UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q3(c). The doctor test for petroleum distillates
Question
A sample of petroleum distillate during Doctor Test is shaken with a faint yellow colour solution X. No change in colour is observed. Then some sulphur is added and the solution turns dark brown. This happens when Y species is present in a petroleum distillate. What are X and Y?
Answer in brief
X=Na₂PbO₂ (sodium plumbite); Y=mercaptans (thiols), RSH.
Key points
- a few mL of sodium plumbite solution (PbO in NaOH) are shaken with the distillate. A black precipitate of PbS shows H₂S; here nothing happens.
- A pinch of sulphur is then added and shaken. Mercaptans form lead mercaptide, 2RSH + Na₂PbO₂ → Pb(SR)₂ + 2NaOH, which reacts with sulphur: Pb(SR)₂ + S → PbS ↓ + RSSR. A dark brown or black colour (PbS) means mercaptans present…
- Mercaptans give the bad smell and corrode copper; sweetening (Merox process) converts them to disulphides.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- Test: a few mL of sodium plumbite solution (PbO in NaOH) are shaken with the distillate. A black precipitate of PbS shows H₂S; here nothing happens.
- A pinch of sulphur is then added and shaken. Mercaptans form lead mercaptide, 2RSH + Na₂PbO₂ → Pb(SR)₂ + 2NaOH, which reacts with sulphur: Pb(SR)₂ + S → PbS ↓ + RSSR. A dark brown or black colour (PbS) means mercaptans present (“sour”). No change means “sweet”.
- Mercaptans give the bad smell and corrode copper; sweetening (Merox process) converts them to disulphides.
Coverssweetening testsThe doctor test for petroleum distillates
All parts of Question 3
- 3(a) Variance and F-test for two methods of Al in bauxite10 marks
- 3(b) Detection limit and unknown by AAS (Fe)10 marks
- 3(c) The doctor test for petroleum distillates5 marks
- 3(d) Resolution of two chromatographic peaks5 marks