UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper I
Q1(g). Eriochrome Black T and calcium
Question
Why can Eriochrome Black T not be used to indicate the end point of a direct EDTA titration of calcium?
Answer in brief
The Ca–EBT complex is too weak: at pH 10 it dissociates well before the equivalence point, so the wine-red → blue change is early and gradual (no sharp end point).
Key points
- Conditional constant at pH 10: log K′CaIn ≈ 5.4 − 1.6 = 3.8. Colour changes at pCa ≈ 3.8 ± 1.
- At equivalence pCa ≈ 6.1 (steep part 4.5 to 8); the indicator turns blue in the flat region, before the jump.
- For Mg, log K′MgIn ≈ 5.4: transition lies inside the jump → sharp change.
- Remedy: add a little Mg–EDTA (substitution titration), or titrate Ca alone at pH 12 with murexide or Patton–Reeder.
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.
CoversEriochrome Black TEDTA titration of calciumconditional formation constantmetallochromic indicatorMg-EDTA substitution
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