Why does colour of \( KMnO_4 \) disappear when oxalic acid is added to its solution in acidic medium?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \({MnO_4}^-\) is reduced to \(Mn^{2+}\) which is colourless
The deep purple colour of \( \ce{KMnO4} \) is due to the \( \ce{MnO4^-} \) ion (Mn in the +7 state). In acidic medium permanganate is a strong oxidising agent and is reduced to Mn2+, which is almost colourless (very pale pink) in dilute solution. Oxalic acid is oxidised to carbon dioxide.
The two half-reactions:
\( \displaystyle \ce{MnO4^- + 8H+ + 5e- -> Mn^2+ + 4H2O} \)
\( \displaystyle \ce{C2O4^2- -> 2CO2 + 2e-} \)
Multiplying the first by 2 and the second by 5 balances the electrons: 10 are given and 10 are taken.
As long as oxalic acid is present, each drop of permanganate is reduced to \( \ce{Mn^{2+}} \) and its colour disappears. So the colour disappears because \( \ce{MnO4^-} \) is reduced to colourless \( \ce{Mn^2+} \).
Why the other options are incorrect
Option (1) \({MnO_4}^-\) is reduced to \(MnO_2\) which is colourless — Incorrect. \( \ce{MnO2} \) is formed in neutral or faintly alkaline solution, and it is a brown-black solid, not colourless.
Option (2) \({MnO_4}^-\) is reduced to \(Mn_2O_4\) which is colourless — Incorrect. There is no such product of permanganate reduction.
Option (3) \({MnO_4}^-\) is oxidized to \({MnO_4}^{2-}\) which is colourless — Incorrect. Going from \( \ce{MnO4^-} \) (+7) to \( \ce{MnO4^2-} \) (+6) is a reduction, and manganate is green; it is formed in strongly alkaline solution.
NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – “If we represent the reduction of permanganate to manganate, manganese dioxide and manganese(II) salt by half-reactions,
MnO4– + e– → MnO42– (Eo = + 0.56 V)
MnO4– + 4H+ + 3e– → MnO2 + 2H2O (Eo = + 1.69 V)
MnO4– + 8H+ + 5e– → Mn2+ + 4H2O (Eo = + 1.52 V)
We can very well see that the hydrogen ion concentration of the solution plays an important part in influencing the reaction. … A few important oxidising reactions of KMnO4 are given below:”
Oxides and Oxoanions of Metals – …
