For solids that do not dissolve in the contacting solution and whose surfaces contain fixed dissociable groups a rigorous thermodynamic treatment shows that when m(i) much greater than m(H)+ or m(OH)-, the expression (partial derivative ln m(H)+/partial derivative T)(p,mi,Gamma p) = -Delta h(p)/RT(2) describes the temperature dependence of potentiometric titration curves both at and away from the zero point of charge (ZPC), where m(H)+, m(OH)-, and mi respectively are bulk concentrations of hydrogen ions, hydroxyl ions, and supporting electrolyte ions, F Gamma(p) is the primary surface charge density, and Delta h(p), the partial molar enthalpy of surface group protonation, is well defined physically and accessible via calorimetry. Consequently the necessary and sufficient condition for temperature congruence of potentiometric titration curves is that Delta h(p), does not depend on Gamma(p) at constant T, p, and m(i). An expression for the enthalpy of forming a diffuse double layer is forwarded that differs from the so-called field energy by a factor of (1 + partial derivative ln epsilon/partial derivative ln T), where epsilon is the dielectric constant of water. It follows therefrom that any contribution to Delta h(p) from the diffuse layer is insignificant. When the method of congruence plots is applied to titration data on hematite and rutile it is found in common with other systems that the BET area per gram is about half the electrochemical surface area. The resultant inner region capacitances at 20 degrees C are 58 and 88 mu F cm(-2), respectively.