Enthalpy and heat capacity changes for the proton dissociation of various buffer components in 0.1 M potassium chloride

Abstract

Enthalpy and heat capacity changes for the deprotonation of 18 buffers were calorimetrically determined in 0.1 M potassium chloride at temperatures ranging from 5 to 45 degrees C. The values of the dissociation constant were also determined by means of potentiometric titration. The enthalpy changes for the deprotonation of buffers, except for the phosphate and glycerol 2-phosphate buffers, were found to be characterized by a linear function of temperature. The enthalpy changes for the second dissociation of phosphate and glycerol 2-phosphate where divalent anion is formed on dissociation were fitted with the second order function of temperature rather than the first order. Temperature dependence of buffer pH calculated by using the enthalpy and heat capacity changes obtained was in good agreement with the temperature variation of the pH values actually measured in the temperature range between 0 and 50 degrees C for all the buffers studied. On the basis of the results obtained, a numeric table showing the temperature dependence of pK values for the 18 buffers is presented.