Radical Intermediates in the Redox Reactions of Tetrazolium Salts in Aprotic Solvents (Cyclovoltammetric, EPR and UV-VIS study)

Tetrazolium Blue (TBCl₂) and Nitrotetrazolium Blue (NTBCl₂) cathodically reduced in non aqueous solvents form radicals with the center of unpaired electron on the tetrazolyl ring (TBH₂, NTBH₂) as detected by EPR spectroscopy. After prolonged reduction, formazans (TBH₂, NTBH₂) are formed and are then further reduced to the nitro-centered anion radical (from NTBH₂) and the azogroup-centered anion radical (from TBH₂). The first cathodic peak in the cyclovoltammetric study in the region from -0.3 to -0.6 V vs. SCE (saturated calomel electrode) is irreversible and indicates an adsorption and diffusion process on the platinum and mercury electrodes. Formation of TBH⁺ and NTBH⁺ is assumed. The second peak, in the region from -0.8 to -1.3 V vs. SCE, is nearly reversible and coupled with the formation of TBH- and NTBH- radicals. UV-VIS spectra measured during the reduction show isosbestic points at the conversions: TB⁺⁺ → TBH⁺, NTB⁺⁺ - NTBH⁺; further, TBH⁺ → TBH₂ and NTBH⁺ → NTBH₂. The characteristic colours of the solutions observed can be used to characterise the reduction state of tetrazolium salts.