Potentiometric titration of metal ions in ethanol

The potentiometric titrations of Zn2+, Cu2+ and 12 Ln(3+) metal ions were obtained in ethanol to determine the titration constants ( defined as the (s)(s)pH at which the [-OEt]/[Mx+](t) ratios are 0.5, 1.5, and 2.5) and in two cases (La3+ and Zn2+) a complete speciation diagram. Several simple monobasic acids and aminium ions were also titrated to test the validity of experimental titration measurements and to establish new (s)(s)pKa constants in this medium that will be useful for the preparation of buffers and standard solutions. The dependence of the titration constants on the concentration and type of metal ion and specific counterion effects is discussed. In selected cases, the titration profiles were analyzed using a commercially available fitting program to obtain information about the species present in solution, including La3+ for which a dimer model is proposed. The fitting provides the microscopic (s)(s)pKa values for deprotonation of one to four metal-bound ethanol molecules. Kinetics for the La3+ catalyzed ethanolysis of paraoxon as a function of sspH are presented and analyzed in terms of La3+ speciation as determined by the analysis of potentiometric titration curves. The stability constants for the formation of Zn2+ and Cu2+ complexes with 1,5,9-triazacyclododecane as determined by potentiometric titration are presented.