Describing the adsorption of potential determining ions on variable charge mineral surfaces

A computer program was developed to elaborate the titration curves of amphoteric surfaces according to
the double layer theory. The zpc, identified from the intersection point of the titration curves carried out
at different ionic strengths, fell at pH 3.15, 9.45 and 7.65 for Si, AI and Fe hydroxides, respectively. The
Stern model described quite satisfactorily the relation between σ0and ψ0. Some problem was presented
by Si surfaces which were then treated by a different calculation procedure.
The δ values, the Stern layer thickness, were very high for the SI surfaces and increased with decreasing
ionic strength. A different behaviour was showed by AI and Fe hydroxides; in these cases δ values were
very low and increased with increasing electrolyte concentration. These results were discussed in terms
of surface structures and activity of the supporting electrolyte.