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

Abstract

The fractionation of 13 C between CO 2 (g) and CO 3 − --HCO 3 − (aq) in solution has been studied in single stage experiments using carbonic anhydrase to catalyze the hydration of CO 2 . 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 13 CO 2 (g)-- 12 CO 3 − (aq) equilibrium constant (K 2 ) has been made using the most recent spectroscopic data on the exchanging species. The variation of the equilibrium constant K 2 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 13 C fractionation in the system may be treated as contributions due mainly to three exchange equilibria whereby 13 CO 2 (g) independently exchanges with 12 CO 2 (aq), H 12 CO 3 (aq),and 12 CO 3 − (aq), the corresponding equilibrium constants being K 0 , K 1 , and K 2 respectively. The best fit values for the equilibrium constants are K 0 = 1.0009, K 1 = 1.0077, and K 2 ≥ 1.013, indicating the order K 0 < K 1 < K 2 .