The extrapolation of experimental equilibrium constant data to zero ionic strength: Critical review and new approach

Different Debye-Hückel expressions for the activity coefficients of species in aqueous solution in the ionic strength range I = 0-3.5m (3M) are used for the extrapolation of equilibrium constants data to I = 0 and the interpolation to unknown I values. This may be accomplished using four or more values of the equilibrium constants that are equally well distributed on the I scale. The interpolated and extrapolated equilibrium constant values obtained are quite satisfactory and within the experimental error of the corresponding equilibrium constants. The values at I 0.1 are very important as they can particularly influence the equilibrium constant value calculated at I = 0 and for which the error can reach 0.1 log unit or more. The values at I 1.5 can also influence the extrapolated value at I = 0 and the interpolated value at a given I when an inadequate extrapolation model is selected. Among the expressions used, only those with two or more unknown parameters are suitable for such calculations.