Permanganate titrations in presence of hydrochloric acid are unsatisfactory as:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Hydrochloric acid is oxidized to chlorine
Potassium permanganate is used as an oxidising agent in titrations in acidic medium, where \( \ce{MnO4^{-}} \) is reduced to \( \ce{Mn^{2+}} \). The acid chosen must not itself be oxidised by permanganate. Dilute sulphuric acid is used for this reason.
Hydrochloric acid is not suitable, because the \( \ce{Cl^{-}} \) ion is a reducing agent and is oxidised to chlorine by acidified permanganate:
\( \displaystyle \ce{2MnO4- + 10Cl- + 16H+ -> 2Mn^2+ + 5Cl2 + 8H2O} \)
Part of the permanganate is then used up by HCl instead of by the substance being titrated, so the titre value is wrong.
Why the other options are incorrect
Option (1) A black ppt of MnO2 is formed — Incorrect. \( \ce{MnO2} \) forms when permanganate is reduced in neutral or weakly alkaline solution; in HCl, \( \ce{Mn^{2+}} \) is formed.
Option (2) Hydrochloric acid does not provide required acidic medium — Incorrect. HCl is a strong acid and supplies plenty of \( \ce{H^{+}} \); acidity is not the problem.
Option (4) Hydrochloric acid is oxidized to hydrogen — Incorrect. Hydrogen would come from the reduction of \( \ce{H^{+}} \), not from oxidation; an oxidising agent like \( \ce{MnO4^{-}} \) cannot produce \( \ce{H2} \).
NCERT (2026-27 edition):
Oxides and Oxoanions of Metals – “… Note: Permanganate titrations in presence of hydrochloric acid are unsatisfactory since hydrochloric acid is oxidised to chlorine.”