Interfacial acid-base reactions of aluminum oxide dispersed in aqueous electrolyte solutions. 2. Calorimetric study on ionization of surface sites

Calorimetric acid-base titration of purified aluminum oxide C (Degussa) was performed under delicate experimental conditions. The initial state of tit ration was fixed at the reference state of aqueous suspensions of the oxide (25 degreesC, pH of suspension at point of zero charge, indifferent electr olyte). Suspensions containing 0.01, 0.1, or 1 MKNO3 were titrated with HNO 3 and KOH solutions, within the dissolution free pH range. The measured dat a were corrected by the heats of mixing and neutralization. The amount of c harged 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 ar row S-O- + H2O, log K-2(int) = 9.2) was calculated assuming constant capaci tance model, and the corrected heats were related to their amounts. The sur face protonation (S-OH + H+ reversible arrow S-OH2+) reaction is exothermic , while the deprotonation (S-OH = S-O- + H+) is endothermic. With increasin g 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 diffe rence (DeltaH(depr) - DeltaH(pr)) is a constant value (69.2 +/- 1.2 kJ/2 mo l 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 thermodyna mic treatment of Hall(12) for electrostatic enthalpy contribution resulted in a square root ionic strength dependence of enthalpy changes. This theore tically expected linear relationship proved to be valid for our data. The l inear extrapolation of partial molar enthalpy (DeltaH(pr) and DeltaH(depr)) vs (c(el))(1/2) functions to zero ionic strength gives the electrostatic f ree, standard reaction enthalpy of surface ionization processes, DeltaH(pr) degrees = -34.0 kJ/mol for protonation and DeltaH(depr)degrees = 34.6 kJ/mo l for deprotonation reactions, in a very good agreement with the calculated standard enthalpy DeltaH(ch)degrees of the protolysis reaction.