Scan, anodic

Anodic Protection On the reverse anodic scan there will be a low current region (LCB) in the passive range. The passive potential range of the LCB is generally much narrower than the passive region seen on a forward slow scan. In anodic protection (AP) work the midpoint of the LCB potential is the preferred design range. This factor was verified for sulfuric acid in our laboratory and field studies. 

Reactivation Ratio EPR Test (Fig. 19.20c) This is a simpler and more rapid method than the single or double loop tests, and depends on the fact that the value of determined during the anodic scan of a double loop test (which produces general dissolution without intergranular attack on sensitised material) is essentially the same for all AlSl Type 304 and 304L steels. 

The anodic scans of the three related octanuclear anionic clusters Re7Pd(/4(,-... 

FIGURE 10.9 Galvanostatic charging curve for a platinized platinum electrode in 0.1 M H2SO4 solution (1) anodic scan, (2) cathodic scan. 

It is basically irrelevant in steady-state measurements in which direction the polarization curves are recorded that is, whether the potential is moved in the direction of more positive (anodic scan) or more negative (cathodic scan) values. But sometimes the shape of the curves is seen to depend on scan direction that is, the curve recorded in the anodic direction does not coincide with that recorded in the cathodic direction (Eig. 12.3). This is due to changes occurring during the measurements in the properties of the electrode surface (e.g., surface oxidation at anodic potentials) and producing changes in the kinetic parameters. 

Solutions (12.7) and (12.11) are valid when the initial scan potential, E-, in an anodic scan is at least 0.1 to 0.2 V more negative (in a cathodic scan more positive) than Ey2 or f (i.e., when the current in the system is still very low). Under these conditions the current during the scan is still independent of the value selected for E, . 

Figure 12.13 Electrochemistry and kinetics of CO resulting from methanol decomposition on polycrystalline Pt with O.IM H2SO4 electrol3de and 0.1 M methanol, (a-d) Current, SFG amphtude, frequency, and width of adsorbed CO, scanning the potential in both directions as indicated with the solid hne and fiUed circles denoting the forward (anodic) scan and the dashed hne and unfilled circles denoting the back (cathodic) scan, (e-g) Starting at 0.6 V, where the adsorbed CO is rapidly electro-oxidized, the potential is suddenly jumped to 0.2 V. The reformation of the CO layer (CO chemisorption) due to methanol decomposition occurs in about 20 s. The adsorbed CO molecules are redshifted, and have a broader spectrum at shorter times, when the adlayer coverage is low.

In the anodic scan, the oxidation of the H adlayer formed below 0.1 V and the re-formation of OHad/Oad (both in peak A) are shifted to markedly higher potentials compared with the Oad/OHad removal and Hupd formation (peak A ) in the cathodic scan (Fig. 14.2b). Furthermore, it overlaps with the peak B (OHad oxidation) observed for a cathodic scan limit of 0.1 V. At low scan rates, peak A starts at 0.1-0.15 V and reaches up to 0.48 V. Hence, compared with a scan with a cathodic limit of > 0.1 V, the equilibration of the Oad/OHad adlayer is shifted from 0.28 to 0.48 V. The charge in peak A integrated in the range 0.1-0.48 V corresponds to 1.5 e per surface atom, which is equal to the sum of the charges in peaks B and A in the negative-going scan. 

In the reverse anodic scan, similar arguments hold true for the removal of the H pd adlayer in peak A, which is likely to proceed in a comparable two-step process ... 

Figure 2.13 pH dependence of the electrocapillary maximum, Emill(. The solutions were 0 5 M Na2S0 /H2S04, except for the pH 14 electrolyte which was 1 M NaOH, The open circles represent data obtained from the anodic scan and the filled circles from the cathodic scan. From... 

