Half-wave potentials, sulfur

The ESR spectrum of the pyridazine radical anion, generated by the action of sodium or potassium, has been reported, and oxidation of 6-hydroxypyridazin-3(2//)-one with cerium(IV) sulfate in sulfuric acid results in an intense ESR spectrum (79TL2821). The self-diffusion coefficient and activation energy, the half-wave potential (-2.16 eV) magnetic susceptibility and room temperature fluorescence in-solution (Amax = 23 800cm life time 2.6 X 10 s) are reported. 

Kihara et al. employed flow coulometry to study the electrode reactions for Np ions in various acidic media [49]. Flow coulometry has an inherent advantage over the conventional hulk coulometry methods in that the electrolysis can be achieved rapidly to aid in the characterization of unstable electrode products. The resulting coulopo-tentiograms for the Np02 /Np02 and Np /Np " " couples indicate reversible processes in nitric, perchloric, and sulfuric acids. The differences in potentials between the various acids are attributed to the associated stability constants of the electrode products with the anion of the acid in each case. Table 2 contains the half-wave potentials for each couple in the various acids. 

The chromium(II)-edta system is powerfully reducing (the half-wave potential is -1.48 V at pH 12 vs. SCE) and has been used in the reduction of iron-sulfur clusters.292 No solid complex has been isolated because of its instability to oxidation, but Cru-edta is high-spin in aqueous solution (/ieff = 5.12 BM) and its stability constant has been determined. The edta is believed to be pentadentate with H20 in the sixth position.293... 

The oxidation-reduction potentials of metal ions differ in different solvents due chiefly to differences in the strength of coordination of the solvents to the metal ions. Thus, Schaap and coworkers,33 who measured reduction potentials polarographically in anhydrous ethylenediamine, found the order of half-wave potentials to be Cd2+ > Pb2+ > Cu2+ - Cu+ > Ti+, whereas, in aqueous solution, the order is Cd2+ > Ti+ > Pb2+ > Cu2+ -> Cu+. Oxidation—reduction potentials have been measured in a great variety of non-aqueous solvents, both protonic and non-protonic. Among the former are liquid ammonia and concentrated sulfuric acid.34 Among the latter are acetonitrile, cyanopropane, cyanobenzene, dimethyl sulfoxide, methylene chloride, acetone, tet-rahydrofuran, dimethylformamide and pyridine.34... 

In 0.1M tetraethylammonium perchlorate in acetonitrile, half-wave potentials of -1.38 and -1.52 were observed for sulindac. The reductions involve the C=C bonds and not the sulfur functionalities by analogy to the work of Hawson et al an< 1 ). ... 

The polarographic and potentiometric methods are not HPLC-run. The polarographic method relies upon the measurement of half-wave potentials of various sulfur compounds reacting with a mercury electrode. It is sensitive to submicromolar concentrations (Luther, pers. com.). While sulfide, thiosulfate, polysulfide and polythionates can be measured, the initial sample must be subdivided and pretreated in different ways. The disadvantages are that sample preparation ana analysis are time-consuming and there is no way to preserve samples for later analysis nor to study organic thiols with precision., ... 

The half-wave potentials are dependent on the nature of the substituents (R ) and of the heteroatom. Thus, electron-withdrawing groups or substituents that interfere with the coplanarity of the sulfur lone-pair electrons and the ethylene tt system tend to increase [163, 164] while electron-donating groups decrease [163, 165, 166] these values. The potentials also become higher by exchanging the sulfur atoms of TTF by Se, while the opposite effect is observed for Te [167, 168] (see Tables 4 and 5). 

The half-wave potential in alcoholic sulfuric acid solution was found to be 0.714 (volt vs, normal calomel electrode), comparable to that... 

Bozsai and Vastagh (54) determined primidone by polarography after nitrating it with a sulfuric-nitric acid mixture. The pher l group nitrates to form the 3 -nitrophenyl derivative (12), which is reducible at the dropping mercury electrode. The method was applied to tablets containing primidone, as well as to the raw material. The observed half-wave potential was -0.17 volt vs. the saturated calomel electrode. 

The reduction of pyocyanine (a phenazine derivative) in acidic solution has been investigated in [13]. In this case two one-electron waves are obtained, and the first (as in the case of phenazine) is higher than the second. In 0.01 mole/liter sulfuric acid, under the conditions used in [13], this difference amounted to 0.5 pA. With fairly high concentrations of pyocyanine (more than3 10 mole/liter) a prewave appears before the second wave, but the difference between the second wave and the first wave (together with the prewave) remains equal to 0.5 mA. The height of the prewave of the second wave does not depend on the concentration of the reactant, but its half-wave potential moves to the positive values with increase in the pyocyanine concentration. This prewave is rather better defined in hydrochloric acid solution than in sulfuric acid. 

The half-wave reduction potentials (HWP) of dibenzothiophene and some of its derivatives have been measured for comparison with those of dibenzofuran and dibenzoselenophene. A shift to more negative HWP was observed for all of the methyl derivatives studied, the magnitude of which depended on the position of substitution. These shifts are in accord with LCAO molecular orbital theory predictions if the sulfur d orbitals are excluded from the calculations (Section III, A). ... 

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