Potentials of zero charge

Department of Physical Chemistry and Electrochemistry, University of Milan, 20133 Milan, Italy 

Institute of Physical Chemistry, University of Tartu, 2400 Tartu, Estonia 

As a metal is brought in contact with an electrolyte, various phenomena occur that result in the onset of an electric potential difference (0M -0s), where M and S stand for metal and solution (the most usual electrolyte), respectively. The kind of phenomenon depends on the nature of the 

Modem Aspects of Electrochemistry, Number 33, edited by Ralph E. White etal. Kluwer Academic /Plenum Publishers, New York, 1999. 

Thermodynamically, all metal/solution interfaces are nonpolarizable, i.e., they can exchange electrical charges freely across the phase boundary. It is the extreme slowness of these exchanges that turns a nonpolarizable into a polarizable interface. Therefore polarizable interfaces are a limiting case of nonpolarizable interfaces.2 

---

An important point of the electrocapillary curve is its maximum. Such maximum value of y, obtained when q = 0, corresponds to the potential of zero charge (E ). The surface tension is a maximum because on the uncharged surface there is no repulsion between like charges. The charge on the electrode changes its sign after the... 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

If the concentration of the metal ion is not negligible at the potential of zero charge, the electrode potential varies linearly with log c according to Eq. (2) and there is no distinctive sign of the situation where the charge at the interface vanishes. The Nemst approach is obviously unsuitable for defining the nature and the amount of the charge at an interface. If the concentration of the metal ion at the pzc is small or very small, the behavior of the interface becomes that of a polarizable electrode. 

Since a metal is immersed in a solution of an inactive electrolyte and no charge transfer across the interface is possible, the only phenomena occurring are the reorientation of solvent molecules at the metal surface and the redistribution of surface metal electrons.6,7 The potential drop thus consists only of dipolar contributions, so that Eq. (5) applies. Therefore the potential of zero charge is directly established at such an interface.3,8-10 Experimentally, difficulties may arise because of impurities and local microreactions,9 but this is irrelevant from the ideal point of view. 

The notion of pzc is absent in early textbooks. A table with pzc values for about 10 metals (but for only 5 are reliable values claimed) was given by Parsons in 1954 in the first volume of this series.4 After a more complete attempt by Frumkin in 196520 to compare work function, extensive work on pzc was reported by Perkins and Andersen9 in this series and by Frumkin etal.8 in another series. Compilations of pzc values were also made by Campanella, Trasatti, Frumkin et al., and Frumkin and Petrii14 up to 1979. A book by Frumkin10 devoted entirely to the potential of zero charge was published posthumously in 1979. 

If M and R are in the same solvent S containing only an inert, surface-inactive supporting electrolyte, AE equals the difference in the potentials of zero charge between the two metals ... 

Equation (17) expresses the cell potential difference in a general way, irrespective of the nature of the electrodes. Therefore, it is in particular valid also for nonpolarizable electrodes. However, since

interfacial structure, only polarizable electrodes at their potential of zero charge will be discussed here. It was shown earlier that the structural details are not different for nonpolarizable electrodes, provided no specifically adsorbed species are present. 

On the other hand, surface physicists often measure 0 which represents the work function of metals as modified by adsorption of polar (water) molecules.35-39 What they are measuring (although they may not realize it) is precisely the potential of zero charge of the given metal in the UHV scale. The value of 0 is exactly known in that case, but the relevance of the value of A0 is in doubt.32,33 In fact, only a few layers of a solvent... 

The contact potential difference between Hg and water (actually a dilute aqueous solution of a surface-inactive electrolyte) has been measured42,43 to be -0.25 V. The negative sign means that the work function of Hg decreases upon contact with water. Since 4.50( 0.02) cV is the currently accepted5 value for 0 of Hg, the value of 0 for the uncharged metal (at the potential of zero charge) is 4.25 eV. 

On the other hand, if the 0 of Hg in the stream is modified by contamination in the cpd measurement, this should not be the case during the measurement of the potential-of-zero charge. If the value of 4.8 eV is accepted for the SHE in the UHV scale, the value of 4.61 eV for < of Hg at the pzc would imply that for 0 to decrease upon water adsorption, the 0 of clean Hg should be substantially higher than 4.61 eV. No experimental evidence exists for this for the time being. 

Relation of the Potential of Zero Charge to Other Quantities... 

For an electrochemical cell consisting of a metal at the potential of zero charge in a solution of surface-inactive electrolyte and a reference electrode (let us assume that any liquid junction potential can be neglected), the electrode potential is given by (cf. Eq. (20)]... 

Figure 5. Sketch of a work function-potential of zero charge plot. The line through the point of Hg has unit slope. The horizontal distance of Mi and M2 from the line measures AX in Eq. (28).

The temperature coefficient of the potential of zero charge has often been suggested to indicate the orientation of solvent molecules at the met-al/solution interface. However, this view is based only on the response of a simple two-state model for the interfacial solvent, and on neglecting any contribution from the electronic entropy.76,77 This is in fact not the case. The temperature coefficient of 0in many instances is negative and of the... 

The potential of zero charge depends on the composition of the solution if adsorption takes place. If partial or total charge transfer occurs, the situation becomes more complex than in a perfect condenser,82 as discussed in Section I.l(iii). 

As ionic adsorption takes place, normally the potential of zero charge varies linearly with the amount adsorbed.83 Such a variation is used84,85 as a means of extrapolating to zero concentration of the adsorbing sub-... 

---