CARBON-13 ISOTOPE EFFECTS IN SYSTEMS CONTAINING CARBON DIOXIDE, BICARBONATE, CARBONATE, AND METAL IONS

Abstract

The fractionation of 13C between CO2(g) and CO3−--HCO3−(aq) in solution has been studied in single stage experiments using carbonic anhydrase to catalyze the hydration of CO2. Added metal ions, pH of the equilibrium mixture, and temperature are found to play important roles in the overall fractionation process. Theoretical evaluation of the 13CO2(g)--12CO3−(aq) equilibrium constant (K2) has been made using the most recent spectroscopic data on the exchanging species. The variation of the equilibrium constant K2 with temperature has been calculated for each 25 deg interval from 0 to 975 °C with the help of the McMaster University IBM 7040 computer. It is shown that the overall 13C fractionation in the system may be treated as contributions due mainly to three exchange equilibria whereby 13CO2(g) independently exchanges with 12CO2(aq), H12CO3(aq),and 12CO3−(aq), the corresponding equilibrium constants being K0, K1, and K2 respectively. The best fit values for the equilibrium constants are K0 = 1.0009, K1 = 1.0077, and K2 ≥ 1.013, indicating the order K0 < K1 < K2.