Points of Zero Charge

The isoelectric point is the pH at which the net surface charge is zero, i.e. the positive and negative charges arising from all sources are equal, i.e. 

It is measured by electrophoresis (see section 10.3) and corresponds to the pH at which there is no motion of the particles in an electric field (Fig. 10.6). 

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. 

3 10 M) or chloride ions (o) vs. pH. o chloride contamination, chloride-free hematite (Matijevic, 1980, with permission). 

Investigators tend to report either the pzc or the iep, but ideally, when specifically adsorbing ions are present, both parameters should be measured. 

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Thus, when titrating iodide with silver nitrate, coagulation occurs as soon as a slight excess of silver ion has been added (so that a point of zero charge has been surpassed). 

Figure V-8 illustrates that there can be a pH of zero potential interpreted as the point of zero charge at the shear plane this is called the isoelectric point (iep). Because of specific ion and Stem layer adsorption, the iep is not necessarily the point of zero surface charge (pzc) at the particle surface. An example of this occurs in a recent study of zircon (ZrSi04), where the pzc measured by titration of natural zircon is 5.9 0.1...

Material Potential-Determining Ion Point of Zero Charge... 

Fig. 11. Effects of pH in the colloidal siUca-water system (1), where A represents the point of zero charge regions B, C, and D correspond to metastable gels, rapid aggregation, and particle growth, respectively. Positive and negative correspond to the charges on the surface of the siUca particle.

To a first approximation, the ions in both Helmholtz layers can be considered point charges. They induce an equal and opposite image charge inside the conductive electrode. When the electrode is negative to the point of zero charge, cations populate the inner Helmholtz layer. 

The above formulas combined with Eqs. (74) and (75) taken at zero charge density yield Eq. (54) for the differential capacitance. Eq. (82) can be used recursively to generate the derivatives of the differential capacity at zero charge density to an arbitrary order, though the calculations become rather tedious already for the second derivative. Thus, in principle at least, we can develop capacitance in the Taylor series around the zero charge density. The calculations show that the capacitance exhibits an extremum at the point of zero charge only in the case of symmetrical ions, as expected. In contrast with the NLGC theory, this extremum can be a maximum for some values of the parameters. In the case of symmetrical ions the capacitance is maximum if + — a + a, < 1. We can understand this result... 

This does not imply that this double layer is at its point of zero charge (pzc). On the contrary, as with every other double layer in electrochemistry, there exists for every metal/solid electrolyte combination one and only one UWr value for which this metal/gas double layer is at its point of zero charge. These critical Uwr values can be determined by measuring the dependency onUWR of the double layer capacitance, Cd, of the effective double layer at the metal/gas interface via AC Impedance Spectroscopy as discussed in Chapter 5.7. 

It must be emphasized that the effective double layer is overall neutral, as the backspillover species (O6, Na6+) are accompanied by their compensating (screening) charge in the metal.32,3,35,36 It must also be clarified that this backspillover formed effective double layer is not in general at its pzc (point of zero charge). This happens only at a specific value of the electrode potential, as in aqueous electrochemistry.37... 

Is the gas exposed catalyst-electrode surface always at the point of zero charge (pzc) ... 

The close similarity between (CO)j species formed, either directly or indirectly, on metal electrodes and the comparable species observed on the same metals following adsorption of CO from the gas phase has already been noted. There is, however, some effect due to the presence of the electrolyte. For example, an Isolated CO molecule in the gas phase, an isolated CO molecule adsorbed onto a Pt surface from the gas phase, and an isolated (CO) g on a Pt electrode at the point of zero charge give rise to bands attributed to C-0 stretching fundamentals at 2143, 2064, and 2030 cm respectively. Thus the metal/... 

At a definite value of the electrode potential E, the charge of the electrode s surface and hence the value of drop to zero. This potential is called the point of zero charge (PZC). The metal surface is positively charged at potentials more positive than the PZC and is negatively charged at potentials more negative than the PZC. The point of zero charge is a characteristic parameter for any electrode-electrolyte interface. The concept of PZC is of exceptional importance in electrochemistry. 

Specific adsorption of ions changes the value of E, hence, one distinguishes the notion of a point of zero charge, in solutions of surface-inactive electrolytes, which depends on the metal, from that of a point of zero charge, in solutions of surface-active ions, which in addition depends on the nature and concentration of these ions. The difference between these quantities. 

ElectrocapiUary curves have a maximum. At this point, according to Eq. (10.32), the surface charge Qg = 0. The potential, E, of the maximum is called the point of zero charge (PZC). Knowing the charge density Qgyi, one can calculate the interfacial potential contained in Eq. (10.1). This is insufficient, however, for a calculation of the total Galvani potential, since other terms in this equation cannot be determined experimentally. 

A condition for inhibitor action is its adsorption on the metal at the open-circuit potential. Nentral inhibitor molecnles wiU not adsorb when this potential is far from the metal s point of zero charge (see Section 10.4.2). In this case, inhibitors forming ions are nsed cations (e.g., from amino compounds) or anions (from compounds with suKo groups), depending on the sign of surface charge. Inhibitor action is often enhanced greatly when mixtures of several substances are used. 

FIGURE 27.32 Cyclic voltammogram of the Ag(lll) electrode in 1 roM NaOH + 0.5mM NaF (pH 11). Sweep rate is 0.05 V/s. The arrows indicate the positions of the voltammetric peaks and the point of zero charge (PZC) of the Ag(lll) in neutral NaF electrolyte. (From Savinova et al., 2000, with permission from Elsevier.)... 

All factors influencing the potentials of the inner or outer Helmholtz plane will also influence the zeta potential. For instance, when, owing to the adsorption of surface-active anions, a positively charged metal surface will, at constant value of electrode potential, be converted to a negatively charged surface (see Fig. 10.3, curve 2), the zeta potential will also become negative. The zeta potential is zero around the point of zero charge, where an ionic edl is absent. 

If the interface is in the zero charge state, named also the point of zero charge (pzc), the Galvani potential should be equal to the dipolar term [19-21] ... 

Gratifyingly (see below), various theories for the metal in the interface have found values close to this one for this quantity.] From a number of experiments, it is suggested that xHl° is between 0.08 and 0.13 V, which means that gH2°(dip) is between 0.02 and 0.07 V on a mercury electrode at the point of zero charge (oxygen end of the water molecules toward the metal). 

Thus, only the tail of the electron density (outside the jellium) contributes. The last term above may further be related11 to the position of the image plane zim so, at the point of zero charge,... 

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