Titrimetric determination of microgram amounts of lead by thiocyanate amplification after precipitation as lead hexathiocyanatochromate(III)

Lead (25–200 μg) is precipitated as Pb₃[Cr(SCN)₆]₂. The filtered precipitate is treated with 10% carbonate solution, and the thiocyanate dissolved is oxidized by iodine to sulphate at pH 8.2. After acidification, the excess of iodine is extracted into chloroform, and the iodide ion retained in the aqueous solution is amplified by bromine oxidation and subsequent treatment with more iodide. The method provides 152 iodine atoms for each original lead ion. Only Bi³⁺ and Cu²⁺ interfere seriously.