Point of zero proton charge

The lack of coincidence between CIP and lEP of (hydr)oxides and related materials indicates the presence of strongly adsorbing species, added intentionally or present as impurities. Specific adsorption induces shifts in the lEP on the one hand and in the point of zero proton charge on the other in two opposite directions. When a shift in the CIP and lEP is addressed, the pristine PZC of given material (cf. Tables... 

Specific adsorption of anions is accompanied by adsorption of protons and specific adsorption of cations is accompanied by proton release. Consequently the point of zero proton charge is shifted to high pH in the presence of specifically adsorbed anions, and to low pH in the presence of specifically adsorbed cations. This is illustrated in Figs 4.11 (anions) and 4.12 (cations) for typical experimental conditions at which titrations are carried out (alumina, 2600 m /dm ). 

The pH value corresponding with zero charge, in the absence of outside cations and anions (Figure 2.19, left-hand column) is the point of zero proton charge PZC. 

Figure 3.19, Effect of ionic strength, /, on the point of zero net charge (PZNC) (a) and the point of zero net proton charge (PZNPC) (b) of a soil containing both permanent (P) and variable (V) charge minerals.

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

Pressure of gas i Point of zero net charge Point of zero net proton charge Point of zero net intrinsic charge Point of zero salt effect Isoelectric point Dipolar moment of species i... 

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. 

Table 2.2 Point of Zero Charge caused by Binding or Dissociation of Protons )...

The net charge at the hydrous oxide surface is established by the proton balance (adsorption of H or OH" and their complexes at the interface and specifically bound cations or anions. 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 point of zero charge (pzc) or the isoelectric point but lower (raise) the pH of the zero net proton condition (pznpc). 

The point of zero charge is the pH at which net adsorption of potential determining ions on the oxide is zero. It is also termed the point of zero net proton charge (pznpc). It is obtained by potentiometic titration of the oxide in an indifferent electrolyte and is taken as the pH at which the titration curves obtained at several different electrolyte concentrations intersect (Fig. 10.5). It is, therefore, sometimes also termed the common point of intersection (cpi). The pzc of hematite has been determined directly by measuring the repulsive force between the (001) crystal surface and the (hematite) tip of a scanning atom force microscope, as a function of pH the pzc of 8.5-8.S was close to that found by potentiometic titration (Jordan and Eggleston, 1998). This technique has the potential to permit measurement of the pzc of individual crystal faces, but the authors stress that the precision must be improved. 

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