Anodic Voltammetry and EPR Studies of Isomeric Phenylenediamines

The Effect of H-bonding and Proton Transfer on the Voltammetry of 2,3,5,6-Tetramethyl-p-phenylenediamine in Acetonitrile. An Unexpectedly Complex Mechanism for a Simple Redox Couple

The voltammetry of 2,3,5,6-tetramethyl-p-phenylenediamine, H₂PD, has been studied and compared to that of its isomer N,N,N'N'-tetramethyl-p-phenylenediamine, Me₂PD. Both undergo two reversible electron transfer processes in acetonitrile that nominally correspond to 1e- oxidation to the radical cations, Me₂PD⁺ and H₂PD⁺, and a second 1e- oxidation at more positive potentials to the quinonediimine dications, Me₂PD²⁺ and H₂PD²⁺. While the voltammetry of Me₂PD agrees with this simple mechanism, that of H₂PD does not. The second voltammetric wave is too small. UV/Vis spectroelectrochemical experiments indicate that the second wave does correspond to oxidation of H₂PD⁺ to H₂PD²⁺ in solution. The fact that the second wave is not present at all at the lowest concentrations (5 µM), and that it increases at longer times and higher concentrations, indicates that H₂PD⁺ is not the initial solution product of the first oxidation. A number of lines of evidence suggest instead that the initial product is a mixed valent, H-bonded dimer between one H₂PD in the the full reduced, fully protonated state, H₄PD²⁺, and another in the fully oxidized, fully deprotonated state, PD. A mechanism is proposed in which this dimer is formed on the electrode surface through proton transfer and H-bonding. Once desorbed into solution, it breaks apart via reaction with other H₂PD's, to give 2 H₂PD⁺, which is the thermodynamically favored species in solution.

Electrochemical consideration on the optimum pH of bilirubin oxidase

Steady-state current-potential curves were obtained for the direct electron transfer (DET) of bilirubin oxidase (BOD) at a highly oriented pyrolytic graphite electrode, and the theoretical analysis based on nonlinear regression enabled us to determine the formal redox potential (E degrees') of BOD in a wide pH range of 2.0 to 8.5. Cyclic voltammetric measurements were also performed for substrates, including p-phenylenediamine (PPD), o-aminophenol (OAP), and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and their E degrees ' values or the anodic peak potentials (for OAP) were determined at various pH values. The difference in the redox potentials between BOD and substrates (DeltaE degrees') showed a maximum at pH 6.5 to 8.0, pH 6.5 to 8.0, and pH 3.5 to 4.5 for PPD, OAP, and ABTS, respectively. These pH ranges should be thermodynamically most favorable for the electron transfer between BOD and the respective substrates. In practice, the pH ranges showing a maximum DeltaE degrees' corresponded well with the optimum pH values for the O(2) reduction activity of BOD: pH 6.5 to 7.5, pH 8.0 to 8.5, and pH 4.0 for PPD, OAP, and ABTS, respectively. Thus, it was suggested that DeltaE degrees ' should be one of the primary factors determining the activity of BOD with the substrates.

Determination of mitoxantrone by flow injection analysis using an amperometric detector

Mitoxantrone was determined by flow injection analysis using a flow cell modified in the laboratory and fitted with carbon paste as an amperometric detector. The sample solution (100 microliters, 5 x 10(-8)-1 x 10(-5) M) was injected into the carrier stream of 0.1 M perchloric acid (pH 1.12). Mitoxantrone was determined by oxidation at the carbon paste electrode (CPE) at +0.90 V. A 60-cm delay coil (0.5 mm i.d.) was incorporated just before the detector (a canal thin layer) and a flow rate of about 4 ml min-1 was used. The system was successfully applied to the determination of mitoxantrone in a pharmaceutical preparation; the method was fast and reproducible.