Studies were made of the H2O-Na2O-SiO2 system at its vapor pressure at 200, 250, and 300??. Three different sodium trisilicate hydrates were encountered in the investigation. At 300??, Na2Si3O7??5H2O is found: at 250??, Na2Si3O7??6H2O; and at 200??, Na2Si3O7??11H2O. The liquid immiscibility previously reported to exist in the system was found to be a quenching phenomenon caused by the decomposition of the hydrates to unstable, supersaturated, viscous liquids. Under conditions where equilibrium is maintained, as temperature is lowered, the hydrates decompose to quartz, sodium disilicate, and liquid. The retrograde solubility of sodium disilicate and its tendency to form supersaturated solutions during heating from 25 to 250?? account for higher solubilities reported by others than were found in this study. The solubility of sodium disilicate in water is 26% at 200??, 9% at 250??, and 5% at 300??. Sodium metasilicate solubility is 38% at 200?? and 34% at 250??; this compound is incongruently soluble at 300??.
Additional Publication Details
The system water-sodium oxide-silicon dioxide at 200, 250, and 300??