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Environmental fate & pathways

Hydrolysis

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In view of their chemical structure and inorganic nature, the hydrolytic stability of soluble silicates is to a large extent depending on the pH regime. The basic consideration is that silica dissolves according to: SiO2+ H2O = Si(OH)4. At low concentrations most species are present as monomers, at higher concentrations polymerisation will occur. Most soluble silicates are in the form: M2O x mSiO2x nH2O where M = alkali metal, predominantly Na, but also K or Li. The index m (molar ratio) ranges between 0.5 - 4, most commonly m = 3.3 for sodium and potassium silicates, and above 4 for lithium silicates. Above pH 10.6 the solutions are chemically stable. The increase of ionic strength accelerates nucleation and deposition and decreases the SiO2 solubility. Coating of surfaces by organic matter may hamper dissolution, but at the same time Si(OH)4may form complexes with organic matter, a process which favours dissolution (Falcone 1997).

Silicic acid, lithium salt is an inorganic solid, in water its lithium component is dissolved while the silica component stays undissolved. Due to its chemical nature and stability, hydrolysis is not expected. However, ion exchange processes are possible depending on the surrounding environment to transfer the substance in another, as very limited, not quantifiable hydrolysis is involved in the dissolution of silicates in water. It is limited to a gel-layer surrounding the silicate particle in aqueous medium. This layer is in equilibrium with the outer aqueous phase and constitutes a diffusion barrier for ions and water.