The constitution of aqueous silicate solutions

Abstract

The distributions of anions in solutions of lithium, sodium, potassium, rubidium, caesium, and tetramethyl-, tetra-n-propyl-, and tetra-n-butyl-ammonium silicate solutions have been determined by trimethylsilylation and gas chromatographic analysis of the resulting volatile derivatives over a range of concentrations, temperatures, and silica–base ratios; and with sodium silicate solutions, the effects of adding salts and other alkalis were also studied. With the exception of tetramethylammonium silicate solutions, which at low temperatures and low silica–base ratios contained substantial quantities of the double-four-ring anion Si8O208–, two values were sufficient to characterise the anion distributions: the fraction of total silicon present found as monomer, and the fraction found in the form of anions with charges of –10 or more. The degree of polymerisation increased with increasing cation radius, silica–base ratio, and concentration, and decreased with temperature; of these, the concentration had much the largest effect. In sodium silicate solutions, addition of alkali metal salts increased the degree of polymerisation but tetramethylammonium chloride had almost no effect; addition of alkali metal hydroxides reduced the degree of polymerisation, NaOH having a greater effect than KOH.