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) cathodical ly reduced in non aqueous solvents form radicals with the center of un paired electron on the tetrazolyl ring (TBH., NTBH.) as detected by EP R spectroscopy. After prolonged reduction, formazans (TBH2, NTBH2) are formed and are then further reduced to the nitro-centered anion radic al (from NTBH2) and the azogroup-centered anion radical (from TBH2). T he first cathodic peak in the cyclovoltammetric study in the region fr om -0.3 to -0.6 V vs. SCE (saturated calomel electrode) is irreversibl e and indicates an adsorption and diffusion process on the platinum an d mercury electrodes. Formation of TBH+ and NTBH+ is assumed. The seco nd peak, in the region from -0.8 to -1.3 V vs. SCE, is nearly reversib le and coupled with the formation of TBH. and NTBH. radicals. UV-VIS s pectra measured during the reduction show isosbestic points at the con versions: TB++ --> TBH+, NTB++ --> NTBH+ further, TBH+ --> TBH2 and NT BH+ --> NTBH2. The characteristic colours of the solutions observed ca n be used to characterise the reduction state of tetrazolium salts.