Statement I: Permanganate titrations in the presence of hydrochloric acid are satisfactory.
Statement II: In a neutral or faintly alkaline solution, permanganate oxidises thiosulphate almost quantitatively to sulphate.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Only Statement II is correct.
Potassium permanganate is a strong oxidising agent. What it oxidises, and what it is itself reduced to, depends on the medium in which the reaction is carried out.
Statement I: Permanganate titrations in the presence of hydrochloric acid are satisfactory. — False. Permanganate also oxidises hydrochloric acid to chlorine. Part of the permanganate is then used up by the acid and not by the substance being titrated, so such titrations are unsatisfactory.
Statement II: In a neutral or faintly alkaline solution, permanganate oxidises thiosulphate almost quantitatively to sulphate. — True. In this medium permanganate is reduced to MnO2, and sulphur of thiosulphate (+2) is oxidised to sulphate (+6):
\( \displaystyle \ce{8MnO4^- + 3S2O3^2- + H2O -> 8MnO2 + 6SO4^2- + 2OH^-} \)
NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “2. In neutral or faintly alkaline solutions:” … “(b) Thiosulphate is oxidised almost quantitatively to sulphate: 8MnO4– + 3S2O32– + H2O → 8MnO2 + 6SO42– + 2OH–”
“Note: Permanganate titrations in presence of hydrochloric acid are unsatisfactory since hydrochloric acid is oxidised to chlorine.”