USE OF METAL-ION INDICATORS TO DETERMINE COMPLEX STABILITY-CONSTANTS - THE METHOD OF COMPETITIVE EQUILIBRATION

The work presented illustrates the feasibility of using metal ion indi cators to study various aspects of complexation reactions. The overall aim of the work was to develop a simple method for studying metal-lig and complexation relevant to the environmental migration of radionucli des. The reaction of Xylenol Orange with zinc(II) ions is investigated as a simple model system, initially determining the variation of equi librium constant with ionic strength. The data are used to show the va lidity of the Davies equation to predict equilibrium constants in syst ems 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. Th e experimental data (giving K-0 = 218.8) show good agreement with lite rature values, thus illustrating the validity of this approach to meas ure complex stability constants in systems where other methods are dif ficult or even impossible.