Chemistry · Redox Equilibrium · NEET
Because KMnO4 has an intense purple colour of its own and its reduced product Mn2+ is nearly colourless. As long as a reducing agent (like oxalic acid or Fe2+) is present, every drop of purple MnO4- is instantly decolourised. The moment the reducing agent is finished, the next drop of purple KMnO4 stays unreacted and gives a permanent light pink colour. This colour change comes from the reactant itself, so no external indicator is needed.
KMnO4 is usually taken in the burette. Inside the flask the solution stays colourless while the reducing agent is being oxidised. At the endpoint, one extra drop of KMnO4 makes the whole solution turn a faint but permanent pink (light pink). The endpoint is the first appearance of a pink tint that does not fade on swirling.
KMnO4 is normally kept in the burette because it is the coloured titrant, and the endpoint is read as the first permanent pink in the flask. If KMnO4 were in the flask, the solution would start purple and end colourless (purple to colourless), which is harder to judge, so the burette arrangement is preferred in NEET-level questions.
In acidic medium MnO4- gains 5 electrons and is reduced fully to colourless Mn2+ (Mn goes +7 to +2). This gives a sharp colourless-to-pink change. In neutral or basic medium MnO4- is reduced only to brown MnO2 (Mn +7 to +4), which makes the solution muddy brown and hides the endpoint. So dilute sulphuric acid is used, not HCl (HCl gets oxidised) and not HNO3 (it is itself an oxidiser).
At the endpoint only one extra drop of KMnO4 is present in excess, so its concentration is tiny and gives only a faint pink. If you keep adding KMnO4, the colour deepens to purple, but that is past the endpoint and would give a wrong (higher) reading. The correct endpoint is the first lasting light pink.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A self-indicator is a reagent that signals the endpoint of a titration by a change in its own colour, so no separate indicator chemical is needed. KMnO4 is the standard example in NEET redox titrations.
Manganese changes from +7 in MnO4- to +2 in Mn2+, a gain of 5 electrons per manganese. This 5-electron change is why the n-factor of KMnO4 in acidic medium is 5.
Dilute H2SO4 provides the H+ needed to reduce MnO4- to Mn2+ without being oxidised. HCl is avoided because KMnO4 can oxidise Cl- to Cl2, consuming extra KMnO4 and giving a wrong result.
The faint pink is permanent for the purpose of the titration - it should not fade on swirling for about 15-30 seconds. Over long standing it may slowly fade due to reaction with air or dust, but at the endpoint moment it marks a reliable stop point.