Chemical analyses were performed by ICP at the Vemaison Center of Chemical Analysis of the CNRS. XRD patterns were obtained on a diffractometer with a copper anode. Scan was taken at 28 rate of 0.2°/min and structural data for reference compounds were taken from the ASTM X-ray powder data file. 

Shuman and Michael [326,327] introduced a technique that has sufficient sensitivity for kinetic measurement at very dilute solutions. It combines anodic scanning voltammetry with the rotating-disk electrode and provides a method for measuring kinetic dissociation rates in situ, along with a method for distinguishing labile and non-labile complexes kinetically, consistent with the way they are defined. 

A deviation from linearity is observed in the calibration curve at higher lead concentrations. The estimated value for the original sample was found to be 0.51 xg/l, with confidence limits at the 95% confidence level of 0.036 pg/1, compared with a value of 0.65 0.08 ig/l obtained by anodic scanning voltammetry. This value is well within the normal range reported in the literature for the natural lead content of unpolluted seawater. A detection limit of 0.03 ng ml"1 was obtained. 

Brugmann et al. [680] compared three methods for the determination of copper, cadmium, lead, nickel, and zinc in North Sea and northeast Atlantic waters. Two methods consisted of atomic absorption spectroscopy but with preconcentration using either freon or methyl isobutyl ketone, and anodic stripping voltammetry was used for cadmium, copper, and lead only. Inexplicable discrepancies were found in almost all cases. The exceptions were the cadmium results by the two atomic absorption spectrometric methods, and the lead results from the freon with atomic absorption spectrometry and anodic scanning voltammetric methods. 

Because of differing sensitivities and the natural levels of free metal or the anodic scanning voltammetric labile metal, cadmium, and copper in seawater are analysed using a 10 minute plating time, a -1.0V plating potential, and scanning in 6.67 mV/s increments. Zinc determinations can be made on a fresh aliquot of sample to eliminate any possible effects due to Cu-Zn inter-... 

Batley [780] examined the techniques available for the in situ electrodeposition of lead and cadmium in seawater. These included anodic scanning voltammetry at a glass carbon thin film electrode and the hanging drop mercury electrode in the presence of oxygen, and in situ electrodeposition on mercury-coated graphite tubes. 

Clem and Hodgson [783] discuss the temporal release of traces of cadmium and lead in bay water from EDTA, ammonium pyrrolidine diethyldithiocarba-mate, humic acid, and tannic acid after treatment of the sample with ozone. Anodic scanning voltammetry was used to determine these elements. 

Pongratz and Hunmann [19], using differential pulse anodic scanning voltammetry, found low levels of methyl cadmium compounds in the Atlantic Ocean. Levels in the South Atlantic were approximately 700 pg/1, and those in the North Atlantic were below the detection limit of the method, i.e., below 470 pg/1. It is believed that these compounds were formed as a result of biomethylation of inorganic cadmium. 

We consider again the redox reaction Ox + ze = Red with a solution initially containing only the oxidized form Ox. The electrode is initially subjected to an electrode potential Et where no reaction takes place. For the sake of simplicity, it is assumed that the diffusion coefficients of species Ox and Red are equal, i.e., D = D()s = DRcd. Now, the potential E is linearly increased or decreased with E(t) = Ei vt (v is a potential scan rate, and signs + and represent anodic scan and cathodic scan, respectively.) Under the assumption that the redox couple is reversible, the surface concentrations of Ox and Red, i.e., c()s... 

The fact that the normalized current ratio becomes negative at intermediate values of X with the ECE mechanism and not with the DISP mechanism stems from the same phenomenon as the one causing the tracecrossing behavior in cyclic voltammetry (Figure 2.9) (i.e., continuation of the reduction of C during the anodic scan). 

One can see immediately that the easy oxidation of mercury renders it of little use for anodic scans. Note that to construct a solid mercury electrode one can simply immerse a gold electrode in mercury for a few seconds. The amalgam that forms produces a mercury electrode much more manageable than the dropping electrode used in polarography. 

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