Polysilicate equilibria in concentrated sodium silicate solutions have been studied using potentiometric (glass electrode) and Si n.m.r. methods. Experimental data cover the ranges: 11.36 [less-than-or-eq] pH [less-than-or-eq] 14.09, 2.4 [less-than-or-eq][Na]/mol dm[less-than-or-eq] 9.6, 2.1 [less-than-or-eq][Si]/mol dm[less-than-or-eq] 9.3 with a variation of SiO/NaO between 3.3 and 1.0. From available pH and n.m.r. data an equilibrium model is derived describing polymerization and protonation of silicate species in these water-glass solutions. With SiO/NaO = 1.0, in the most alkaline solutions (pH [greater, similar] 13), dimeric as well as cyclic forms of tri- and tetra-meric silicate species predominate. At higher SiO/NaO, polymerization leads to the formation of polysilicate species containing branching groups with nuclearities 6-8 dominating. With SiO/NaO [greater, similar] 3.3, close to the precipitation boundary of amorphous silica, cage-like polysilicate species dominate.The average charge per Si atom for the different polysilicate species varied between -2 in the most alkaline solutions down to -0.5 close to the precipitation boundary. The buffering capacity of the different water-glass solutions was found to decrease with increasing SiO/NaO.The change in the different formation constants (log ) due to variation in total Si and SiO/NaO, was found to be linear with respect to the Na concentration, log = log +[Na].Distribution diagrams describing speciation and equilibria in water-glass solutions of different SiO/NaO and total Si concentrations are derived using the computer program .
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