The rates of the spontaneous hydration of CO2 and the reciprocal reaction in neutral aqueous solutions between 0° and 38°

Abstract 1. 1. The course of the hydration reaction and the reciprocal dehydration reaction have been determined in 0.01 M phosphate buffers by means of a pH-stat method. The measurements were performed in the temperature range 0–38° at pH values between 7 and 8. The rate constants were calculated by means of integrated rate equations. Both reactions followed first-order kinetics. 2. 2. The effect of temperature at pH 7.0 on the observable first-order rate constant in the dehydration reaction can be described by the equation: logk′d = 8.60 − 3213/T. Ea was found to be 14.6 kcal·mole−1. The corresponding results for the hydration reaction are: log k′h = 11.63 − 3870/T and Ea = 17.7 kcal·mole−1. A comparison with the values obtained from the literature for the first apparent dissociation constant of carbonic acid, K′1, indicates that the ratio k′ h k′ d has been determined with an error of less than 10%. 3. 3. By combining the present results with the well-established values for the second-order rate constant in the reaction: CO2 + OH− → HCO3−, it appears that, in contrast to the earlier findings, the contribution of this reaction to the total decreases with increasing temperature. 4. 4. No difference could be demonstrated in the values of k′d and k′h as determined at pH 7 and 18° in solutions of 0.1 M phosphate, 0.01 M imidazole and 0.01 M diemal.