Zero proton condition

The point of zero charge pHpzc corresponds to the zero proton condition at the surface ... 

The pHpZc (zero proton condition, point of zero charge) is not affected by the concentration of the inert electrolyte. As Fig. 2.3 shows, there is a common intersection point of the titration curves obtained with different concentrations of inert electrolyte. 

As we have seen, the net surface charge of a hydrous oxide surface is established by proton transfer reactions and the surface complexation (specific sorption) of metal ions and ligands. As Fig. 3.5 illustrates, the titration curve for a hydrous oxide dispersion in the presence of a coordinatable cation is shifted towards lower pH values (because protons are released as consequence of metal ion binding, S-OH + Me2+ SOMe+ + H+) in such a way as to lower the pH of zero proton condition at the surface. 

Thus, according to this interpretation the zero proton condition is at pH = 8.6. Furthermore, an ion exchange reaction... 

V Specifically adsorbed species are those that are bound by interactions other than electrostatic ones. To what extent SO and Ca2+ can form inner-sphere complexes is not yet well established. SO2 is able to shift the point of zero proton condition of many oxides. 

Specific adsorption of ions other than protons causes the pzc and the iep to shift along the pH scale (Stumm, 1992). Specifically adsorbed cations (anions) shift the titration curve and the point of zero proton condition at the surface (pznpc) to lower (higher) pH values, whereas the iep is moved to higher (lower) pH values. The shift of the iep of hematite to a lower pH by adsorbed EDTA and Cl" is shown in Figure 10.8. 

H-AcidUy If a natural water contains more protons than that given by the zero proton condition (or the H2CO3 equivalent point), then this water contains H-acidity (H-Acy) also called mineral acidity ... 

The pHpzc (zero proton condition, point of zero charge) is not affected by the concentration of the inert electrolyte in the absence of a different specific supporting electrolyte ion boundary for cation and anion. The computations of Dzombak and Morel (1990) employ a difftise layer model coupled with acid-base surface reactions to describe Q versus pH. (This acid-base model incorporates variable capacitance.) As Figure 9.8 shows, there is a common intersection point of the titration curves obtained with different concentrations of inert electrolyte. 

The acid-hase characteristics of the surface groups (relative speciation of surface groups as a function of pH in upper figure) determine the pH of zero potential (point of zero proton condition MeOHt = =MeO ). The Nernst equation—a surface potential dependence on pH of (RT/Fj In 10 (= 59 mvolt at 25°C)—is not fulfilled. The lines in the lower figure were calculated from alkalimetric and acidimetric titration curves using... 

The proton balance at the surface Fh — Fqh obtained from alkalimetric or acidimetric titration permits the determination of that portion of the charge which is attributable to H or OH". With the help of these curves, the acidity and basicity of the =MeOH groups and the pH of zero proton condition can be determined (see Figures 9a, b, and c). 

Surface Coordination of 7-AI2O3 with and S04 . Figures 5a and 9a illustrate the titration of 7-AIOOH with base and acid. In the absence of specifically adsorbable ions the point of zero proton condition... 

This charge can be determined from an alkalimetric-acidimetric titration curve and from a measurement of the extent of adsorption of specifically adsorbed ions. Specifically adsorbed cations (anions) increase (decrease) the pH of the lEP but lower (raise) the pH of the zero proton condition. 

The point of zero proton condition (the pH of fixed charge = 0 in the absence of specifically sorbed ions = lEP) is given by... 

In a similar way, the shift caused by S04 on lEP and zero proton condition can be derived from ... 

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