which requires the use of an activity coefficient model. Unlike the case for the deprotonation reaction, it is problematic to extrapolate results obtained for the carbamate reaction using Eq. ( ) to infinite dilution, since in CO 2 -loaded aqueous amine solutions, the carbamate reversion reaction cannot be isolated and the bicarbonate and carbamate ions co-exist with other ions (i.e, OH -, H 3 O + , CO -2 3 etc). At relatively low CO 2 loadings (low ionic strength), the activity coefficients of the bicarbonate and carbamate ions approach unity and at low amine weight fractions, only the activity coefficient data of binary amine/water system (at relatively low amine weight fraction, where only amine infinite dilution activity coefficients are required) along with the equilibrium composition maybe used in the Eq. ( ) to estimate the equilibrium constant. An approximation often made for isocoulombic reactions (with ions on each side of the reaction having the same charge, which is the case for the carbamate reversion reaction) is to assume that the activity of water is unity and the activity coefficients of RNHCO - 2 and HCO - 3 are equal (as is the case for the Debye-Hückel; and other simple activity coefficient models). In this case, the activity coefficient terms in Eq. ( ) cancel at all concentrations.