Inorganic Chemistry Laboratory, University College of Science, Calcutta-700009. Manuscript received 22 August 1975 ; accepted 22 October 1975. The synthesis of 2-2' (benzenesulphonamido)-ethylene dibenzimidazole (RbeβbH) has been reported and the uv and ir spectral data are recorded. Preparation of 2-2' (\(\alpha\)-benzenesulphonamido)- trimethylene dibenzimidazole (Rb\(p\gamma\)bH) has also been described. The stepwise acid dissociation constants of the ions Rb\(p\gamma\)bH32+ and

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... bH32+ in (1;1) water-dioxan at constant ionic strength (µ= 0.5) at 30±0.5° have been determined by potentiometric titration. The pk3*, pk2 and pk1* values are computed by graphical methods and are found to be 2.77, 4.89, 10.43 and 3.51, 5.59, 10.74 respectively.