Zero Charge Conditions

1 Potentiometric titration of oxide dispersion at three different ionic strengths. 

The above discussion refers to a somewhat idealized situation. For instance, what should we do when CIP/IEP First, the level of the experimental error should be considered as an important factor in the assessment whether or not CIP = IEP. The number of significant digits reported in the literature often exceeds the actual 

Potential pitfalls of specific experimental methods are diseussed in more detail in Section I.B. A proper experimental procedure with sufficiently pure materials leads to CIP = IEP = PZC ( 0.1 pH unit) for crystalline oxides in dilute solutions of some electrolytes. This equality has been challenged on grounds of different theoretical models (for detailed discussion of the models and their parameters cf Chapter 5). TLM with unsymmetrical counterion binding gives PZC IEP [10]. The following expressions for PZC and lEP in the TLM framework were proposed by Zhukov [11]. 

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Another possibility is obtained by first reducing the measured information and then implementing the zero charge condition. 

The further evaluation of these two options is treated elsewhere (13). The error coefficient matrix of eq 20 has now become meaningless, and so the standard deviation of the fit (eq 21), since these are only valid when having equal variances V for all the measurements. The variances are not equal anymore, since the zero charge condition will have another variance than the actual conductivity measurements. 

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. 

The non-steady-state optical analysis introduced by Ding et al. also featured deviations from the Butler-Volmer behavior under identical conditions [43]. In this case, the large potential range accessible with these techniques allows measurements of the rate constant in the vicinity of the potential of zero charge (k j). The potential dependence of the ET rate constant normalized by as obtained from the optical analysis of the TCNQ reduction by ferrocyanide is displayed in Fig. 10(a) [43]. This dependence was analyzed in terms of the preencounter equilibrium model associated with a mixed-solvent layer type of interfacial structure [see Eqs. (14) and (16)]. The experimental results were compared to the theoretical curve obtained from Eq. (14) assuming that the potential drop between the reaction planes (A 0) is zero. The potential drop in the aqueous side was estimated by the Gouy-Chapman model. The theoretical curve underestimates the experimental trend, and the difference can be associated with the third term in Eq. (14). 

Potential of zero charge Electrode potential on absolute scale Electrode potential at standard conditions Electrode potential at equilibrium Galvani potential... 

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. 

Points of Zero Charge. Points of zero charge (pzc) are pH-values where the net surface charge is zero. We consider here above all the surface conditions where the... 

As we have seen, the net surface charge of a hydrous oxide surface is established by proton transfer reactions and the surface complexation (specific sorption) of metal ions and ligands. As Fig. 3.5 illustrates, the titration curve for a hydrous oxide dispersion in the presence of a coordinatable cation is shifted towards lower pH values (because protons are released as consequence of metal ion binding, S-OH + Me2+ SOMe+ + H+) in such a way as to lower the pH of zero proton condition at the surface. 

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). 

In electrochemical conditions, the electrons are transferred from the metal to the solution rather than to a vacuum. Moreover, the metal/solution interface is charged and the potential difference between the metal and the solution should be taken into account. The situation is simplified when the work function and uncharged interface are considered. The relationship between the work function and potential of zero charge was propos nearly 30 years ago by Bockris and Argade and by Frumkin (see e.g., Ref. 66) and later intensively discussed by Trasatti (e.g., Refs. 5, 21, 67). The relationship is given by the equation... 

The potential of zero charge, ac, can be obtained from the condition at which Om = - (dy/d.E)=O.This is the potential at which the interfacial tension is maximum in an electrocapillary curve (yvs.E) and is called the electrocapillary maximum. Figure 5-17 illustrates the electrocapillary curves observed for a liquid mercury electrode in aqueous solutions of varioxis anions. It is found that the greater the adsorption affinity of the anions (Cl" < Br" < I") on mercury, the more negative is the potential of zero charge (the potential of electrocapillary maximum). 

If there exist no free charges within the domain of interest, the time-harmonic electromagnetic held must also satisfy zero divergence conditions... 

The vanadium oxide species is formed on the surface of the oxide support during the preparation of supported vanadium oxide catalysts. This is evident by the consumption of surface hydroxyls (OH) [5] and the structural transformation of the supported metal oxide phase that takes place during hydration-dehydration studies and chemisorption of reactant gas molecules [6]. Recently, a number of studies have shown that the structure of the surface vanadium oxide species depends on the specific conditions that they are observed under. For example, under ambient conditions the surface of the oxide supports possesses a thin layer of moisture which provides an aqueous environment of a certain pH at point of zero charge (pH at pzc) for the surface vanadium oxide species and controls the structure of the vanadium oxide phase [7]. Under reaction conditions (300-500 C), moisture desorbs from the surface of the oxide support and the vanadium oxide species is forced to directly interact with the oxide support which results in a different structure [8]. These structural... 

Under specific conditions, the potential of zero charge does not appear to be constant during electrochemical experiment, which makes the double-layer effect more complex. For example, the shift of the potential of zero charge during electroreduction of S20g , combined with the Frumldn-type double-layer effect, has been proposed [26] as an explanation for the oscillatory reduction of peroxodisulfate on Au(llO) in diluted solutions of NaF. 

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