Goethite surface charge density

Fig. 10.5 Surface charge density of hematite and goethite as a function of pH in an inert electrolyte of various concentrations (KCI for hematite NaNOs for goethite) as obtained by acid/base titration (hematite Atkinson et al., 1967, with permission goethite S. Glasauer, unpubl.).

Using Eq. 11 -34 (Box 11.1) and the goethite surface site density, its pA i and pK values, and the ionic strength of the solution, find the intensity of surface charging of the goethite at the three pH values of interest (note that [Pg.446]

Figure 3. The surface charge density at the goethite/electrolyte interface a function of pH in a presence and absence of cadmium ions.

Acid-base constants/proton-affinity constants These parameters cannot, at present, be established experimentally. Because there are probably several distinct sites and electrostatic effects, the overall surface-charge curves do not give any hint as to the nature and reactivity of these sites. Experimental back-titration data by Schulthess and Sparks [103], which indicate steps in the proton adsorption isotherm (i.e., the surface-charge density versus pH curve), were interpreted by these authors to be indicative of individual sites with individual reactivities. However, such data are not reproduced by the bulk of research groups. In the previous subsection, the back-titration technique has been discussed in some detail and, in particular, the results on goethite obtained with base titrations at low pH were found to show significant scatter compared to data obtained with the coulometric approach. 

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