Europium silicate complexation at 25 °C and 0.7 molar ionic strength

Previous determinations of rare earth element (REE) complexation by silicate have been confined to observations of Eu(SiO(OH)3)2+ formation. Critical assessment of these observations reveals order-of-magnitude ambiguities in the stability constants derived from a variety of experimental designs. A salient feature of prior Eu(SiO(OH)3)2+ stability constant assessments is that the largest silicate stability constants were obtained in the presence of substantial silica polymerization. In the present work, Eu(SiO(OH)3)2+ formation is described under conditions where polymerization should be negligible. Stability constants were obtained via observations of the following equilibrium: Eu3++Si(OH)40⇌Eu(SiO(OH)3)2++H+ with total silicon concentrations on the order of 0.5mM or less. The formation constant for this equilibrium at 25°C and 0.7M ionic strength is