Complexation of europium(III) by aqueous orthosilicic acid

The interaction of europium(III) with orthosilicic acid, Si(OH)(4), was studied in aqueous solutions of 0.1 M ionic strength. Radiotracer-solvent extraction of Eu-152, 154 and time-resolved laser-induced fluorescence spectroscopy of Eu(III) were used to measure the stability constants for the reactions Eu3+ + n OSi(OH)(3)(-) reversible arrow Eu(OSi(OH)(3))(n)((3-n)+) at pcH 4, 6, and 8.8, where n is 1 or 2. Measurable amounts of the 1:1 complex begin to form as low as pcH 3 in solutions containing millimolar orthosilicate concentrations and the europium bisorthosilicate complexes are present in such solutions when pcH greater than or equal to 8. The measured values of the stability constants under these conditions an log beta(1) = 7.26+/-0.20 and log beta(2) = 11.7 +/- 0.4.