UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper I
Q1(f). Colour change interval of diphenylamine
Question
Determine the colour change interval for the indicator, diphenylamine, at 298 K. [Given: E° for diphenylamine is +0.756 V]
Answer in brief
Colour change interval = 0.756 ± 0.030 V, i.e. 0.726 V to 0.786 V (colourless → violet).
Key points
- Nernst equation: E = E°_In − (0.0591/n) log([Inred]/[Inox]).
- Eye sees the change when the ratio moves from 10 to 1/10: E = E°_In ± 0.0591/n.
- Diphenylamine → diphenylbenzidine violet is a 2-electron change: 0.0591/2 = 0.0296 V.
- For Fe²⁺ by dichromate, H₃PO₄ is added to complex Fe³⁺ and lower the Fe potential, so the jump covers this interval.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversredox indicatordiphenylaminecolour change intervalNernst equationtransition potential
All parts of Question 1
- 1(a) Defect in silver chloride5 marks
- 1(b) Isomerism in [(NH₃)₅CoONO]Cl₂ and [(NH₃)₅CoNO₂]Cl₂5 marks
- 1(c) Trans effect: Niᴵᴵ, Ptᴵᴵ, Pdᴵᴵ5 marks
- 1(d) Phenolphthalein in dilute and in concentrated alkali5 marks
- 1(e) Solubility product of silver chromate5 marks
- 1(f) Colour change interval of diphenylamine5 marks
- 1(g) Eriochrome Black T and calcium5 marks
- 1(h) Tendency to form metal clusters5 marks
- 1(i) Complete the nuclear equations5 marks
- 1(j) First ionisation enthalpy of 5d elements5 marks