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

This stability constant coupled with the first ionization constant of Si(OH)40 yields logSiβ1 = 6.18 ± 0.18 for the equilibrium Eu3 + + OSi(OH)3− ⇌ Eu(SiO(OH)3)2 +. Both of these constants are more than an order of magnitude smaller than results obtained under conditions conducive to silica polymerization (i.e., total silica concentrations between 2 and 20 mM). These new stability constants indicate that Eu(SiO(OH)3)2 + is a significant fraction of the total dissolved europium in deep waters of the Atlantic and Pacific but a very minor species in surface waters. In contrast to predictions obtained via the recent speciation model of Akagi (2013), concentrations of REE-silicate complexes throughout the oceans are much smaller than those of REE-carbonate complexes.