Zero proton condition at the surface

The point of zero charge pHpzc corresponds to the zero proton condition at the surface ... 

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. 

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. 

The proton balance at the surface Fh — Fqh obtained from alkalimetric or acidimetric titration permits the determination of that portion of the charge which is attributable to H or OH". With the help of these curves, the acidity and basicity of the =MeOH groups and the pH of zero proton condition can be determined (see Figures 9a, b, and c). 

The acid-hase characteristics of the surface groups (relative speciation of surface groups as a function of pH in upper figure) determine the pH of zero potential (point of zero proton condition MeOHt = =MeO ). The Nernst equation—a surface potential dependence on pH of (RT/Fj In 10 (= 59 mvolt at 25°C)—is not fulfilled. The lines in the lower figure were calculated from alkalimetric and acidimetric titration curves using... 

Procedures of data treatment were as follows. Firstly, the special integration of experimental voltammograms, transformed into i-t functions, is performed with Eq. (11.9) and Acj [ quantities, as E functions, are obtained. The second step consists in determination of distribution of proton donors at the electrode surface under cathodic polarization conditions. For this purpose, material balance equations are used. When increasing cathodic overvoltage, surface decreases from the bulk value (initial state) to zero (limiting current region). It must be emphasized that the surface concentration of the electrically active substance (hydronium ions) does not fall to zero, in contrast to common redox processes. In the absence of proton donors, a neutral medium is created at the electrode surface ([H+]j = [OH ]j), otherwise an alkalization occurs(see Eq. (11.7)atCH = 0). 

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). 

Supported metal oxide species are hydrated when exposed to moist environments and low temperatures (<230°C). Thus, all calcined supported metal oxide species are hydrated at ambient conditions (room temperature and air exposed) [34,35]. The hydrated surface layer corresponds to a thin aqueous film that corresponds to multiple layers of moisture [32]. The hydrated surface metal oxide species equilibrate with the pH of the aqueous layer. The pH of the aqueous film is determined by the pH at point zero charge (PZC) of the hydrated surface [36,37]. The net pH at PZC is defined as the equilibrated pH of a hydrated surface when the net charge is zero (protonated positive surface sites are balanced by an equal number of depro-tonated negative surface sites). At pH values above the PCZ, the hydrated surface becomes negatively charged, while for pH values below the PCZ, the hydrated surface becomes positively charged. Thus, hydrated surfaces always equilibrate at the pH at PZC in order to preserve charge balance. 

The essence of this reference electrode is that it presents a well-known and stable potential, in comparison with which the potential of the working electrode can be measured. Thus, suppose one has a reference electrode which is some arbitrary standard set up so that the conditions which lead to its reproduction become easy. In practice, two reference electrodes are chosen. The first and more fundamental is the hydrogen electrode, which is the electrode produced by bubbling hydrogen gas at one atmosphere over a highly catalytically active surface (which is in the solution of proton, of activity unity) for the reaction of hydrogen to equilibrate protons in solution and electrons (platinized platinum is often used) with a hydrogen gas. The potential of this electrode (it could be used as a reference) is arbitrarily taken to be zero volt. 

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