Simultaneous evaluation and correlation of thermodynamic data

A computer-oriented strategy has been developed for simultaneous evaluation and correlation of thermodynamic data on groups of chemically related species. A general empirical equation for the temperature dependence of the heat capacity of a species and derived functional relations for entropy, enthalpy, free energy, equilibrium constant, and electrochemical potential provides a mathematical model, whereby internally consistent analytical representations of thermodynamic functions may be obtained by multiple regression on experimental data. Programmining is simplified by defining the free energies of elements to be zero at 298.15/sup 0/K and the selected reference pressure. Conversion to other conventions is readily made after regression is completed. Error plots obtained from the initial parameter adjustment help to identify inconsistent data. Where justifiable, such data are rejected, and additional regressions are made to obtain an internally consistent description of the data. The species-related parameters from the final evaluation are then used with the mathematical model to calculate the thermochemical properties of the species considered. Application of this procedure is described, using, as examples, studies in the system Fe-C-H-O to derive thermodynamic functions for iron and its oxides and analysis of the system N-H-O to correlate the aqueous solubility of N/sub 2/ and NH/sub 3/ and themore » equilibrium between aqueous NH/sub 3/ and NH/sub 4//sup +/.« less