A TEST OF A FUNCTIONAL GROUP-CONTRIBUTION SCHEME FOR PREDICTING THERMODYNAMIC PROPERTIES OF AQUEOUS ORGANIC SPECIES

Despite extensive experimental work and the efforts of geochemical modelers (e.g. Shock 1995 Am J. Sci. 295: 496-580) and builders of compilations (Plyasunova et al., 2004 Int. J. Thermophys. 25: 351-360) situations still arise where thermodynamic values for simple organic aqueous species are needed, but are not available readily. In these circumstances a robust functional group-contribution model is useful. We recently applied the Sedlbauer-O'Connell-Wood (SOCW) functional group-contribution model (Sedlbauer et al. 2000 Chem. Geol. 163: 43-63; Yezdimer et al., 2000 Chem. Geol. 164: 259-280) and in doing so tested it against a number of experimental measurements and results obtained from other models. The published functional group contributions available do not include a value for the carbonyl group; a functional group required for the calculation of Δ_fG for pyruvic acid, for example. One small modification was made to the model, therefore; the contribution of a carbonyl group was approximated by the subtraction of an alcohol from the carboxylic acid group (COOH - OH = CO). This approximation was justified because the model considers carbon atoms individually, without consideration of neighboring groups.

All calculations were performed as described in the original publication (Yezdimer et al., 2000 Chem. Geol. 164: 259-280) after changing the value of 3.91×10^-4 in Equation A22 to -3.91×10^-4 and the value of HΔ_b*10¹ in Table 4 from 1.0604 to -1.0604 (Sedlbauer, pers. comm., 2004). Three separate quality control tests were performed. 1)Calculations of Δ_fG for butanol and K_a for propanoic acid were performed in order to replicate calculations presented in Yezdimer et al. (2000). 2)Values of Δ_fG for acetaldehyde calculated here were compared with those predicted by Schulte and Shock (Geochim. Cosmochim. Acta 57:3835-3846) thus testing for the technique for estimating the contribution from carbonyl functional groups used here. 3)Equilibrium dissociation constants calculated from Δ_fG values for the acid of interest and its conjugate base were compared with a suite of values derived from potentiometric experiments. In each case the agreement between the value obtained from the SOCW model and the experimental data or more established model data was excellent.