– ions of alkali suppress the ionisation of MeOH so that the solution of MeOH is yellow in colour. When an acid is added to MeOH. the H⁺ ions furnished by acid combine with MeOH to form feebly ionised water along with the formation of MeCl. The McCl. is strongly ionised to give a large concentration of Me⁺ ions, thereby producing a pink-red colour. Thus, in the acid solution, methyl orange gives a pink-red colour.



Ostwald’s theory does not explain the cause of colour changes in the indicator.