Ionic Strength Dependence of Rare Earth – NTA Stability Constants at 25°C

Observations of competitive complexation of NTA by Cu2+ and rare earth element (REE) ions are used to determine REE-NTA stability constants at ionic strengths between 0.1 and 5.0 molar. Although REE stability constants change markedly with ionic strength, differences in the ionic strength dependence of REE-NTA stability constants across the rare earth element series are small. The ionic strength dependence of logβ1 for Y and REEs with NTA at 25 °C can be described as: logβ1(M) = logβ1(M)0 - 9.198 I1/2/(1+B I1/2)+C I + D I3/2, where β1(M) = [MNTA°][M3+]-1[NTA3-]-1, I is ionic strength, B = 1.732, C = 0.1596, D = 0.0816, and logβ1(M)° is the metal-NTA stability constant at zero ionic strength.