Point 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. 

On a broad scale, separation of the conditional equilibrium constant into contributions from an intrinsic affinity [K" (int)] and electrostatic repulsion or attraction (T g) accounts for the overall charge properties of most oxides (Fig. 8). Wieland et al. [38] compiled charge data for oxide minerals with well-defined surfaces and showed that, on an areal basis and a logarithmic scale, the oxide minerals and latex beads fall in a similar trend where the concentration of positive charge increases as pH is decreased from the point of neutrality (the point of net zero proton charge, or PZNPC). This PZNPC is interpretable as a measine of the fundamental affinity of the surface for protons apart from electrostatic repulsion and attraction. That these... 

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. 

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. 

Oxide surfaces have zero net charge and zero net surface potential at pH0 when immersed in an indifferent electrolyte solution. Therefore, the second term of the right side of equation 4.14 vanishes. Any change in the concentration of the supporting electrolyte does not change the surface potential and thus pH0 is independent of the electrolyte concentration. This independence is evidenced by a crossing point of the proton adsorption curves and pH0 coincides with PZC. 

The distinction between the isoelectric and isoionic states of a protein was first made in a classic paper by S0rensen et at. (1926). Three definitions of the isoionic point were proposed, one of these being the stoichiometrically defined point which we have called the point of zero net proton charge. The other tw o were operational definitions (summarized by Linderstr0m-Lang and Nielsen, 1959). The term isoionic point, as used here, corresponds to one of these two operational definitions, chosen because it always permits calculation of the point of zero net proton charge, which is the only parameter of real interest in the analysis of titration curves. The same choice has been made by Scatchard and Black (1949). 

It is to be noted that the point of zero net proton charge can be determined only for proteins which can be deionized without precipitation or other change. The point would also have little significance (and probably could not in any event be determined unequivocally) if the pH lies in a region where the titration curve is not behaving reversibly. 

If the point of zero net proton charge is known, then a count is available... 

Fig. 20. Approach to the acid end point of the titration curves of /3-lactoglobulins A and B, and for the normal equimolar mixture of the two, at 25°C and ionic strength 0.15. The value of 2b is calculated relative to the point of zero net proton charge, which occurs at a different pH for each of the three samples (Tanford and Nozaki, 1959).

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. 

This is often referred to as the isoelectric point. It is the condition where particles do not move in an applied electric field. If one wants to specify that the pzc is established solely due to binding of or OH", one may specify point of zero net proton charge (or condition) (pznpc). Furthermore we can define a point of zero salt effect (pzse)... 

For an oxide surface on which H and OH" are the only specifically adsorbed ions, at the pH of the point of zero net proton charge or pHpzNPc (see Table 10.3) the net surface potential and net... 

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.

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