Science Serving Maryland's Coasts
Johan Schijf, Ph.D.
Chesapeake Biological Laboratory
Determination of the Stability Constants for the Complexation of Yttrium Rare Earth Elements with Silicate at Seawater Ionic Strength
Inorganic ligands dominate aqueous complexation of yttrium and rare earth element (YREE) within the marine environment. Therefore, the stability constant of several important ligands has been characterized to discover more about YREE complex speciation. Little attention has been directed towards evaluating silicate. This may be due to silicate undergoing polymerization at pH 8 making it difficult to work with. Here, we present a method to determine the logβ1 and logβ2 for Eu-silicate complexes through conducting potentiometric titrations at seawater ionic strength (I=0.7). To prevent Eu-hydroxide precipitation, titrations have to be conducted in the presence of the strong ligand desferrioxamine B (DFOB). Using a computer program called FITEQL 4.0, non-linear regressions were produced from the raw titration data. The acid dissociation constants for silicic acid are pKa1 = 9.45±0.02 and pKa2 = 11.4±0.1. The stability constants were found to be logβ1 = 6.1±0.4 and logβ2 = 11.4±0.8. These values show a good correlation with stability constants obtained from static potentiometry, a linear free energy relationship (LFER) with Fe3+, and reanalyzed data from the literature. These results indicate that silicate only affects the speciation within the deep Pacific Ocean. ndicate
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