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
- 4xNaCl +ySiO2 + xO2 = 2xNa2O,ySiO2 + 2xCl2;
- 2xNaCl +ySiO2 + xH2O = xNa2O,ySiO2 + 2xHCl;
- 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. |