5-amino-2-benzimidazolethiol as an analytical reagent—I: Spectrophotometric determination of palladium

Abstract

The orange coloured complex formed by palladium II with 5-amino-2-benzimidazolethiol in the pH range 2–4 forms the basis of a very sensitive colorimetric method for the determination of trace amounts of the metal. The complex shows maximum absorption at 390 m sm , where the reagent has negligible absorption, and it obeys Beer's law from 1 to 8 ppm of palladium II, the optimum range being 2–7 ppm where the percentage relative error per 1% absolute photometric error is 2.95. The sensitivity of the reaction is 0.008 μg cm 2 . Most of the common cations and anions are without effect on the complex. The interferences of some of the platinum metals are avoided by developing the complex in the cold and using EDTA (disodium salt) as a masking agent. A composition study by the molar ratio method shows that at pH 3.42 the solution contains both 1:1 and 1:2 complexes, the dissociation constant of the latter being 1.32 × 10 −11. However, above pH 5.5 incipient precipitation of the orange-red palladium complex occurs, and the precipitation is quantitative above pH 10. The analysis of this precipitate shows a ratio of 1:2 between the metal and the reagent.