Polarization of a graphite

All of the fat-soluble vitamins, including provitamin carotenoids, exhibit some form of electrochemical activity. Both amperometry and coulometry have been applied to electrochemical detection. In amperometric detectors, only a small proportion (usually <20%) of the electroactive solute is reduced or oxidized at the surface of a glassy carbon or similar nonporous electrode in coulometric detectors, the solute is completely reduced or oxidized within the pores of a graphite electrode. The operation of an electrochemical detector requires a semiaqueous or alcoholic mobile phase to support the electrolyte needed to conduct a current. This restricts its use to reverse-phase HPLC (but not NARP) unless the electrolyte is added postcolumn. Electrochemical detection is incompatible with NARP chromatography, because the mobile phase is insufficiently polar to dissolve the electrolyte. A stringent requirement for electrochemical detection is that the solvent delivery system be virtually pulse-free. 

In a recent investigation31 of TaC(lll) the surface shifted Ta 4f levels were investigated before and after depositing a monolayer of graphite on the surface. A dense carbon overlayer on the polar TaC(lll) surface might be expected to quench the surface Ta core level and to leave only a bulklike core level. The spectrum recorded after deposition of a graphite monolayer, however, showed an attenuated and shifted surface Ta core level, shifted about 0.3 eV closer to the bulk peak. It thus appears that not even a dense graphite monolayer is sufficient to produce a bulk-like electronic environment for the surface Ta atoms on TaC(lll). 

The same authors also studied the polarization behavior of a rotating-disc electrode and the chronopotentiometry of a graphite disc electrode. Figure 15.11 illustrates the situation in the vicinity of the cathode. A cupric chloride ion, CUCI3, moves to the cathode through its diffusion layer (Step 1) and is electrochemically reduced to CuClg (Step 2). 

Anodic evolution of chlorine at graphite has been investigated many times mainly for applied value (see, for example, the review articles[308-311]). Ksenzhek and Stender[312-315] were the first to analyze the effect of porosity of a graphite anode on the kinetic parameters of the process. Proceeding from the idea formulated by Ioffe[316] about the relation between the IR drop in electrolyte in the pores of the anode and a decrease in the true current density the authors analyzed the shape of the polarization curve and showed, in particular, that in the Tafel region the true current density on the outer surface of the electrode is proportional to the square of the apparent current density, which results in the doubling of the slope b. This result is valid for any true value of the Tafel slope[317]. 

FIGURE 15.7 Polarization curves for anodic chlorine (1) and oxygen (2) evolution at a graphite electrode, and the current yields of chlorine as a function of potential (3). 

Figure 7. In-situ AFM imaging of synthetic graphite flakes (a, b), MCMB particles (c, d) and natural graphite particles (e,f during the first cathodic polarization of the electrodes in the probe solution (LiClO/EC-PC), measured at the indicated potentials vs. Li/Li. The arrows and circles point to the relevant morphological processes, as detailed in the text (see ref 26).

Orientation Determination. While polarized edge studies, together with a known sample orientation, can provide information about the electronic structure of the absorber, one can also use polarized edges to probe ordered systems of unknown orientation. This sort of approach was used in a study of B adsorbed on graphite (26,27). In this case, the orientational dependence of an edge transition was used to calculate the degree of orientational purity of the graphite surface. 

In the case of the on graphite, polarized EXAFS was used to determine molecular orientation (26,27). The system consisted of a stack of parallel graphite sheets in which the EXAFS could be measured parallel or perpendicular to the plane of the sheets. The authors were able to show that the bromine was bound as Br and to determine the orientation of the Br molecules with respect to the plane of the graphite sheets. Polarized EXAFS proved to be a useful complement to LEED, which had been used to probe the long-range order in this system. 

---