VKC ScienceChemistry for written exams Telegram

UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2016 · Mains Paper III

Q1(e). Auxiliary complexing agent

Question

State the purpose of an auxiliary complexing agent and illustrate with an example.

Answer in brief

It keeps a metal soluble under titration conditions while allowing EDTA to displace the auxiliary ligand.

Key points

  • At alkaline pH some metals precipitate as hydroxides. A weaker soluble complex prevents this precipitation.
  • For example, ammonia holds zinc(II) in ammine complexes in an ammonia/ammonium buffer near pH 10.
  • EDTA forms the more stable zinc–EDTA complex and displaces ammonia during titration. The auxiliary ligand must not bind so strongly that it prevents quantitative reaction.

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.

Model answer to Q1(e), 2016 Mains Paper III: solution pages 3 and 4
Solution pages 3 and 4 of 22
Read the whole 2016 solution2016 question paper

CoversAuxiliary complexing agentComplexometry

All parts of Question 1