Q1(d). Cold-vapour atomisation for mercury

Answer in brief

Cold-vapour technique: Hg²⁺ is reduced to Hg⁰ by SnCl₂, swept by a gas stream into a quartz-windowed cell at room temperature, and its absorbance at 253.7 nm is measured.

Key points

• Mercury is the only metal with an appreciable pressure of monatomic vapour at room temperature, so no flame or furnace is needed.
• Sample is digested with HNO₃–H₂SO₄ and KMnO₄ to oxidise organo-mercury to Hg²⁺; excess oxidant is destroyed with hydroxylamine hydrochloride.
• Reduction with SnCl₂ in acid: Hg²⁺ + Sn²⁺ → Hg⁰ + Sn⁴⁺; NaBH₄ reduces organically bound Hg as well.
• Ar, N₂ or air sweeps Hg vapour through a drying tube into a long-path quartz-windowed cell; absorbance of the 253.7 nm line is measured.
• Detection limit about 1 ng; matrix isolation, as the analyte leaves the matrix as vapour.