Methylene blue, oxidation with

The observed rate laws for the methylene blue oxidations of both mercaptoethanol and dithioerythritol at the different pH s are consistent with the mechanisms proposed for these oxidations. The most significant effect of the pH on the reaction is that of establishing the relative amounts of thiol and sulfide ion. Initiation of the chain sequence by the reaction of a sulfide ion with MB+ would be expected to be more rapid, and therefore the overall oxidation. faster, if the pH of the medium is increased. However, at the higher pH s where the relative amount of undissociated thiol is small, the reaction rate diminishes indicating not only that the thiol is likely a reactant in the overall reaction but that it is involved in a rate limiting step of the reaction at higher pH s. While not as supportive as the... 

The effects of pH, ionic strength, reagent concentration ratios and deuterium substitution of the sulfur-bonded hydrogens on both the rates and rate laws for the methylene blue oxidations of mercaptoethanol and dithioerythritol have been determined. A free radical chain mechanism consistent with the observed kinetic behavior of the oxidation reactions is proposed. A key feature of the proposed mechanism is the formation of the sulfur-sulfur linkage of the disulfide in the reversible formation of a disulfide radical anion (RSSR) as a chain propagating step in the chain sequence. 

Several variations of the chemical method are in use. In the one described below, a freshly prepared Fehling s solution is standardised by titrating it directly against a standard solution of pure anhydrous glucose when the end-point is reached, I. e., when the cupric salt in the Fehling s solution is completely reduced to cuprous oxide, the supernatant solution becomes completely decolorised. Some difficulty is often experienced at first in determining the end-point of the reaction, but with practice accurate results can be obtained. The titrations should be performed in daylight whenever possible, unless a Special indicator is used (see under Methylene-blue, p. 463). 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Photochemical irradiation of dimethyl and diethyl sulphoxides yields the corresponding sulphone in the presence of air and a photosensitizer such as methylene blue in yields up to 99% . Sulphoxides are also oxidized when they act as traps for persulphoxides, the intermediate formed on reaction of a sulphide with photochemically generated singlet oxygen - , equation (9). Isotope studies have shown that such reactions proceed through a linear sulphurane intermediate . Persulphones also react with sulphoxides in a similar manner , equation (10). 

We can now explain how an electrochromic car mirror operates. The mirror is constructed with II in its colourless form, so the mirror functions in a normal way. The driver activates the mirror when the anti-dazzle state of the mirror is required, and the coloured form of methylene blue (MB+) is generated oxidatively according to Equation (7.24). Coloured MB+ blocks out the dazzling reflection at the mirror by absorbing about 70 per cent of the light. After our vehicle has been overtaken and we require the mirror to function normally again, we reduce MB+ back to colourless MB0 via the reverse of Equation (7.24), and return the mirror to its colourless state. These two situations are depicted in Figure 7.6. 

Photolysis of nicotine (173) in the presence of oxygen and methylene blue with light wavelengths greater than 300 nm gave the pyrrolidone (174, 30%), nicotyrine (175, 23%) and TV-oxide (176, 7%). Under nitrogen, with eosin as photosensitizer, only nicotyrine (14%) was identified in the photodegraded mixture [112]. 

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