Point of zero net proton charge

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. 

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).

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.

The point of zero net proton charge (PZNPC) is the pH value of the soil solution at which ctjj, defined in Eq. 1.24, is equal to zero. As can be inferred from Eq. 1.25, the PZNPC can be measured by potentiometric titration, provided only proton-selective surface functional groups on the soil solids are titrated. 

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... 

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