Thermodynamics of coupled reactions by the analysis of chemical relaxation signals: the nickel(II)–murexide system

Abstract

A temperature jump study of the Ni^II–murexide system, has been performed in buffered aqueous solution. The aim of the work is to provide a simple method for the evaluation of thermodynamic parameters of the involved proton transfer and complex formation steps by an analysis of the amplitudes of the relaxation signals. This method is based on a description of the system in terms of three ‘normal modes of reaction’ and it is demonstrated here that the slowest of them coincides with the concept of ‘apparent reaction’ introduced by Schwarzenbach in his theory of complexometric titrations. In analogy with classical titrations the experiments have been carried out by changing the metal ion concentration at constant ligand concentration and pH. In this way the amplitude of each relaxation curve represents a point of a ‘dynamic titration’ and a procedure is formulated whereby the analysis of the data provides simultaneously the equilibrium constant and the enthalpy of the slow normal reaction from simple linear plots.

The equilibrium constants have been evaluated also by spectrophotometry and from the relaxation times. The results show that for the investigated system the accuracy of the dynamic method is comparable with that of conventional techniques. The circumstances where the dynamic titration becomes more informative than classical titration are discussed.