When a biochemical reaction is studied at a specified pH, the transformed Gibbs energy G’ provides the criterion of equilibrium, and the apparent equilibrium constant K’ yields the standard transformed reaction Gibbs energy Delta(r)G’(0). The Legendre transformation is required to make pH a natural variable. The concentrations of reactants in the expression for the apparent equilibrium constant are sums of concentrations of species, and the reactants each have a standard transformed Gibbs energy of formation Delta(f)C’(0) and a standard transformed enthalpy of formation Delta(f)H’(0) at the specified T, P, pH, and ionic strength. These formation properties of a reactant can be calculated if the standard Gibbs energies and standard enthalpies of formation of the species involved are known. Here the standard formation properties of the species CO2, H2CO3, HCO3-, and CO32- are calculated using data from the NBS tables and Delta(r)G(0) and Delta(r)H(0) for the reaction H2O + CO2 = H2CO3. These values are then used to calculate Delta(f)G’(0)(TotCO(2)) and Delta(f)H’(0)(TotCO(2)), where TotCO(2) represents the sum of the four species in aqueous solution at a specified pH. The same values for the standard transformed properties of TotCO(2) can be calculated from the NBS tables alone, but the advantage of using information on the hydration reaction in addition is that the equilibrium concentrations of the species CO2 and H2CO3 can be calculated and are of interest kinetically. This analysis leads to values for a pH-dependent Henry’s law constant. The application of these ideas to the formate dehydrogenase reaction is discussed in detail.