Equilibrium and structural studies of silicon(IV) and aluminium (III) in aqueous solution. 28. Formation of soluble silicic acid-ligand complexes as studied by potentiometric and solubility measurements

Abstract

Amorphous silica solubilities in the presence of organic (mannitol, oxalate, citrate, catechol) and inorganic (SO42-) ligands have been studied in 0.6 M Na(Cl) medium at 25 °C. Apart from the catechol system, the experiments were performed at neutral pH with an equivalent ligand concentration of ≤0.6 M. In the presence of mannitol, the solubility of SiO2(am) was found to decrease with increasing ligand concentration. With oxalate, citrate and sulphate, an increased solubility was observed. However, the effects were small and it was concluded that the formation of soluble Si complexes of these ligands under natural water conditions could be neglected. With catechol, the measurements were performed under slightly alkaline conditions and a significant solubilization was noticed. Composition and stability of the species causing this solubilization was evaluated from precise potentiometric titration data in homogeneous Si(OH)4-catechol solutions. These data cover the ranges 2 ≤-log [H+] ≤ 10; 0.002 ≤ B ≤ 0.004 M and 0.002 ≤ C ≤ 0.015 M, with C/B ratios of 1, 2,2.5, 3 and 5 (B and C denote the total concentration of Si and ligand, respectively). With-log [H+] ≥ 7, a hexacoordinated complex, SiL32- is formed with log K [Si(OH)4 + 3H2L ⇌± SiL32- + 2H+ + 4 H2O] =-10.44 ± 0.029 (3σ). The recorded solubilization of amorphous silica was fully explained by the formation of this species.