Petroleum products sweetening: The traditional "Doctor's solution" used for sweetening sour petroleum fractions is chemically which compound?

Introduction / Context:
Many straight-run and cracked petroleum fractions contain mercaptans and other sulphur species. Sweetening converts objectionable sulphur compounds into less odorous, less corrosive, or less reactive forms. The historical “Doctor test” and “Doctor sweetening” are classic quality checks/processes in petroleum refining.

Given Data / Assumptions:

• Question asks the composition of the traditional Doctor’s solution.
• Focus is on the reagent used in mercaptan sweetening tests/processes.
• Answer options list various sodium and lead compounds.

Concept / Approach:
The classical Doctor process employs a solution of sodium plumbite (Na2PbO2) in caustic. In the presence of elemental sulphur, mercaptans are oxidized to disulphides while lead sulphide forms. The associated Doctor test checks whether a petroleum sample turns “sweet” (passes) under set conditions.

Step-by-Step Solution:

Verification / Alternative check:
Standard petroleum textbooks and test methods repeatedly associate the Doctor test and process with sodium plumbite as the specific reagent in an alkaline medium.

Why Other Options Are Wrong:

• Sodium thiosulphate: Used as a reducing/complexing agent in various chemistries, not the Doctor reagent.
• Sodium sulphite: A different sulphur oxy-salt; not the historical Doctor reagent.
• Lead sulphate: A product or precipitate type in some lead reactions, not the reagent solution itself.

Common Pitfalls:
Confusing the “test” name with end products or with other sulphur treatment chemistries (e.g., caustic wash alone, copper strip corrosion test).

Final Answer:
Sodium plumbite