In the calorimetry experiment requirements of Eq. (19) should be respected so that reaction extents A4, A4, and A4 may be calculated by Eqs. (20) and (21), respectively. The next step is to evaluate the difference in reaction enthalpies by the relationship which comes from Eq. (11).

As stated in (22), the experiment in which initial and final pH values are related to p.z.c. by (19) yields the difference in standard enthalpies describing "chemical" interactions only and does not incorporate the electrostatic effects, just as in the case of the value obtained from p.z.c. dependence on temperature. The compensation of the electrostatic effect is based on the assumption of "symmetrical" surface potential with respect to p.z.c. and does not necessarily assume Nernstian behavior. It is sufficient that surface equilibria results, e.g., half pH unit below p.z.c., positive surface potential equal in magnitude to that one half pH unit above p.z.c. If such a behavior is found within a certain temperature range, then the derivative of potential with respect to temperature is again the same in magnitude but opposite in sign. Since the electrostatic contribution to the total enthalpy exhibits the same behavior as a derivative of the potential, the compensation of electrostatic contribution to the enthalpies takes place.

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