Chemical reactions near critical points: The dissociation of weak acids

We have reanalyzed published data on the conductance as a function of temperature near the liquid-liquid critical points of isobutyric acid + water [A. Stein and G. F. Allen, J. Chem. Phys. 59, 6097 (1973)] and of phenol + water [C.-H. Shaw and W. I. Goldburg, J. Chem. Phys. 65, 4906 (1976)]. The aim was to test whether the anomalous decrease in the conductance near the critical point could be ascribed to a critical anomaly in the extent of acid dissociation and/or in the rate of proton transfer. In this new analysis, background contributions due to the normal temperature dependence of the dissociation constant and due to confluent critical singularities are given consideration. We find that the leading critical anomaly in the conductance is characterized by a critical exponent 1-α, where α is the exponent associated with the specific heat at constant pressure and composition. The exponent 1-α is expected for an anomaly in the extent of the dissociation reaction and also for an anomaly in the proton-transfer rate.