Thermochemistry and kinetics of silica dissolution in NaOH aqueous solution

Dissolution of increasing amounts of silica (α-cristobalite) in the same volume of a 4% weight of sodium hydroxide solution was followed by microcalorimetry in a closed cell at temperature up to 130 °C. The process led to the formation of a dissolved entity whose formula depends on the Na2O/SiO2 ratio. At the beginning dissolution gave [3Na2O, 4SiO2] entity and when increasing dissolved silica amounts, the latter transforms into [Na2O, 2SiO2]. Kinetic study showed that dissolution starts by a very rapid step with about 10% of the whole heat energy released and continued with a very slow process. Kinetic analysis showed that the latter agrees with two successive silica first-order reactions scheme. The resulting activation energy values are of the same magnitude order of that deduced from the isoconversional model.