The work presented illustrates the feasibility of using metal ion indicators to study various aspects of complexation reactions. The overall aim of the work was to develop a simple method for studying metal–ligand complexation relevant to the environmental migration of radionuclides. The reaction of Xylenol Orange with zinc(II) ions is investigated as a simple model system, initially determining the variation of equilibrium constant with ionic strength. The data are used to show the validity of the Davies equation to predict equilibrium constants in systems where I<0.1 M. By the introduction of a second ligand (sulfate) to the zinc–Xylenol Orange mixture, competitive equilibration is used to determine the equilibrium constant of the zinc–sulfate reaction. The experimental data (giving K0=218.8) show good agreement with literature values, thus illustrating the validity of this approach to measure complex stability constants in systems where other methods are difficult or even impossible.
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