TEMPERATURE-DEPENDENCE OF THE PH-VALUE AND EFFECTS ON THE KINETICS OF THE HYDROLYTIC DECOMPOSITION OF BENZOCAINE

The dissociation constant of water increases from about 10(-14) mol l-1 at 25-degrees-C to about 10(-12) mol l-1 at 120-degrees-C. If a pH-stable buffer is used, the hydroxyl ion concentration of the solution increases inevitably, because the product of proton and hydroxyl ion concentration must correspond to the dissociation constant of water. The kinetics of pH-dependent decomposition reactions is influenced by this temperature-dependent variability of proton and hydroxyl ion concentration. An exact theoretical prediciton, of how the pH-value changes with increasing temperature, is hardly possible: the pH-value must be measured in each decomposition solution at each observed temperature. It could be shown, that the pH-temperature-variation of a solution affects clearly the effective kinetics parameters (frequency factor and activation energy), being estimated by the Arrhenius function. A physical significance is given to the Arrhenius parameters only, if the proton concentration in acid hydrolysis and the hydroxyl ion concentration in basic hydrolysis does not change with increasing temperature. Otherwise, the frequency factor itself is a function of the temperature and can no longer be a temperature-independent constant in the Arrhenius equation. As far as heat-sterilization processes are concerned, often it is not realized, that acid and base catalysed decomposition reactions happen simultaneously. Because of their clearly different activation energies, a sum of two independent Arrhenius functions is formed, that cannot be expressed by a single exponential function. The present paper is concerned with the question of which preconditions are necessary to make sure that parameters, estimated by an Arrhenius function, correspond to a frequency factor and an activation energy.