Sodium silicates

The metasilicate, Na₂SiO₃, is formed by the interaction of calculated quantities of sodium hydroxide and freshly precipitated silicic acid, and on addition of alcohol separates in monoclinic crystals, m.p. 1007° C. or 1056° C., or 1088° C. Its aqueous solution is strongly alkaline. A rhombic nonahydrate melting at 47° C., a monoclinic hexahydrate melting at 63.5° C., and a hexagonal tetrahydrate melting at 83° to 85° C. have been described. An octahydrate possibly exists at low temperature. With excess of sodium hydroxide another silicate, probably the ortho-form, Na₄SiO₄, is produced. According to Niggli, fusion of sodium carbonate and silica develops the equilibrium indicated by the scheme

Na₂CO₃+Na₂SiO₃ ⇔ Na₄SiO₄+CO₂,

the proportion of orthosilicate increasing with rise of temperature.

Fusion of silica with sodium carbonate yields a mixture of silicates, excess of silica producing the so-called " water-glass," a thick, syrup-like liquid employed as a cement and in the preservation of eggs. The commercial product contains 3 or 4 molecules of silica to each molecule of sodium oxide.

The interaction of silica and sodium chloride, characteristic of such processes as salt-glazing, coking salty coals, and chloridizing roasts, has been investigated up to 1000° C. by Clews and Thompson. There are three distinct reactions corresponding with the equations

1. 4xNaCl +ySiO₂ + xO₂ = 2xNa₂O,ySiO₂ + 2xCl₂;
2. 2xNaCl +ySiO₂ + xH₂O = xNa₂O,ySiO₂ + 2xHCl;
3. 4HCl + O₂=2H₂O + 2Cl₂.

In moist air all three reactions occur, the second predominating, although up to 1000° C. the magnitude of reaction is very small. The first reaction also takes place in dry air, and the second in moist nitrogen.