Carbon electrode potentials

HPLC with ED, vitreous carbon electrode, potential range 0.72-0.85 V. 

FIGURE 5.17 CVs for a Oj-saturated 0.10 M Bu4NPFyMeCN solution at (a) unmodified and (b) Cu-MOF-modified glassy carbon electrode. Potential scan rate, 50 mV/sec. 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

In contact with a solid material, eg, steel, exceeding the carbon electrode potential for a gas means that carbon can precipitate as pure graphite on the solid. The solid saturates with carbon, until the excess settles out on the surface. 

Carbazide, diphenyl-in chromiuin(III) analysis, 523 liquid-liquid extraction, 545 Carbon electrodes potential range aqueous solution. 480 Carbon monoxide... 

Figure 2. Scanning electron micrograph of polymeric (TMHPP)Ni (a). An impedance spectrum for thin-layer polymeric film on a solid electrode (b). An impedance spectrum for polymeric-(TMHPP)Ni film on a glassy carbon electrode (potential 0.52 V, film thickness 0.4 j im) (c).

Another important feature for lithium graphite intercalation compounds in Li -containing electrolytes is the formation of solid electrolyte interface (SEI) film. During the first-cycle discharge of a lithium/carbon cell, a part of lithium atoms transferred to the carbon electrode electrochemically will react with the nonaque-ous solvent, which contributes to the initial irreversible capacity. The reaction products form a Lb-conducting and electronically insulating layer on the carbon surface. Peled named this film as SEI. Once SEI formed, reversible Lb intercalation into carbon, through SEI film, may take place even if the carbon electrode potential is always lower than the electrolyte decomposition potential, whereas further electrolyte decomposition on the carbon electrode will be prevented. 

Iodine has the lowest standard electrode potential of any of the common halogens (E = +0.54 V) and is consequently the least powerful oxidising agent. Indeed, the iodide ion can be oxidised to iodine by many reagents including air which will oxidise an acidified solution of iodide ions. However, iodine will oxidise arsenate(lll) to arsenate(V) in alkaline solution (the presence of sodium carbonate makes the solution sufficiently alkaline) but the reaction is reversible, for example by removal of iodine. 

The purity of a sample of K3Fe(CN)6 was determined using linear-potential scan hydrodynamic voltammetry at a glassy carbon electrode using the method of external standards. The following data were obtained for a set of calibration standards. 

Another important potential appHcation for fuel cells is in transportation (qv). Buses and cars powered by fuel cells or fuel cell—battery hybrids are being developed in North America and in Europe to meet 2ero-emission legislation introduced in California. The most promising type of fuel cell for this appHcation is the SPEC, which uses platinum-on-carbon electrodes attached to a soHd polymeric electrolyte. 

One criterion for the anode material is that the chemical potential of lithium in the anode host should be close to that of lithium metal. Carbonaceous materials are therefore good candidates for replacing metallic lithium because of their low cost, low potential versus lithium, and wonderful cycling performance. Practical cells with LiCoOj and carbon electrodes are now commercially available. Finding the best carbon for the anode material in the lithium-ion battery remains an active research topic. 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

There are some other matters that should be considered when comparing metallic lithium alloys with the lithium-carbons. The specific volume of some of the metallic alloys can be considerably lower than that of the carbonaceous materials. As will be seen later, it is possible by selection among the metallic materials to find good kinetics and electrode potentials that are sufficiently far from that of pure lithium for there to be a much lower possibility of the potentially dangerous forma-... 

FIGURE 4-5 Accessible potential window of platinum, mercury, and carbon electrodes in various supporting electrolytes. 

S.2.2 Carbon Electrodes Solid electrodes based on carbon are currently in widespread use in electroanalysis, primarily because of their broad potential window, low background current, rich surface chemistry, low cost, chemical inertness, and suitability for various sensing and detection applications. In contrast, electron-transfer rates observed at carbon surfaces are often slower than those observed at metal electrodes. The electron-transfer reactivity is strongly affected by the origin... 

Fig. 17. Cyclic voltammogram of the water-soluble Rieske fragment from the bci complex of Paracoccus denitrificans (ISFpd) at the nitric acid modified glassy carbon electrode. Protein concentration, 1 mg/ml in 50 mM NaCl, 10 mM MOPS, 5 mM EPPS, pH 7.3 T, 25°C scan rate, 10 mV/s. The cathodic (reducing branch, 7 < 0) and anodic (oxidizing branch, 7 > 0) peak potentisds Emd the resulting midpoint potential are indicated. SHE, standEU d hydrogen electrode.

The redox properties of a series of heterometal clusters were assessed by electrochemical and FPR measurements. The redox potentials of derivatives formed in D. gigas Fdll were measured by direct square wave voltammetry promoted by Mg(II) at a vitreous carbon electrode, and the following values were determined 495, 420,... 

Glassy carbon electrodes polished with alumina and sonicated under clean conditions show activation for the ferrl-/ ferro-cyanlde couple and the oxidation of ascorbic acid. Heterogeneous rate constants for the ferrl-/ ferro-cyanlde couple are dependent on the quality of the water used to prepare the electrolyte solutions. For the highest purity solutions, the rate constants approach those measured on platinum. The linear scan voltammetrlc peak potential for ascorbic acid shifts 390 mV when electrodes are activated. 

Most phenolic compounds are readily oxidized at carbon electrodes. The oxidation potentials vary widely depending upon the number of ring hydroxyl groups and their positions on the ring. Many compounds of biomedical and industrial interest are phenolic and LCEC based trace determinations are quite popular. 

While most amino acids are not electroactive at analytically usable potentials at carbon electrodes, much work is currently directed at general methods of LCEC amino acid detection by electrode surface modification or derivatization of the amino acid. Kok et al. have directly detected amino acids at a copper electrode. Several derivatization methods for amino acids have also been reported 227.228)... 

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