Question

Phenolphthalein and methyl orange are themselves:
A. Weak acid and weak base respectively
B. Both weak acids
C. Both weak bases
D. Weak base and weak acid respectively

Answer 1

Hint: In the quantitative analysis of the concentration of any substance, titration method is employed that uses a given known concentration of substance (titrant) with unknown substance. These are then titrated with a standard solution. Indicators are the substances that give the endpoint of titration. Their nature depends on their pH after dissociation.

Complete answer:
Titration is the technique used for the quantitative analysis of any solution. This determines the concentration of any substance in a solution. At the equivalence point of titration, where the reactants are fully utilized, then there is a color change that is indicated by an indicator.
Most common indicators are phenolphthalein and methyl orange indicators. These indicators work with the change in pH and are termed as acid – base indicators. Indicators are organic molecules of weak acids and bases. The dissociation of these indicators can tell their nature.
Phenolphthalein is dissociated to give hydronium ions at a pH from 8.3 to 10 as:
$H{{I}_{n}}+{{H}_{2}}O\rightleftharpoons {{I}_{n}}^{-}+{{H}_{3}}{{O}^{+}}$
Therefore, it is a weak acid.
Methyl orange dissociates to give hydroxide ions at a pH from 3.1 to 4.4 as:
${{I}_{n}}OH+{{H}_{2}}O\rightleftharpoons {{I}_{n}}O{{H}_{2}}^{+}+O{{H}^{-}}$
Therefore, it is a weak base.
Hence, phenolphthalein and methyl orange are themselves weak acids and weak base respectively.

So, option A is correct.

Note:
The dissociation of any compound can tell us it is acidic or basic. If it liberates hydrogen or hydronium ions then it is acidic, while the liberation of hydroxide ions tells that it is basic. The pH range can tell that it is a weak or a strong acid or base. The range is from 0 to 14, where lower ranges are strong acids and weak bases and higher ranges are weak acids and strong bases. pH is the negative logarithm of hydrogen ion concentration.