A CONTRIBUTION TO THE STUDY OF THE SOLUBILITY OF OXIDIC SOLIDS IN WATER - THE DISOLUTION KINETICS OF SILICA-GEL AND ITS INTERPRETATION

The dissolution process of silica gel in water is investigated in its dependence on dissolution time, pH (2 < pH < 10), and temperature (25 degrees C, 45 degrees C, 55 degrees C). The dissolved amount of silica is depicted as a function of lime and pH. A dissolution surface is fo rmed. The saturation concentration at a certain temperature is relativ ely high al low pH-levels. When the pH is increased, a minimum of solu bility is found at pH about 4. A maximum seems to follow. Its position depends on the dissolution temperature. After this maximum a relative ly constant region of amount of dissolved silica with a second small m inimum is observed. If the pH-value is as high as 10, the solubility i ncreases drastically by the formation of an alkali silicate solution. A model is developed for the interpretation of the experimental result s. 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 proto n transfer reactions as well as the corresponding enthalpies and entro pies. The rate constants of the elementary reactions, and the activati on enthalpies and entropies are estimated. The expressions for the (ov erall) rate constants of dissolution and precipitation are derived, wh ich are commonly used in the literature. They depend on the equilibriu m constants of the proton transfer reactions, on the rate constants of the elementary reactions and on pH. Thus the course of the rate const ant as a function of dissolution pH, as reported by several authors, c ould be established and interpreted. As far as appropriate data are av ailable in the literature, a general good agreement of thermodynamic a nd kinetic quantities with the results obtained in this work could be observed. Therefore they could be a base for further investigations.