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 (TBH., NTBH.) 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.