Theories of acid base indicators were given by Ostwald and Quinonoid separately. Acid-base indicators are those organic dyes that help to detect the endpoint of an acid-base reaction by changing their colors. These indicators exhibit one color in acidic solution and different colors in basic solution. Therefore, an equivalent point is detected due to a change in the color of the indicator. Examples of acid-base indicators are Phenolphthalein, Methyl orange, etc.
There are two famous theories to explain how acid base indicator works in acid base titration.
- Ostwald’s Theory of acid base indicator
- Quinoid Theory of acid base indicator
Ostwald theory of acid base indicator
According to Ostwald’s theory, acid-base indicators are weak organic acid (HIn) or weak organic base (InOH). When these indicators undergo ionization, the ionized form In- and unionized from HIn has different colors in the aqueous solution. Moreover, the greater the extent of ionization, the greater will be the visibility of the color.
Let us take the example of Methyl orange and explain its mechanism of action according to Ostwald’s theory. As stated above, Methyl orange is a weak acid that undergoes ionization in an aqueous solution as shown below:
Applying law of mass action,
where, Kin=dissociation constant of indicator, called as indicator constant.
The unionized form of methyl orange is red in color and the ionized form is yellow in color. When acid is added to the solution, the concentration of H+ ion increases. In order to keep the value of the indicator constant (Kin), the equilibrium shift in the backward direction which means the concentration of unionized form (HIn) increases and exhibit red color in the solution.
But, when a base is added to the solution, H+ ions available in the solution combine with OH– from base to water. Therefore, equilibrium is shifted toward the forward direction which means the concentration of In- in the solution increases and hence exhibits yellow color.
Similarly, the mechanism of action of Phenolphthein can be explained.
The unionized form Phenolphthein exhibit no color but an ionized form of it exhibits pink color in the solution.
Quinonoid theory of acid base indicator
Ostwald’s theory has explained the color change mechanism of the indicator but Quinonoid theory has tried to explain the cause of color change of the indicator. This theory is also known as the benzenoid quinonoid theory.
According to Quinoid Theory, the indicator’s unionized form HIn and ionized form In– are tautomeric forms. Among these, one tautomeric form has a quinoid structural unit, called Quinonoid form, and another tautomeric form has benzene rings called Benzenoid form. Moreover, the Quinonoid form exhibit deep color, and the benzenoid form exhibit no color or very light color. Therefore, when one tautomeric form changes from one to another form.
Let’s take the example of Methyl orange.
In the case of Phenolphthalein,
- Arun Bahl, B. S. Bahl & G. D. Tuli, Essentials of Physical Chemistry, S. Chand and CompanyLtd., New Delhi, 2012
- M. L. Sharma & P. N. Chaudhary, A Textbook of B. Sc. Chemistry (Volume II), 2nd Edition, Ekta Books Nepal, 2007