Name: pH Scale
Uploaded: 2019-07-19
Duration: 9 min 45 s
Description: This video discusses the pH of acids, bases, and salts and also the limitations of using a pH scale for identification. The natural indicators do the verific...

Question

Describe Ostwald’s theory of Acid-Base indicators.

Accepted Answer

According to Ostwald's theory:
(a) Acid-base indicators are mostly complex organic molecules which are either weak acids or weak bases. For example, phenolphthalein is a weak organic acid which may be represented in a simple way as $HPh$ and methyl orange is a weak organic base which may be represented as $MeOH$ .

(b) They dissociate in aqueous solution as follows:

$\underset{Colourless}{HPh} ⇌ \underset{Colourless}{H^{+}} + \underset{Pink}{{Ph}^{-}} . . . . . . . (i) \underset{Yellow}{MeOH} ⇌ \underset{Red}{{Me}^{+}} + \underset{Colourless}{{OH}^{-}} . . . . . . . (ii)$

The unionised form has one colour and the ions have different colour. For example, $HPh$ is colourless while $Ph$ ions have pink colour. Similarly, $MeOH$ has yellow colour while $M e^{+}$ ions have red colour.

(c) As the medium changes from acidic to basic or vice versa, the equilibrium shifts either towards right or towards left and so the colour changes, For example, in acidic medium (i.e., in presence of excess of $H^{+}$ ions), equilibrium $(i)$ shifts towards left and the solution is colourless. While in basic medium, $O H^{-}$ ions will combine with the $H^{+}$ ions to from unionised $H_{2} O$ molecules, hence, the equilibrium shifts towards right and the solution has pink colour.
Similarly, in case of methyl orange, in basic medium (i.e., presence of excess $O H^{-}$ ions) the equilibrium $(ii)$ shifts towards left and the colour is yellow while in acidic medium, $H^{+}$ ions combine with the $O H^{-}$ ions and the equilibrium $(ii)$ shifts towards right and the colour is red.

Describe Ostwald’s theory of Acid-Base indicators.

Important Questions on Ionic Equilibrium in Solution

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