Potentiometric titrations

Potentiometric titrations measure the acid-base properties of the sorbent and the ionic exchange properties with regard to H+ and OH — ions. The presence of acid and basic sites determines the sorbent amphoteric properties and, depending on the pH, the functional groups can be either protonated or deprotonated. Active site concentrations are generally determined by acid-base potentiometric titration of the adsorbent and related modeling. Acidity constants found in the literature can be considered as mean values, which are representative of the class of the functional groups. Potentiometric titrations can also be used to determine the pH at the point zero charge (pHpzc) of biomass. pHpzc is the pH at which the sorbent surface charge takes a zero value as the charge of the positive surface sites is equal to that of the negative ones.

The knowledge of pHpzc allows one to hypothesize on the ionization of functional groups and their interaction with metal species in solution; at solution pH’s higher than pHpzc the sorbent surface is negatively charged and could interact with metal positive species while at pHs lower than pHpzc the solid surface is positively charged and could interact with negative species. Carboxylic groups were found to be the most involved, in the majority of cases, where potentiometric titration was used to elucidate the functional groups on biomass responsible for metal ions sorption. This fact is in part expected on the basis of their easiest deprotonation in the 2 — 6 pH range which is the most suitable for metal sorption.

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