3.5. Potentiometric titration
Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). Subsamples of the initial and exhausted materials (∼0.050 g) were added to NaNO3 (0.01 M, 25 mL) and placed in a container maintained at 25 °C overnight for equilibrium. During the titration, the suspension was continuously saturated with N2 to eliminate the influence of atmospheric CO2. The suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant starting from the initial pH of the material suspension up to pH 11. The experimental data was transformed into a proton-binding curve, Q, representing the total number of protonated sites [13] and [26]. These curves were deconvoluted using the SAIEUS procedure [13], and the pKa distributions for the species present on the surface were obtained.