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

Thallium (5–200μg) is precipitated as Tl3Cr(SCN)6. The precipitate is collected, then treated with 10% sodium carbonate solution, and the thiocyanate dissolved is oxidized to sulphate by iodine at pH 8.2. After acidification, the excess of iodine is extracted into chloroform, and the iodide in the aqueous solution is determined by the Leipert amplification procedure. The method provides 76 iodine atoms for each original thallium(I) ion. Copper(II), mercury(II), bismuth, lead and silver interfere seriously, the first three causing negative errors and the other two positive errors.