Calorimetric acid-base titration of purified aluminum oxide C (Degussa) was performed under delicate experimental conditions. The initial state of titration was fixed at the reference state of aqueous suspensions of the oxide (25 degreesC, pH of suspension at point of zero charge, indifferent electrolyte). Suspensions containing 0.01, 0.1, or 1 MKNO3 were titrated with HNO3 and KOH solutions, within the dissolution free pH range. The measured data were corrected by the heats of mixing and neutralization. The amount of charged surface species formed in the H+ and OH- association reactions (S-OH + H+ reversible arrow S-OH2+, log K-1(int) = 6.8; S-OH + OH- reversible arrow S-O- + H2O, log K-2(int) = 9.2) was calculated assuming constant capacitance model, and the corrected heats were related to their amounts. The surface protonation (S-OH + H+ reversible arrow S-OH2+) reaction is exothermic, while the deprotonation (S-OH = S-O- + H+) is endothermic. With increasing ionic strength, the partial molar enthalpy of surface protonation process (DeltaH(pr)) decreases in absolute value from -34 to -28 kJ/mol, and that of deprotonation (DeltaH(depr)) increases from 34 to 41 kJ/mol. Their difference (DeltaH(depr) - DeltaH(pr)) is a constant value (69.2 +/- 1.2 kJ/2 mol of H+) and independent of the ionic strength. Thus, the standard enthalpy for a single surface protolysis reaction (SOH21/2+ reversible arrow SOH1/2- + H+) on alumina is DeltaH(ch)degrees = 34.6 +/- 0.6 kJ/mol. A thermodynamic treatment of Hall(12) for electrostatic enthalpy contribution resulted in a square root ionic strength dependence of enthalpy changes. This theoretically expected linear relationship proved to be valid for our data. The linear extrapolation of partial molar enthalpy (DeltaH(pr) and DeltaH(depr)) vs (c(el))(1/2) functions to zero ionic strength gives the electrostatic free, standard reaction enthalpy of surface ionization processes, DeltaH(pr)degrees = -34.0 kJ/mol for protonation and DeltaH(depr)degrees = 34.6 kJ/mol for deprotonation reactions, in a very good agreement with the calculated standard enthalpy DeltaH(ch)degrees of the protolysis reaction.