Speciation calculations and ‘best’ stability constant choice


It is well known that serious discrepancies exist between published thermodynamic parameters of chemical reactions. As the factors that cause such discrepancies are numerous (such as experimental error, inadequate theory, and carelessness), they can be very difficult to pinpoint and thus to eliminate. It therefore takes a significant effort to critically assess all the relevant primary measurements and transform them into thermodynamically consistent datasets. Most of speciation codes have their own database of equilibrium constants and enthalpy values. Such databases unfortunately contain only a tiny fraction of all available data. Howeve, of even greater concern is the manner in which their thermodynamic data have stored; (i) standard state values are stored without the corresponding extrapolation functions and parameters used in their derivation, (ii) the reasons why particular values have been selected from published work in preference to others and the consequences of these decisions are not recorded properly. A thorough discussion of the problems involved, as well as a new procedure intended to achieve database thermodynamic consistency automatically, can be found in JESS-related publications that can be found at: http://jess.murdoch.edu.au/jess_home.htm.

 Examples of application of JESS in the selection of equilibrium constant data sets:

M. Filella, M. García Bugarín and P.M. May (2001) Obtaining the "best" values of stability constants: the protonation constants of five thioether carboxylates as a case study. THE ANALYST, 126, 2093-2100.
M. Filella and P.M. May (2003) Computer simulation of the low-molecular-weight inorganic species distribution of antimony(III) and antimony(V) in natural waters. GEOCHIM. COSMOCHIM. ACTA, 67, 4013-4031.
M. Filella and P.M. May (2005) Critical appraisal of available thermodynamic data for the complexation of antimony(III) and antimony(V) by low-molecular-mass organic ligands. J. ENVIRON. MONITORING, 7, 1226-1237.