5-Amino-2-benzimidazolethiol as an analytical reagent—II : Spectrophotometric determination of rhodium and the simultaneous determination of rhodium and palladium

Abstract

5-Amino-2-benzimidazolethiol has been found to be a highly sensitive reagent for the colorimetric determination of rhodium III. The orange-yellow complex of rhodium III with the reagent, developed by heating in the pH range 2–5, shows maximum absorption at 370 mμ; it is practicable to use wavelength 390 mμ, where the reagent absorption is negligible. The colour system obeys Beer's law from 0.5 to 4.0 ppm of rhodium III, the optimum concentration range being 1.0–3.5 ppm where the percentage relative error per 1% absolute photometric error is 2.88. Sandell's spectrophotometric sensitivity of the reaction is 0.004 μg cm 2 . Most of the common ions and rare metal ions, including uranium VI, are without effect on the complex. Palladium II interferes, producing a colour, but a simultaneous colorimetric method for determining rhodium III and palladium II in a mixture has been developed by determining rhodium and palladium together in hot solution and then subtracting the palladium determined in the cold. Interferences from ruthenium III and larger amounts of iridium IV are avoided by using EDTA (disodium salt) as a masking agent. The continuous variation and the molar ratio methods, applied at pH 3.42, indicate that rhodium III forms with the reagent both 1:2 and 1:3 complexes, their dissociation constants being 2.7 × 10 −12 and 6 × 10 −17, respectively.