Ionization equilibriums of silicic acid and polysilicate formation in aqueous sodium chloride solutions to 300.degree.C

The ionization behavior of silicic acid and polysilicate formation in basic solutions have been studied by precise potentiometry using titration techniques in a hydrogen-electrode concentration cell. Polysilicate formation was studied in 1 m NaCl solutions at temperatures from 60 to 290/sup 0/C and at Si(IV) concentrations 0.005 to 0.05 m. At the lowest silica concentration only mononuclear species occur over wide temperature and pH ranges. At hydroxyl numbers from about 0.7 to 1.0 (average charge per silicon) small polysilicates which equilibrate rapidly occur at higher Si(IV) concentrations. Polysilicate formation decreases with increasing temperature. The equilibrium quotient for the most significant reaction, Si(OH)/sub 4/(aq) + OH/sup -/ broken arrows SiO(OH)/sub 3//sup -/ + H/sub 2/O, has been precisely determined from 0.1 to 5.0 m NaCl and to about 300/sup 0/C. Values of the logarithm of the equilibrium quotient for the reaction are 3.96 and 2.20 at 50 and 300/sup 0/C in 1 m NaCl and 4.32 and 2.26 at the same two temperatures in 5 m NaCl. An analytical expression from which the thermodynamic quantities for the reaction can be computed is presented. The small effect of salt concentration is interpreted as evidence for little or no sodium ion complexing.