Calculating the Equilibrium Constants for All Monoazide Lanthanide Complexes in Aqueous Solution Based on the Formation of

Interest in lanthanide (Ln)/azide (N−3) chemical interactions, especially those involving Eu(III), has recently increased because N−3 quenches the luminescence of some lanthanides. Consequently, accurate equilibrium constant values for Ln/N−3 complexes are useful thermodynamic data. Herein, potentiometrically determined equilibrium constant data for the Eu(N3)2+ complex in aqueous solution at constant ionic strength (2.0 mol L−1, with NaClO4) and temperature (25.0 ± 0.1 °C) are reported. Data for the average number of ligands (n¯¯¯) were obtained by measuring the pH of the Eu(III)/N−3/HN3 system. The relationship between the integrated n¯¯¯ versus [N−3] curve, built by consecutively incrementing [N−3], provided the Fronaeus function, F0(L). This relationship (in the 0.010–0.10 mol L−1 [N−3] range) at four different Eu(III) concentrations (28.6–45.2 mmol L−1) revealed a single mononuclear complex (β1 = 4.51 mol−1 L). This β1 value was used in a mathematical approach to calculate the β1 values of all other lanthanides.