Polysilicate equilibria in concentrated sodium silicate solutions
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 βpq) due to variation in total Si and SiO2/Na2O, was found to be linear with respect to the Na+ concentration, i.e. log βpq= 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.