UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper I
Q4(c). Working principle of redox indicators
Question
Explain the working principle of redox indicators with example.
Answer in brief
A redox indicator is a reversible redox couple whose oxidised form (Inₒₓ) and reduced form (In_(red)) have different colours. The colour changes when the solution potential passes through the indicator's own formal potential E^°_(In), which is made to coincide with the sharp potential jump at the equivalence point.
Key points
- Inₒₓ + ne⁻ ⇌ In_(red) (colour 1 ⇌ colour 2); E_(system)=E_(In) at every moment, so the ratio [Inₒₓ]/[In_(red)] is set by the titration potential.
- the eye detects colour 1 when the ratio is ≥10 and colour 2 when ≤0.1: E_(change)=E^°_(In)±(0.0591)/(n) (≈±0.03 V for n=2)
- the indicator range must lie within the steep jump of the titration curve. For Fe²⁺ with Ce⁴⁺ the jump (0.94–1.30 V) takes in ferroin (1.06 V).
- the colour change is sharp if E^°_(In) lies near the equivalence potential and the reaction of the indicator is fast and reversible.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- Equilibrium: Inₒₓ + ne⁻ ⇌ In_(red) (colour 1 ⇌ colour 2); E_(system)=E_(In) at every moment, so the ratio [Inₒₓ]/[In_(red)] is set by the titration potential.
- Visible change: the eye detects colour 1 when the ratio is ≥10 and colour 2 when ≤0.1: E_(change)=E^°_(In)±(0.0591)/(n) (≈±0.03 V for n=2)
- Choice: the indicator range must lie within the steep jump of the titration curve. For Fe²⁺ with Ce⁴⁺ the jump (0.94–1.30 V) takes in ferroin (1.06 V).
- Sharpness: the colour change is sharp if E^°_(In) lies near the equivalence potential and the reaction of the indicator is fast and reversible.
| Indicator | E^°_(In) (V) | Colour change | Used in |
| methylene blue | 0.53 | blue (ox) → colourless | Fe²⁺/Ce⁴⁺ |
| diphenylamine | 0.76 | colourless → violet (diphenylbenzidine violet) | Fe²⁺ with Cr₂O₇²⁻ (with H₃PO₄) |
| ferroin | 1.06 | red [Fe(phen)₃]²⁺ → pale blue [Fe(phen)₃]³⁺ | Ce⁴⁺, Cr₂O₇²⁻ |
KMnO₄ is a self-indicator (not a redox indicator) because MnO₄⁻ itself is purple; starch is a specific indicator for I₂: its blue I₃⁻–starch complex is the visible change.
CoversindicatorsWorking principle of redox indicators
All parts of Question 4
- 4(a) Indicators for 0.1 M ethylenediamine with HCl10 marks
- 4(b) Homogeneous precipitation10 marks
- 4(c) Working principle of redox indicators10 marks