UPSC Combined Geo-Scientist, Mains · 2021 · Mains Paper I
Q1(d). Indicator choice for weak-acid titration
Question
Why is methyl orange not a suitable indicator for titration of a strong base against a weak acid?
Answer in brief
The equivalence point is alkaline, whereas methyl orange changes at approximately pH 3.1-4.4; its colour change does not locate the equivalence point.
Key points
- At equivalence the solution contains the salt of the weak acid: A⁻ + H₂O ⇌ HA + OH⁻.
- Hydrolysis makes the equivalence pH greater than 7 at 25 °C. The useful steep part of the titration curve lies mainly in the alkaline region.
- Methyl orange changes in the acidic region: if acid is added to the base it changes only after excess acid; in the reverse direction it changes too early.
- Phenolphthalein is usually suitable because its range, about pH 8.2-10, overlaps the sharp region. Extremely weak acids may require instrumental detection.
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.
Coversmethyl orangephenolphthaleinweak acid strong base titrationindicator rangesalt hydrolysis
All parts of Question 1
- 1(a) Quartz-type aluminophosphate5 marks
- 1(b) Fifth ammonia ligand on copper(II)5 marks
- 1(c) Why hydride and phosphine are strong-field ligands5 marks
- 1(d) Indicator choice for weak-acid titration5 marks
- 1(e) Postprecipitation versus coprecipitation5 marks
- 1(f) Role of mercuric chloride in iron titration5 marks
- 1(g) Chelators in complexometric titrations5 marks
- 1(h) Carbonyl C-O bond strengths5 marks
- 1(i) Balancing nuclear reactions5 marks
- 1(j) Orbital contribution to magnetic moments5 marks