Oxidation current

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Electrochemical Structure closure under cathodic prepolarizatiuon Anodic potential step or scan Oxidation current... 

Dissociation equilibriums in both electrolyte and polymer gels and the ionic concentration partition (Donnand potential) between solutions and polymer gels allow189 the relaxation-oxidation current to be obtained as a function of the perchlorate concentration ... 

Figure 8.40. Transient effect of electrochemical O2 pumping to (a) and from (b) a Ag catalyst film on selectivity and yield to ethylene oxide. Current applied at t=0 pC2H4" - Pa, Poj-lO kPa T=400°C.45 Reprinted with permission from Academic Press.

OS 59] [R 34] [P 42] The furan and dimethoxylated product concentrations were monitored as a function of the cell voltage [71]. The product concentration follows the sigmoidal shape of the bromide oxidation current. The furan concentration shows the inverse behavior. For a cell voltage of 3 V and a current of 30 mA, a concentration of 50% is approached. The product formation is limited by mass transfer of bromine generation. 

Continuous CO Oxidation on Piatinum The main difference between CO stripping and continuous CO oxidation is the CO (re-)adsorption Reaction (6.3). In contrast to CO stripping, this leads a steady-state CO oxidation current because of the continuous supply of CO. In modeling the continuous CO oxidation, we also need to consider the mass transport of CO from the bulk of the solution to the electrode surface. The temporal change in the CO coverage is now given by... 

Figure 11.18 Linear cyclic voltammograms of a FePc/C disk electrode and corresponding oxidation current of a Pt ring electrode maintained at 1.2 V vs. RHE, recorded at 2500 rev min in an 02-saturated 0.5 M H2SO4 electrolyte (temperature 20 °C, sweep rate 5 mV s ).

In this section, we present results of potentiodynamic DBMS measurements on the continuous (bulk) oxidation of formic acid, formaldehyde and methanol on a Pt/ Vulcan catalyst, and compare these results with the adsorbate stripping data in Section 13.3.1. We quantitatively evaluate the partial oxidation currents, product yields, and current efficiencies for the respective products (CO2 and the incomplete oxidation products). In the presentation, the order of the reactants follows the increasing complexity of the oxidation reaction, with formic acid oxidation discussed first (one reaction product, CO2), followed by formaldehyde oxidation (two reaction products) and methanol oxidation (three reaction products). 

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

Conversion of the m/z = 44 ion current into a partial faradaic reaction current for formaldehyde oxidation to CO2 (four-electron reaction) shows that, under these experimental conditions, formaldehyde oxidation to CO2 is only a minority reaction pathway (dashed line in Fig. 13.6a). Assuming CO2 and formic acid to be the only stable reaction products, most of the oxidation current results from the incomplete oxidation to formic acid (dotted hne in Fig. 13.6a). The partial reaction current for CO2 formation on Pt/Vulcan at 0.6 V is only about 30% of that during formic acid... 

In the original proposal of the dual-pathway mechanism (for formic acid oxidation, see [Capon and Parsons, 1973a, b, c] for methanol oxidation, see [Parsons and VanderNoot, 1988 Jarvi and Stuve, 1998 Leung and Weaver, 1990 Lopes et al., 1991 Herrero et al., 1994, 1995]), both pathways lead to CO2 as the final product, as illustrated in the reaction scheme depicted in Fig. 13.8a [Jarvi and Smve, 1998]. In this mechanism, desorption of incomplete oxidation products was not included. The existence of a direct reaction pathway for methanol oxidation, following the dual-pathway mechanism, was justified by the observation of a methanol oxidation current at potentials where COad oxidation is not yet active [Sriramulu et al., 1998, 1999 Herrero et al., 1994, 1995]. The validity of this interpretation was questioned, however, by Vielstich and Xia (1995), who claimed that CO2 formation is observed only with the onset of COad oxidation and that the faradaic current measured at lower potentials is due to the formation of the incomplete oxidation products formaldehyde and formic acid. The latter findings were later confirmed by Wang et al. [2001], Korzeniewski and Childers [1998], and Jusys et al. [2001, 2003]. In more... 

By convention the reduction current is indicated as positive and the oxidation current as negative. 

Bertilsson, L. (1995). Geographical/interracial differences inpolymorphic drug oxidation current state of knowledge of cytochrome P450 (CYP) 2D6 and 209. Clin. Pharmacokinet., 29, 192— 209. 

After Ej the current is fairly constant (the limiting current) as C is reduced as soon as it reaches the electrode, so that the rate of reduction of C, and therefore the current, is limited by the rate at which C can get to the electrode from other parts of the solution. At 3 there is a large increase in current as the solvent is reduced. Similar considerations apply to the anodic part of the graph. Oxidation of B (easier than A) begins at 4 and of A at 5 (the oxidation current of A is superimposed on the limiting current of B). Finally the solvent is oxidised at . ... 

Mihovilovic, M.D. (2006) Enzyme mediated Baeyer-Villiger oxidations. Current Organic Chemistry, 10 (11), 1265-1287. 

Figure 3.35 shows the potential dependence of the integrated band intensity of the linear CO observed in the experiment described above and the corresponding variation in the methanol oxidation current. The latter was monitored as a function of potential after the chemisorption of methanol under identical conditions to those employed in the IRRAS experiments. As can be seen from the figure the oxidation of the C=Oads layer starts at c. 0.5 V and the platinum surface is free from the CO by c. 0.65 V. The methanol oxidation current shows a corresponding variation with potential, increasingly sharply as soon as the CO is removed strong evidence in support of the hypothesis that the adsorbed CO layer established at 0.4 V acts as a catalytic poison for the electro-oxidation of methanol. 

Figure 3.35 Potential dependence or the ft) integrated band intensity of the linear COad< derived from methanol at 0,4 V vs. RHE in I M CH3OH/0.5M H SO and (2) the methanol electro-oxidation current observed after the adsorption of methanol at 0.4 V. From K. Kunimatsu, Berichte der Bunsen-Ceseiischaft jur Phy-sitcafische Chemie. 1900.94. 1025 1030-...

H.I. Magazine, Detection of endothelial cell-derived nitric oxide current trends and future directions. Adv. Neuroimmunol. 5, 479-490 (1995). 

---