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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 · 2021 · Mains Paper III

Q1(d). Gravimetric estimation of zinc in brass

Question

How will you estimate Zn in brass gravimetrically?

Answer in brief

After tin, lead, copper and iron are removed, zinc is precipitated from a neutral solution as zinc ammonium phosphate, ZnNH₄PO₄, weighed as such or, after ignition, as Zn₂P₂O₇.

Key points

  • Dissolve about 0.5 g in 1 : 1 HNO₃: tin separates as hydrated SnO₂; fume with H₂SO₄: lead separates as PbSO₄.
  • Copper removed electrolytically on a platinum cathode; iron removed as Fe(OH)₃ with NH₄Cl + NH₃.
  • Make the filtrate just neutral (pH 6–7), add excess (NH₄)₂HPO₄ near boiling: white ZnNH₄PO₄.
  • Dry at about 105 °C and weigh as ZnNH₄PO₄ (factor 0.3665), or ignite at about 900 °C to Zn₂P₂O₇ (factor 0.4291).

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(d), 2021 Mains Paper III: solution pages 5 and 6
Solution pages 5 and 6 of 39
Read the whole 2021 solution2021 question paper

Topic notes: analysis of ores, alloys, petroleum, coal and coke (dolomite, bauxite, brass, Ni in steel, aniline point, proximate and ultimate analysis, calorific value), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversgravimetric estimation of zincbrass analysiszinc ammonium phosphatezinc pyrophosphategravimetric factor

All parts of Question 1