Methyl viologen solutions

Gurunathan, K., Maruthamuth, R, Sastri, M.V.C. 1997. Photocatalytic hydrogen production by dye-sensitized Pt/SnOj and Pt/SnOj/RuO, in aqueous methyl viologen solution. Int J Hydrogen Energy 22 57-62. 

The divalent cation methyl viologen is highly colored in reduced form and is used as an electrochromic in solution cells (Eig. 11). 

Methyl viologen (l,T-dimethyl-4,4 -bipyridylium dichloride, MV " ) promotes photoanodic dissolution in aerated CdS solution Figure 8 shows how the rate of dissolution depends on the concentration. The colloid has a weak fluorescence at 620 nm which is quenched by. The curves for fluorescence and dissolution in Fig. 8 are symmetric, which indicates that the two processes have a common intermediate that reacts with M. These effects are explained by the following mechanism ... 

Photoanodic dissolution in the presence of air and its promotion by methyl viologen was also observed for alkaline solutions of colloidal cadmium phosphide, CdjPj and cadmium arsenide, CdjAsj Bismuth sulfide, Sb Sj, photo-dissolves in the presence of air mainly according to... 

The metal formed is deposited on the colloidal particles and causes the absorption of the solution to increase at all wavelengtlK. The depositel cadmium is very reactive. It is reoxidized when air is admitted to the illuminated solution. Addition of methyl viologen to the illuminated solution under the exclusion of air yields the blue colour of the semi-ceduced methyl viologen, MV". As the absorption coefficient of MV" is known, the concentration of reduced cadmium can be readily calculated. reoxidises cadmium atoms ... 

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... 

Instead of postulating Zn," as intermediate, as it has a highly negative potential and is possibly unstable in ZnO, one may write the above mechanism with Zn e pairs. The blue-shift in the absorption upon illumination was explained by the decrease in particle size. The Hauffe mechanism was abandoned after it was recognized that an excess electron on a colloidal particle causes a blue-shift of the absorption threshold (see Fig. 19). In fact, in a more recent study it was shown that the blue-shift is also produced in the electron transfer from CH2OH radicals to colloidal ZnO particles When deaerated propanol-2 solutions of colloidal ZnO were irradiated for longer times, a black precipitate of Zn metal was formed. In the presence of 10 M methyl viologen in the alcohol solution, MV was produced with a quantum yield of 80 %... 

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. 

The second-generation 02" biosensors are mainly based on the electron transfer of SOD shuttled by surface-confined or solution-phase mediators, as shown in Scheme 2(b). In 1995, Ohsaka et al. found that methyl viologen could efficiently shuttle the electron transfer between SOD and the glassy carbon electrode and proposed that such a protocol could be useful for developing 02 biosensors [125], Recently, Endo et al. reported an 02, biosensor based on mediated electrochemistry of SOD [148], In that case, ferrocene-carboxaldehyde was used as the mediator for the redox process of SOD. The as-developed 02 biosensor showed a high sensitivity, reproducibility, and durability. A good linearity was obtained in the range of 0 100 pM. In the flow cell system, tissue-derived 02 was measured. 

Fig. 11 Transient spectra obtained upon the application of a 200-fs laser pulse to a solution of [Ph2C(0H)C02, MV2+] charge-transfer salt showing the simultaneous formation of benzophenone ketyl radical (dashed line) and the reduced methyl viologen (dotted line). The inset is the authentic spectrum of ketyl radical. Reproduced with permission from Ref. 92a.

Also, the carboxylation of oxoglutaric acid to form isocitric acid catalyzed by isocitric acid dehydrogenase (ICDH) can be driven electrochemically using methyl viologen as mediator without reduced pyridine nucleotides as cofactors. Thus, electrolysis at — 0.95 V vs SCE of a 0.2 M tris buffer solution (pH 7.7)... 

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... 

In the presence of earlier generations, the luminescence quenching of aqueous solutions of Ru(L)32+ by methyl viologen (Figure 13.11) was found to follow Stern-Volmer bimolecular kinetics with a quenching constant /cq 5 x 109 M-1s-1, typical for the bimolecular quenching in homogeneous solutions. This... 

Methyl viologen (MV) is used as a mediator during the one-eiectron reduction of an enzyme. The concentration of the enzyme is thought to be about 10 moi dm. How iong will it take the MV to reduce ail the enzyme if added to 1 dm of enzyme solution, and the maximum current passed is 1 pA ... 

Worked Example 8.1. A reductive current is passed through an aqueous solution of methyl viologen dication to effect the reaction + e MV ". The... 

When the methyl viologen reached its reductive equilibrium, noted by the start of hydrogen evolution, the enzyme in the side arm was tipped into the main compartment, and the reaction stopped after 10 min. The solution in the main compartment of the flasks were analyzed for ferrous ion concentration, which is a measurement for metal corrosion. 

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