Effects of pH and acid anions on the dissolution kinetics of MgO

The kinetics of the dissolution of particulate sintered polycrystalline MgO under advanced kinetic regime conditions were investigated, with special regard to the effects of concentration of H+ ions, temperature and the type of the inorganic acid used (HCl, HNO3 and H2SO4). It was observed that the rate of chemical dissolution of MgO accelerated with increase in both concentration of H+ ions (from 10−4 to 10−2 M) and temperature (from 25 to 60 °C). The overall process of MgO dissolution was found to be controlled by the surface chemical reaction of MgO with H+ ions. The presented values of the reaction order for H+ ions are consistent, allowing for experimental error, with current kinetic theories and most other experiments. However, the rate-controlling step could not be assigned unambiguously, because the same value of the reaction order for H+ ions can be predicted by various theories for different rate-determining steps. Measured values of the reaction order for H+ ions are indicative that the rate is limited by the second protonation reaction, i.e. the slow reaction of surface hydroxide with a second proton. Though the reaction order for H+ ions seems not to be affected by the type of acid used, the apparent activation energy changes in the order HCl ≤ HNO3 < H2SO4. Higher values of activation energy obtained for H2SO4 could indicate that surface chemical reaction is controlled by different reactions between surface anions and protons created by dissociation of H2SO4 and HCl (or HNO3).