Polysilicate equilibria in concentrated sodium silicate solutions

Abstract

Polysilicate equilibria in concentrated sodium silicate solutions have been studied using potentiometric (glass electrode) and ²⁹Si n.m.r. methods. Experimental data cover the ranges: 11.36 ⩽ pH ⩽ 14.09, 2.4 ⩽[Na⁺]_tot/mol dm^–3⩽ 9.6, 2.1 ⩽[Si]_tot/mol dm^–3⩽ 9.3 with a variation of SiO₂/Na₂O 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 SiO₂/Na₂O = 1.0, i.e. in the most alkaline solutions (pH ≳ 13), dimeric as well as cyclic forms of tri- and tetra-meric silicate species predominate. At higher SiO₂/Na₂O, polymerization leads to the formation of polysilicate species containing branching groups with nuclearities 6–8 dominating. With SiO₂/Na₂O ≳ 3.3, i.e. 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 SiO₂/Na₂O.

The change in the different formation constants (log β_pq) due to variation in total Si and SiO₂/Na₂O, was found to be linear with respect to the Na⁺ concentration, i.e. log β_pq= log β^°_pq+k[Na⁺].

Distribution diagrams describing speciation and equilibria in water-glass solutions of different SiO₂/Na₂O and total Si concentrations are derived using the computer program SOLGASWATER.