Potentiometric analysis of humic substances and other colloids

Abstract

The acid–base behaviour of colloidal suspensions of γ-Al2O3, TiO2 (rutile), high surface-charge latex, and humic substances in water is reexamined by using a Nernst-like model and two electrostatic models to simulate the proton-affecting surface potential. The Nernst-like model is based on the assumption that the proton-affecting surface potential is proportional to the Nernst potential. The other models are based on conventional double-layer assumptions. All models yield numerically consistent information about (i) number of chemically discernible functional groups (or collections of functional groups), (ii) their dissociation constants and concentrations, (iii) the free H+- (or OH−-) concentrations in the suspending medium, (iv) surface area of colloidal substance, and (v) surface potential (and charge) as function of ph. Potentiometric analyses are performed on documented titration curves by employing a modified version of the ACBA (acid–base) program developed by Arena etal. The goodness-of-fit information provided by this program suggests that the Nernst-like model is particularly useful for generating and verifying estimates for surface capacities as function of ph.