I1. Phase boundary processes as rate determining steps in reactions between solids and liquids: The inhibitory action of metal ions on the formation of calcium bicarbonate by the reaction of calcite with aqueous carbon dioxide

Abstract

The inhibitory action of metal ions on the formation of calcium bicarbonate by the reaction of powdered calcium carbonate (calcite) with aqueous carbon dioxide has been studied. It has been shown that in the presence of the inhibitors the reaction approaches an apparent equilibrium state, very different from the real one. The rates of reaction have been correlated by the empirical formulae ▪ where c * represents the real or apparent equilibrium concentration of calcium ions. The effectiveness of metal ions as inhibitors increases with decreasing solubilities of the metal carbonates. The inhibitors are effective in concentrations below 10 −6 mole/l. with only 5 per cent or less of the calcite surface covered by metal salt. The fraction of the surface covered appears to decrease towards zero when the real equilibrium is approached. Interference with the growth steps on the crystal faces is not a likely explanation of the inhibitory effect, and it is suggested that the adsorbed inhibitors increase the surface concentration of carbonate ions and thereby increase the rate of the reverse reaction. A kinetic equation developed on this basis agreed reasonably well with the observations. The proposed theory indicates that this type of inhibition might be a general phenomenon likely to occur whenever an ionic crystal is being dissolved in a liquid containing metal ions which form less soluble salts with the anion of the crystal than the cation, provided the reaction is not diffusion controlled.