The electrical double layer and electrochemical properties

The earliest models used to describe the distribution of charges in the edl are, besides the Helmholz model, the Gouy-Chapman diffuse layer model and the Stem-Graham model. Details of these models are given in Westall and Hohl (1980), Schindler (1981, 1984) and Schindler and Stumm (1987). 

Their electrochemical properties serve to regulate the coagulation rates, catalysis behaviour and electron transfer reactions of iron oxides (Mulvaney et ah, 1991). Two major methods of characterizing electrochemical behaviour are potentiometric titration and electrophoresis. 

Electrophoresis involves movement of particles in suspension under the influence of an electric field the direction of movement depends on the charge of the particles 

Particle mobility and zeta potential can now be measured by more sophisticated techniques. With photoelectrophoresis, particle mobility is measured as a function of pH under the influence of ultraviolet radiation. At pH 8, the electrophoretic mobility of irradiated hematite particles (A = 520 nm) was markedly different from that measured in the absence of UV irradiation. This was attributed to the development of a positive surface charge induced by photo-oxidation of the surface Fe-OH° sites to (Fe-OH) sites (Zhang et al., 1993). The electroacoustic technique involves generation of sound waves by the particles in the colloidal dispersion and from this data. 

The total charge of the Fe oxide surface has several components, i. e. 

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The Electrical Double Layer and Electrochemical Properties 235 Tab. 10.6 Isoelectric points and points of zero charge of iron oxides. 

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