Various dyestuffs which are sensitive to thallium, such as rhodamine Br--3, methyl violet*, ethyl violet6 and brilliant geen 6, have been suggested for the determination of tl~allium(III)~ These methods are based on the formation of acoloured ionassociation complex between a halide compfex anion of tha~ium{ III) and a large cationit dyestuff. Chelate cations such as the 2,2'-bipyridyl-iron(I1) chelate? and the I,IOphenanthroline-iron(H) &elate cation 8.0, have been proposed for the spectrophotometric determination of perchlorate, in place of cationic dyestuffs such as methyiene bluer* and crystal violet". Moreover, it has been shown12 that 2,2'-bip~idyl-jron(I1) &elate cations can be extracted with halide complex anions of mercury(H) into x,2dichloroethane. The absorbance of the organic layer showed a linear relationship to the quantity of mercury(H) initially present in the aqueous solution.
In the present paper, a similar method is suggested for the determination of th~lium(II1) by extracting it into x,z-dichloroethane with the 2,2'-bipyridyl-iron(I1) chelate as a counter ion, from aqueous solution in the presence of a large excess of bromide. The proposed method is convenient and accurate for the determination of thallium in the range 7.9' IO -0-3.5 l 10-6 M. The species predominating in the extraction was also ascertained. EXPERIMENTAL The spectrophotometric measurements were made with a Hitachi Model EPU-2A spectrophotometer with xo-mm glass cells. The shaking was done with an Iwaki Model KM shaker with a time switch, The PH measurements were made with a Hitachi-Horiba M 3 PH meter. Reagents Thatlizrm sotzdion. A standard solution containing I rng of thallium per ml was prepared by dissolving thallium(I) nitrate in distilled water-z. This standard solution was then diluted to the concentration required for the experiments. 2,2'-Bipy)Lidyd_iro~~(~~) chdute solution. Iron(I1) ammonium sulfate and 2,2'bipyridyl solutions were prepared as described previously7. The two solutions were Anal. Chim. Ada, 42 (x968) 225-231: 226 K, KOTSUJI,