Total oxidation current

When the metals are coupled, conservation of charge requires that the total oxidation current must equal the total reduction current, LIox = SIre(i. Thus> the two oxidation and the two reduction curves must... 

Correcting for Residual Current In any quantitative analysis the signal due to the analyte must be corrected for signals arising from other sources. The total measured current in any voltammetric experiment, itot> consists of two parts that due to the analyte s oxidation or reduction, and a background, or residual, current, ir. 

When a polymer relaxes at a constant anodic potential, the relaxation and partial opening of the polymeric structure involve a partial oxidation of the polymer. Once relaxed, the oxidation and swelling of the relaxed polymer goes on until total oxidation is reached this is controlled by the diffusion of the counter-ions through the film from the solution. This hypothesis seems to be confirmed by the current decay after the chronoam-perometric maximum is reached. We will focus now on the diffusion control. 

In the following, after a brief description of the experimental setup and procedures (Section 13.2), we will first focus on the adsorption and on the coverage and composition of the adlayer resulting from adsorption of the respective Cj molecules at a potential in the Hup range as determined by adsorbate stripping experiments (Section 13.3.1). Section 13.3.2 deals with bulk oxidation of the respective reactants and the contribution of the different reaction products to the total reaction current under continuous electrolyte flow, first in potentiodynamic experiments and then in potentiostatic reaction transients, after stepping the potential from 0.16 to 0.6 V, which was chosen as a typical reaction potential. The results are discussed in terms of a mechanism in which, for methanol and formaldehyde oxidation, the commonly used dual-pathway mechanism is extended by the possibility that reaction intermediates can desorb as incomplete oxidation products and also re-adsorb for further oxidation (for the formic acid oxidation mechanism, see [Samjeske and Osawa, 2005 Chen et al., 2006a, b Miki et al., 2004]). 

The current efficiencies for CO2 formation and formic acid formation during poten-tiodynamic formaldehyde oxidation, calculated from the data in Fig. 13.3b as the ratio of the partial currents to the total faradaic current (in %), are plotted in Fig. 13.4a. 

The calibrated m/z = 44 and m/z = 60 ion currents were converted into the respective partial reaction faradaic currents as described above, and are plotted in Fig. 13.3c as dashed (m/z = 44) and dash-dotted (m/z = 60) lines, using electron numbers of 6 electrons per CO2 molecule and 4 electrons per formic acid molecule formation. The calculated partial current for complete methanol oxidation to CO2 contributes only about one-half of the measured faradaic current. The partial current of methanol oxidation to formic acid is in the range of a few percent of the total methanol oxidation current. The remaining difference, after subtracting the PtO formation/reduction currents and pseudocapacitive contributions as described above, is plotted in Fig. 13.3c (top panel) as a dotted line. As mentioned above (see the beginning of Section 13.3.2), we attribute this current difference to the partial current of methanol oxidation to formaldehyde. This way, we were able to extract the partial currents of all three major products during methanol oxidation reaction, which are otherwise not accessible. 

The current efficiencies for the different reaction products CO2, formaldehyde, and formic acid obtained upon potential-step methanol oxidation are plotted in Fig. 13.7d. The CO2 current efficiency (solid line) is characterized by an initial spike of up to about 70% directly after the potential step, followed by a rapid decay to about 54%, where it remains for the rest of the measurement. The initial spike appearing in the calculated current efficiency for CO2 formation can be at least partly explained by a similar artifact as discussed for formaldehyde oxidation before, caused by the fact that oxidation of the pre-formed COacurrent efficiency. The current efficiency for formic acid oxidation steps to a value of about 10% at the initial period of the measurement, and then decreases gradually to about 5% at the end of the measurement. Finally, the current efficiency for formaldehyde formation, which was not measured directly, but calculated from the difference between total faradaic current and partial reaction currents for CO2 and formic acid formation, shows an apparently slower increase during the initial phase and then remains about constant (final value about 40%). The imitial increase is at least partly caused by the same artifact as discussed above for CO2 formation, only in the opposite sense. 

Figure 10.7 Total ion current chromatograms obtained after headspace SPME for (a) incense from Mount Athos and (b) B. papyrifera olibanum. Peak labels correspond to compound identification given in Table 10.3. The occurrence of the following biomarkers of B. papyrifera olibanum in the incense from Mount Athos are a clear indication of its botanical origin n octanol (18), n octylacetate (40), incensole (127), incensole acetate (129), incensole oxide (130) and incensole oxide acetate (131). Artefacts. Reproduced from S. Hamm, J. Bleton,). Connan, A. Tchapla, Phytochemistry, 66, 1499 1514. Copyright 2005 Elsevier Limited...

The correlation between the total oxidation charge and the coverage was studied using data shown in Figs. 3-17. The oxidation current (= current with methanol — current without methanol ) was integrated to obtain the total oxidation charge ... 

The accumulated ozone dose since June 1, which is associated with the current-year and 1-yr-old needle injury expressed on the left of Figure 12-17, can be roughly estimated by transferring the injury score to the right. For example, the current-year needle score of 2.2 on September 7 is associated with a total-oxidant dose of 2.75 x 10 Mg/m -h. These results assume that all the air monitoring data were available, but... 

Again, points on the curve were the measured acrolein production rates, and the line is the predicted production rate based on the current and the stoichiometry according to eq 9. At higher conversions, we observed significant amounts of CO2 and water, sufficient to explain the difference between the acrolein production and the current. It should be noted that others have also observed the electrochemical production of acrolein in a membrane reactor with molybdena in the anode. The selective oxidation of propylene to acrolein with the Cu—molybdena— YSZ anode can only be explained if molybdena is undergoing a redox reaction, presumably being oxidized by the electrolyte and reduced by the fuel. By inference, ceria is also likely acting as a catalyst, but for total oxidation. 

Upon further increase of voltage, the current reaches I Ed and begins to increase again. This increase is not due to the oxidation of the primary iron species, but rather results from the contribution of the reduction of protons (5.4) or oxidation of water (5.5) to the total cell current. 

This has in turn been related to the relative stability of the OMME compared to the ethylene reactant and the epoxide product [11]. It has been argued that the relative instability of the OMME intermediate on Ag compared to Group VIII metals is the main origin of the unique activity of Ag as an effective epoxidation catalyst. Whether this simple interpretation is correct remains to be seen and will require considerable further investigations. In our current studies, we propose to shed light on the competitive partial oxidation and total oxidation channels with ab initio derived microkinetic modeling [61]. 

For each monomer and ionic liquid, measurement of the total cathodic charge passed during reduction of the polymers in the final post-polymerization CVs, compared to the peak polymer oxidation currents from the final growth cycles, allows comparison of the film electrochemical activities while taking into account the relative amounts of the polymer. The former value is often used as an indication of the amount of polymer grown, but this assumes that the electrochemical activities of the films are identical. 

Cation vacancy and anion vacancy currents can similarly lead to oxide growth. To generalize, we can state that the total oxide growth rate will be the algebraic sum of all such contributions. Mathematically, this can be written as... 

---