Methyl viologen oxidation

It is of interest to link ki to the rate of hydrogen peroxide decomposition. As shown in Figure 17.5, for a constant pH, the rate of oxidation correlates well with the rate of H2O2 decomposition. This observation is consistent with the idea of the presence of two channels for oxidation involving oxygen, and hydrogen peroxide as the second electron acceptor. The total rate of methyl viologen oxidation is the sum of the reaction rates in these two channels ... 

Figure 17.5 Initial rate of methyl viologen oxidation (right scale) and hydrogen peroxide destruction (left scale) as a function of added hydrogen peroxide concentration pH = 10. (Reproduced with permission from I. N. Martyanov et al.. Kinetics and Catalysis 1997, 5S(1), 70. Copyright 1997 Pleiades Publishing, Inc.)...

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

With respect to using methyl viologen as electron relay, it might be of interest to note tlmt MV " can be oxidized by positive holes produced in illuminated colloidal semiconductors such as Ti02 Two oxidation products of MV are 1, 2 -di-hydro-l,r-dimethyl-2 -oxo-4,4 -bipyridylium chloride and 3,4-dihydro-l,r-dime-thyl-3-oxo-4,4 -bipyridylium chloride, which can readily be detected by their strong fluorescences at 516 nm and 528 nm, respectively. These products are also produced in the direct photolysis of MV " solutions and in the reaction of MV "" with OH radicals in homogeneous solution... 

Later, an improved system for C02 photofixation was reported by the same authors.164 The new system consisted of 6.5 x 1(T5 M tris(2,2 -bipyridine)ruthenium(II), Ru(bpy)3, as the photosensitive electron donor, methyl viologen (MV2+, 20 mM) as the electron acceptor, and triethanolamine (TEOA, 0.6 M) as a sacrificial electron donor in a C02-saturated aqueous solution (Fig. 18). Under irradiation with a 300-W high-pressure Hg lamp with a CuS04 chemical filter (A > 320 nm), formic acid, which was detected by isotachophoresis, was produced in quantum yields of ca. 0.01%. Recently, however, Kase et al.165 have repeated this experiment using a 13C02 tracer and have claimed that the formic acid obtained was produced not by C02 reduction but rather by oxidative cleavage of TEOA. 

T. Ohsaka, Y. Shintani, F. Matsumoto, T. Okajima, and K. Tokuda, Mediated electron transfer of polyethylene oxide-modified superoxide dismutase by methyl viologen. Bioelectrochem. Bioenerg. 37, 73-76 (1995). 

Similarly, the pyruvate dehydrogenase complex (PDC) can be activated directly by electrogenerated methyl viologen radical cations (MV +) as mediator. Thus, the naturally PDC-catalyzed oxidative decarboxylation of pyruvic acid in the... 

Because Nafion film can preconcentrate organic cations at a substrate electrode surface, Nafion-coated electrodes can be used as sensitive and selective sensors for electroactive organic cations. The film-entrapped ion is then either oxidized or reduced and the resulting current is recorded and related to the concentration of ions in the aqueous solution (qualitatively similar to anodic stripping voltammetry). It was shown that ion-exchange voltammetric determination of the dication methyl viologen at a Nafion-film-coated electrode results in an improvement of three orders of magnitude in detection limit relative to an uncoated electrode.46... 

Reactions may be followed by observing the oxidation of D or rednction of A, by spectrophotometry. A simple assay uses the dye methyl viologen, which tnrns bine on reduction. Alternatively, hydrogen prodnction or consumption can be measnred in various ways ... 

However, in certain cases, the rate of electron uptake by a particular species just happens to be slow. For example, electron transfer between the methyl viologen radical cation (MV ) and hydrogen peroxide has a rate constant of 2.0 (mol dm ) s , while the reaction between MV and just about any other chemical oxidant known is so fast as to be dijfusion-controlled. The reason for this is simply not known at the present time. 

A second example of a membrane-bound arsenate reductase was isolated from Sulfurospirillum barnesii and was determined to be a aiPiyi-heterotrimic enzyme complex (Newman et al. 1998). The enzyme has a composite molecular mass of 100kDa, and a-, P-, and y-subunits have masses of 65, 31, and 22, respectively. This enzyme couples the reduction of As(V) to As(III) by oxidation of methyl viologen, with an apparent Kra of 0.2 mM. Preliminary compositional analysis suggests that iron-sulfur and molybdenum prosthetic groups are present. Associated with the membrane of S. barnesii is a h-type cytochrome, and the arsenate reductase is proposed to be linked to the electron-transport system of the plasma membrane. 

Physiologic electron acceptors flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) produced similar effects on cathodic hydrogen evolution from mild steel as achieved with methyl viologen (Bryant and Laishley 1990). These experimental results showed that the mild steel rods reacting with phosphate can preferential act as electron donors for the reduction of low-potential electron carriers. All hydrogenases catalyze a reversible reaction for the formation and oxidation of hydrogen, which requires low-potential electron carriers for the enzyme activity (Church et al. 1988 Fauque et al. 1988). 

Metal oxide, e.g. RUO2 [35,36] and WO3 [151], loaded Cd8 colloidal suspensions with and without Ft functionalization were investigated for water decomposition under visible light. WO3-Pt/Cd8, in an aqueous solution of methyl viologen (MV ) which serves as an electron relay, has been found most efficient to date in terms of water splitting, see reactions (7.4.1) to (7.4.8) [151] ... 

The conduction band electrons of Cd8 move to Ft that in turns reduces MV. The reduced methyl viologen in turn reduces a hydrogen ion to a hydrogen molecule. The valence band boles of WO3 then oxidize water to O2 molecules. 

Methyl viologen (/V, /V - d i m e t h I -4,4 - b i p r i d i n i u m dication, MV2+ ) can function as an electron acceptor.34 When MV2+ is linked to electron donor, photoinduced electron transfer would occur. For example, within molecule 24 the 3MLCT excited state of [Ru(bpy)3]2+ is quenched by MV2+ through oxidative electron transfer process. The excited state of [Ru(bpy)3]2 + can also be quenched by MV" + and MV°. The transient absorption spectroscopic investigations show that the quenching of the excited state of [Ru(bpy)3]2+ by MV + and MV° is due to the reductive electron transfer process. Thus, the direction of the photoinduced electron transfer within molecule 24 is dependent on the redox state of MV2 +, which can be switched by redox reactions induced chemically or electrochemically. This demonstrates the potential of molecule 24 as a redox switchable photodiode.35... 

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