UPSC Combined Geo-Scientist, Mains · 2020 · Mains Paper I
Q1(c). Boric acid titration
Question
How is boric acid titrated using NaOH solution?
Answer in brief
Add excess mannitol or glycerol to form a more acidic borate complex, then titrate with standard NaOH using a suitable endpoint indicator such as phenolphthalein.
Key points
- Boric acid is too weak for a sharp ordinary direct titration: B(OH)₃ + H₂O ⇌ B(OH)₄⁻ + H⁺.
- A polyol forms stable cyclic complexes with borate, shifting the overall equilibrium towards proton release and increasing the effective acidity.
- Use a neutralised polyol reagent, titrate to a faint persistent pink and correct any reagent blank.
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.
CoversBoric acid titrationAcid–base analysis
All parts of Question 1
- 1(a) Normal and inverse spinels5 marks
- 1(b) Trans effect5 marks
- 1(c) Boric acid titration5 marks
- 1(d) Aluminium oxinate5 marks
- 1(e) Iodometry and iodimetry5 marks
- 1(f) Magnesium-assisted EDTA endpoint5 marks
- 1(g) Triiron dodecacarbonyl5 marks
- 1(h) Masking and demasking5 marks
- 1(i) Iodine beta decay5 marks
- 1(j) Transition elements and CuCl5 marks