EXPERIMENTAL DETERMINATION OF CARBON ISOTOPE EQUILIBRIUM FRACTIONATION BETWEEN DISSOLVED CARBONATE AND CARBON DIOXIDE

Abstract

By mixing aliquots of pure Na2CO3 and CO2, solutions with precisely known molal fractions of CO=3, HCO-3, and dissolved CO2 were prepared. The apparent fractionation factor between gaseous CO2 and total carbon in the solution was determined mass spectrometrically from which the CO=3 - CO2 fractionation factor was calculated taking into account the known fraction of HCO-3 and respective isotope fractionation. Although the measurements have been made for a rather narrow temperature range, from 4 to 80°C, a theoretical curve was fitted through the experimental points, and thereby, the isotopic partition function ratio of 13CO=3 and 12CO=3 molecules has been obtained for an extended temperature range. The results obtained for CO=3-CO2 isotope fractionation are significantly lower than those for HCO-3-CO2 exchange (with 103 (α - 1) = 5.0 +/- 0.2 at 25°C and cross-over point at about 63°C).