A Kinetic Study of the Ligand Substitution Reaction of Tetramethylenediaminetetraacetatocobaltate(II) with Trimethylenediaminetetraacetate

The kinetics of the ligand substitution reaction of tetramethylenediaminetetraacetatocobaltate(II)(cobalt(II)–TDTA) with trimethylenediaminetetraacetate (TRDTA) was investigated by two different polarographic methods. The kinetic data at pH 4.9 to 5.6 were obtained by means of a conventional method in which the reaction was initiated by the mixing of two reactant solutions and was followed polarographically. However, this method could not be applied to solutions with pH values lower than 4.9, for the rate of the reaction is immeasurably fast in such a low pH region. The kinetic data at pH values down to 3.7 were obtained by analyses of the polarographic current which was observed in solutions containing TRDTA and the substitution-inert cobalt(III)–TDTA complex. From these data, it was found that the reaction proceeds through four simultaneous reaction paths, two paths of which are the nucleophilic substitution reactions, while the others are the dissociation reactions of cobalt(II)–TDTA complex ions.

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