Conditions have been established for the determination of Thallium(III) (Tl) and Indium(III) (In) individually and in mixtures. In the presence of sufficient chloride ions, Tl(III) forms the tetrachlorothallate(III), TlCl4 –, which forms a stable ion-pair association with the cationic part of methylene blue. Tl(III) is determined alone by direct titration with Ethylenediaminetetraacetic Acid (EDTA) using methylene blue as an ion-pair indicator. In(III) is titrated with EDTA in the presence of a known amount of Tl(III) and sufficient chloride with methylene blue as an ion-pair indicator. Moreover, direct titration of a mixture containing the two metal ions with EDTA results in successive complexation of the ions, first with In(III) and then with Tl(III). The equivalence point is reached in all cases when the pink color of the ion-pair association turns to blue. The titer value for the mixture corresponds to the total of In(III) and Tl(III) and a blank correction corresponding to Tl(III) is applied. In the analysis of the mixtures, the total amount of In(III) and Tl(III) is estimated complexometrically, while the Tl(III) alone is estimated bromatometrically after reducing to Tl(I). The amount of In(III) is calculated by difference.

A redox method has been developed by Sagi and Prasada Rao (1979) for the determination of Thallium (Tl)(III) by titration with Tin (Sn)(II) sulphate using methylene blue as ionpair indicator. This method gives good results, but is relatively cumbersome, time consuming and needs inert atmosphere. The separation of Gallium (Ga), Indium (In) and Tl by extraction with n-octylaniline in chloroform was reported (Shashikant et al., 1988). Determination of Ga, In and Tl complexometrically using Ethylenediaminetetraacetic acid (EDTA) and semimethylthymol blue (Medhat et al., 1994), and the determination of In and Tl in the alloys of these two metals were also reported (Melvin et al., 1971). The extraction-spectrophotometric method was developed for the determination of Tl in high-purity In (Marczenko et al., 1974) and complexometric determination of Tl(III) using ethanethiol as a selective masking agent (Karthikeyan et al., 2006). The determination of In(III) in 1, 2, 5, 8-tetrahydroxyanthraquinone-In(III) complex was reported by Fernandez-Gutierrez et al. (1985). Determination of In and Tl in foods was also reported by Evans et al. (1983). In view of the fact that Tl(III) forms a strong complex with EDTA (Vogel, 1961), a method can be developed to estimate Tl(III) complexometrically using the same ion-pair indicator. Similarly, based on the difference in the stabilities of the EDTA complexes of Tl(III) and In(III), in which the later forms a stronger complex (Vogel, 1961), the present authors envisage the possibility of using ion-pair indicator for complexometric estimation of In(III) with EDTA in the presence of Tl(III). The method can also be extended for the estimation of In(III) and Tl(III) in mixtures.

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