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Category name:
Soluble silicates

Justifications and discussions

Category rationale:
The soluble silicates are structurally very similar. Silicon-oxide tetrahedra as the basic structural units are linked with each other via Si-O-Si bonds resulting in an infinite three-dimensional network. The negative charge of unshared oxygen atoms is balanced by the presence of sodium or potassium cations which are randomly spaced in the interstices. The extent to which balancing alkali ions are present in a given silicate is defined by the molar ratio SiO2/M2O (M = Na or K). The higher the molar ratio, the less sodium or potassium ions are present in the silica network and consequently the less alkaline the silicates are. Whereas the sodium and potassium salts have an amorphous threedimensional structure, the disodium salts (= metasilicates) are crystalline with penta- and nonahydrate differing from the anhydrous form only by their water of crystallisation. Once in aqueous solution, ll soluble silicates are subject to the same molecular speciation resulting in a mixture of monomeric tetrahedral ions, oligomeric linear or cyclic silicate ions and polysilicate ions. At environmental pH values the soluble silicates are present as poorly soluble amorphous silica and monomeric silicic acid. The biological properties of soluble silicates are mainly governed by their intrinsic alkalinity. Based on the available data the members of the soluble silicates category exhibit a similar toxicological profile (OECD SIDS, 2004).

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