Titration calorimetry of aqueous alumina suspensions part I. Results and comparison with similar studies

Titration calorimetry is used to investigate the enthalpies associated with proton adsorption and desorption in aqueous alumina (γ-Al₂O₃) suspensions. Adsorption enthalpies are exothermic and desorption enthalpies endothermic over the pH (4-10) and ionic strength (0.001, 0.01 and 0.1 M NaCl) ranges examined. Below pH 6, enthalpies are constant at about 30 kJ mol^-1 H⁺ adsorbed or desorbed (in absolute magnitude) for both alumina samples investigated and titration directions as well as with ionic strength. Above pH 6, enthalpies increase to 40-60 kJ mol^-1 (in absolute magnitude) at pH 9.75 and significant differences are observed between ionic strengths, titration directions and the 2 alumina samples titrated. Residual heat and H⁺ consumption or production resulting from the titration of supernatant blank solutions were only important above pH 7. Conceivable dissolved Al hydrolysis and solid-phase dissolution and precipitation reactions are also compared with these blank effects so that the precision and accuracy of the data can be ascertained with more accuracy. It is concluded that the data primarily reflect the enthalpy associated with the proton adsorption and desorption process and that data precision is adequately represented by the estimates resulting from averaging several separate titrations over narrow (0.5 pH unit) pH intervals. The enthalpy values also compare favorably with most of the several previous estimates available for alumina and with analogous dissolved Al hydrolysis reactions.