pH-static titrations for kinetic studies of metal-ligand complex formation: The case example of the reaction between Strontium(II) and DOTA

The theory of pH-static (or pH-stat) titrations in kinetic measurements of metal ion+ligand complex formation was resumed, and equations required to fit the experimental data were derived. This method was applied to study the kinetics of Sr2++DOTA reaction (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). pH-static titrations were performed at various pH (from 4.00 to 6.00) and temperatures (5, 15, and 25 °C) and at a constant ionic strength of (Na)Cl 0.15 mol/L. The complex formation involved either a rapidly formed intermediate species which then gave the final complex, or the direct metal+ligand reaction. The corresponding equilibrium constant β(os) and kinetic constants k(os) (first order) and k (second order) were obtained. Both k and k(os) increased by increasing pH and temperature. Kinetic data were recorded by pH-static titrations also for Ni2++DOTA reaction, and other kinetic results were obtained by using a more commonly employed method, proposed by Kasprzyk and Wilkins [S. P. Kasprzyk and R. G. Wilkins, Inorg. Chem., 1982, 21, 3349], which however appears to be theoretically incorrect. The comparison between the kinetic results for Sr2++DOTA and Ni2++DOTA reactions obtained by pH-static titrations, by the Kasprzyk/Wilkins method, and by the well-established spectrophotometric method (literature data), indicated that the Kasprzyk/Wilkins method should be used with caution as sometimes its systematic errors can be statistically relevant with respect to the typical precision of kinetic data