Experimental determination of aqueous sodium-acetate dissociation constants at temperatures from 20 to 240°C

Dissociation constants of sodium acetate ion pair (NaCH3COO0) were determined at the liquid–vapor saturation pressure by Raman spectroscopy at temperatures from 20 to 240°C and by potentiometry at temperatures from 25 to 172°C. Large differences were found between these two experimentally determined data sets; dissociation constants generated from Raman spectroscopic data were found to be ∼1.5 orders of magnitude higher than corresponding constants obtained using potentiometric measurements. Values generated from Raman spectroscopic data are consistent with the high temperature NaCH3COO0 dissociation constants reported by [Oscarson, J.L., Gillespie, S.E., Christensen, J.J., Izatt, R.M., Brown, P.R., 1988. Thermodynamic quantities for the interaction of H+ and Na+ with C2H3O2− and Cl− in aqueous solution from 275 to 320°C. J. Soln. Chem., Vol. 17, pp. 865-885], whereas those obtained by potentiometry are consistent with the low temperature NaCH3COO0 dissociation constants measured by [Archer, D.W., Monk, C.B., 1964. Ion-association constants of some acetates by pH (glass electrode) measurements. J. Chem. Soc., pp. 3117–3122.] and [Daniele, P.G., De Robertis, A., De Stefano, C., Sammartano, S., Rigano, C., 1985. On the possibility of determining the thermodynamic parameters for the formation of weak complexes using a simple model for the dependence on the ionic strength of activity coefficients: Na+, K+, and Ca2+ complexes of low molecular weight ligands in aqueous solution. J. Chem. Soc. Dalton, pp. 2353–2361.], as well as the predictions of [Shock, E.L., Koretsky C.M., 1993. Metal–organic complexes in geochemical processes: Calculation of standard partial molal thermodynamic properties of aqueous acetate complexes at high pressures and temperatures. Geochim. Cosmochim. Acta, Vol. 57, pp. 4899–4922.] and [Bénézeth, P., Castet, S., Dandurand, J.L., Gout, R., Schott, J., 1994. Experimental study of aluminum acetate complexing between 60 and 200°C. Geochim. Cosmochim. Acta, Vol. 58, pp. 4561–4571]. The origin of these differences likely stems in the species being sampled by different experimental techniques; Raman spectroscopy only identifies inner sphere sodium-acetate complexes as associated species whereas potentiometric measurements identify both inner and outer sphere sodium-acetate complexes as associated species. Regression of sodium acetate ion pair dissociation constants obtained from Raman spectroscopy and potentiometry enable characterization of true dissociation constants for both inner and outer sphere complexes. These latter parameters are used to assess the formation of these complexes at temperatures and solution compositions typical of natural fluids.