Dissolution of MgO into the H₂SO₄-HI Acid Mixture

Abstract

Dissolution of MgO into an H₂SO₄-HI acid mixture: was investigated as a constituent reaction of the Mg-S-I thermochemical water splitting cycle.
  MgO (100) single crystals with cubic shape were used to accurately measure the dissolution rates. The single crystal was fixed at the end of a glass rod and dipped in an H₂SO₄-HI acid mixture. The MgO dissolution rate was obtained by the weight loss of the single crytal. Measurements were performed with varying the following conditions: rotation speed of crystal, temperature, (H₂SO₄/HI) ratio, H+ concentration, and extent of reaction of acid mixture. The results were in the following. (i) Initial dissolution rates of MgO into the acid mixture (H₂SO₄: HI=1: 2, 4mol H⁺/kg H2O) showed little dependency on the rotation speed at 70°C and were almost constant above 100rpm. Activation energy obtained from the Arrhenius plot (40-80°C) for the initial rates, ΔE=64.9kJ·mol-1, indicates that the surface reaction is rate controlling. ( ii ) When the acid ratio (H₂SO₄/HI) was larger than O.125 at 4mol H+/kg H2O, the initial rates were found almost constant and equal to the value corresponding to pure H₂SO₄ solution, while they decreased to 60% at pure HI solution side. (iii) As the reaction proceeded, the rates decreased with the decrease in H+ concentration remaining in the solution. This behavior is almost the same as the dependency of the initial dissolution rates on the H⁺ concentration of the fresh acid mixture. (iv) These results were well explained by considering the adsorption of H₂SO₄ and HI on MgO.
  Dissolution experiments for MgO powders with several particle sizes were also carried out by measuring the time requirement for complete dissolution of the powders in a test tube with stirring. The time measured was consistent with the time calculated using the data of single crystals.