Complexometric determination of mercury(II) and thallium(III) using cysteamine hydrochloride as releasing agent

A simple complexometric method is described for the determination of mercur y(II) and thallium(III) using cysteamine hydrochloride as a selective relea sing agent. Hg-II or Tl-II solution with diverse ions are initially complex ed with a known excess of EDTA, the excess EDTA is then back-titrated with lead nitrate solution at pH 5.0-6.0 (hexamine) using xylenol orange as indi cator. A slight excess of the releasing agent is then added and the EDTA re leased from the metal-EDTA complex is quantitatively determined as above. B oth the determinations work well at low and high concentrations of metal io ns (4-125 mg of Hg-II and 5-108 mg Tl-III) with a permissible relative erro r of +/-0.4% and standard deviation less than or equal to 0.04 mg. The vali dity of the method is confirmed by extending it to the determination of Hg- II and Tl-III in several synthetic binary mixtures.