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Environmental fate & pathways

Hydrolysis

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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. The index m (molar ratio) ranges between 0.5 - 4, most commonly m = 3.3. Stability depends to a large extent on pH, above pH 10.6 the solutions are chemically stable. The increase of ionic strength accelerates nucleation and deposition and decreases the SiO2solubility. 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).