UPSC Combined Geo-Scientist, Mains · 2018 · Mains Paper III
Q1(b). Medium for iodimetry
Question
Why are iodimetric titrations usually performed in neutral solutions?
Answer in brief
Neutral or mildly acidic conditions preserve iodine as the intended oxidant and avoid alkaline side reactions.
Key points
- Direct iodimetry titrates a reductant with standard iodine, normally dissolved in excess iodide as triiodide.
- In alkaline solution iodine disproportionates: I₂ + 2OH⁻ → I⁻ + OI⁻ + H₂O.
- Strongly acidic conditions may suppress reaction of some reducing analytes or promote other interferences.
- The optimum pH is analyte-dependent, so neutral conditions are a common practice rather than an absolute rule.
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.
CoversMedium for iodimetryRedox titrimetry
All parts of Question 1
- 1(a) Carbonate titration endpoint5 marks
- 1(b) Medium for iodimetry5 marks
- 1(c) Magnesium and the EBT endpoint5 marks
- 1(d) Source modulation in AAS5 marks
- 1(e) Chromatographic zone broadening5 marks
- 1(f) ICP multielement capability5 marks
- 1(g) Alkaline glass-electrode error5 marks
- 1(h) Electron-capture selectivity5 marks