Abstract
AbstractThe dissolution process of silica gel in water is investigated in its dependence on dissolution time, pH (2 < pH < 10), and temperature (25°C, 45°C, 55°C). The dissolved amount of silica is depicted as a function of time and pH. A dissolution surface is formed. The saturation concentration at a certain temperature is relatively high at low pH‐levels. When the pH is increased, a minimum of solubility is found at pH about 4. A maximum seems to follow. Its position depends on the dissolution temperature. After this maximum a relatively constant region of amount of dissolved silica with a second small minimum is observed. If the pH‐value is as high as 10, the solubility increases drastically by the formation of an alkali silicate solution.A model is developed for the interpretation of the experimental results. The model consists of four reactions with proton transfer which are assumed to be in equilibrium. Furthermore, four elementary reactions of monosilicic acid addition or elimination are considered. The model allows for the determination of the equilibrium constants of the proton transfer reactions as well as the corresponding enthalpies and entropies. The rate constants of the elementary reactions, and the activation enthalpies and entropies are estimated.The expressions for the (overall) rate constants of dissolution and precipitation are derived, which are commonly used in the literature. They depend on the equilibrium constants of the proton transfer reactions, on the rate constants of the elementary reactions and on pH. Thus the course of the rate constant as a function of dissolution pH, as reported by several authors, could be established and interpreted.As far as appropriate data are available in the literature, a general good agreement of thermodynamic and kinetic quantities with the results obtained in this work could be observed. Therefore they could be a base for further investigations.
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