RESUMEN

The experimental dissociation constants of strong acids are notoriously ill-defined, and it is necessary to rely on theoretical methods for their evaluation. We present a methodology for the theoretical evaluation of the dissociation constants, and the values of Ka for perchloric and sulfuric acids have been estimated. It has been shown that the acid dissociation constant Ka can be expressed as a product of two terms, Ka = Ka' × fHA∞, where Ka' is the apparent dissociation constant and fHA∞ is the infinite dilution activity coefficient of undissociated molecule of acid in liquid solution. The values of Ka' can be computed from readily available reference data. The limiting activity coefficients fHA∞ for strong acids can be determined by theoretical methods only. The following estimate for the limiting activity coefficients of perchloric and sulfuric acids has been obtained, -2.5 < log10 fHA∞ < -1.3. The ranges of values of the dissociation constants of HClO4 and H2SO4 at 25 °C have been determined; log10 Ka(HClO4) = 10.8-12.3; log10 Ka1(H2SO4) = 4.5-8.6.