Pznpc

The relationship between the PZNPC and the PZNC for a soil when the experimental conditions of rule 1 are met can be deduced from Eq. 3.3b and Table 3.1. Consider the PZNPC. Equation 3.3b becomes 

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. 

TABLE 10.3 The pH of the point of zero net proton charge PZNPC) of some minerals and organic substances 

The most common procedure to estimate (underlined by MK) the PZNPC is to equate 

Figure 3.1. Experimental examples of the PZC of > Al203, the PZNPC of goethite, the PZSE of hydroxyapatite, and the PZNC of kaolinite. (u is the electrophoretic mobility.)

It should be noted that the relation between the sign of oq and that of PZNPC-PZNC does not depend on equality between the PZC and the PZNC but only on the condition that = 0. 

The intrinsic equilibrium constants have thermodynamic significance, in that they determine the PZNPC through the condition Xso- sohj (pH = PZNPC) applied to Eq. 5.42  

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. 

Figure 5.2, with its two lines of unequal slope (in absolute value), illustrates another typical feature of applications of the constant capacitance model. The value of the capacitance parameter C inferred from each slope is not the same above and below the PZNPC. This result is in conflict with the nonspecific, coulombic nature of indicated in 

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) 

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). 

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