Evaluation of kinetic parameters and redox mechanism of quinoxaline at glassy carbon electrode

Abstract

The electrochemical behavior of a biologically important heterocyclic compound quinoxaline (QUI) was investigated by cyclic voltammetry (CV) in solutions of differing pH, using a glassy carbon electrode (GCE). The reduction of QUI occurs as a quasi-reversible reaction in acid medium, reaching reversibility in alkaline solutions. The kinetic parameters of the electrode process such as ?n?, diffusion coefficient (D) and heterogeneous rate constant (ks), were evaluated and discussed. Redox mechanism of QUI was proposed on the basis of experimental results. Reduction process involves a transfer of two electrons and two protons at the pyrazine ring of QUI forming a dihydro-derivative. In acid solutions, the product of QUI reduction undergoes irreversible oxidation in a one-electron process. The electrode processes was found to be diffusion controlled. [Projekat Ministarstva nauke Republike Srbije, br. 172033]

Voltammetry study of quinoxaline in aqueous electrolytes

“…In lower pH electrolytes (pH 8.6), the measured redox potentials were E À0.19 V vs. RHE. These values (in acid and buffer) are in good agreement with literature [32,33]. In alkaline electrolytes, the redox potential was E %À0.02 V vs. RHE, with slight variation depending on the electrolyte.…”

“…By rounding each entry for n in Table 3, the nearest integer value is always n = 2, providing strong evidence that the quinoxaline redox processes incorporated two electron transfers. This result is in agreement with previous literature which reported two electron transfer behavior in aqueous electrolytes [29][30][31][32][33], although only recent work by Aleksi c et al quantitates the process [32]. Thus, on this basis, we conclude that quinoxaline exhibits a two-electron transfer process during redox reactions, leading to a theoretical capacity of 410 mAh g À1 .…”

“…11, which implies that the quinoxaline redox mechanism involved H + . Similar results of pH dependent redox potentials for quinoxaline derivatives were observed in buffered aqueous experiments [29,32,33] and in a study of quinoxaline voltammetry in DMF / water-mixed acidic media [45].…”

“…Prior studies hypothesized that under acidic conditions, quinoxaline undergoes a coupled 2 e À / 2H + electrochemical reaction where quinoxaline reduces to 1,4-dihydroquinoxaline [29][30][31][32]. Prior work also noted that 1,4-dihydroquinoxaline can undergo a H + catalyzed chemical reaction to produce 1,4-dihydroquinalin-1-ium, followed by an additional electrochemical reaction to an unagreed upon quinoxaline derivative [30][31][32]. Furthermore, spontaneous chemicallydriven protonation events were observed when quinoxaline was dissolved in acidic solutions [44].…”

“…Aqueous quinoxaline was recently applied as a redox active compound in a solar-rechargeable RFB device, however, exploration of quinoxaline electrochemical behavior was not the focus of that study [28]. Finally, several studies investigated quinoxaline electrochemical behavior in acidic and near-neutral electrolytes with a focus on determining kinetic rate parameters and related organic chemistry [29][30][31][32][33].…”