Question

The colour change is due to ionisation of the acid base indicator.
A.Ostwald theory
B.chromophore theory
C.Quinonoid theory
D. Resonance theory
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Answer 1

(C) Quinonoid Theory is your right answer .

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Answer 2

(A) is the correct answer

A substance that determines the endpoint in a titration is called an indicator. Ostwald's theory states the following:

1. Indicators change their color within the range of the pH.
2. The unionized form has a different color than the ionized form and hence the change in color is due to the ionization of the indicator.
3. The ionization of these indicators largely depends on whether it is a weak acid or a weak base. The ionization of weak bases is low in alkalies and high in acids. Similarly, the ionization of weak acids is low in acids and fairly high in bases.
4. This happens due to the common $H^{+}$ and $OH^{-}$ ions.