A Fast Kinetic Study of Formation and Decay of 2,2′-Azinobis(3-ethylbenzothiazole-6-sulfonate) Radical Cation in Aqueous Solution

Abstract

The kinetics of oxidation of 2,2′-azinobis(3-ethylbenzothiazole-6-sulfonate) (ABTS) to the radical cation, ABTS·+ by S2O82−, HSO5−, MnO4−, Cr2O72−, Ce(IV), and molecular Cl2, Br2, and I2 and further oxidation to the dication, ABTS2+ by the above oxidants except HSO5−, Br2, and I2 have been carried out by the stoppedflow spectrophotometric technique. The first-stage oxidation was followed by monitoring the absorption of ABTS·+ at 417 nm and the second-stage oxidation, by following the disappearance of ABTS·+ or the appearance of ABTS2+ at 513 nm. Both the reactions, formation and decay of ABTS·+ were found to obey a total second-order kinetics, first-order each with respect to [ABTS] or [ABTS·+] and [oxidant]. With I2, ABTS underwent reversible reaction, the stoichiometry of which being,I2+2ABTS\ightleftarrows2ABTS·++2I− with K=142.8,whereasBr_2generatedonlytheradicalcationandCl_2gaveadecomposedproductviaradicalcationanddicationintermediates.ABTS^2+formedinpresenceofexcessMnO_4^-,Cr_2O_7^2-,andCe(IV)decayedasinthecaseofCl_2reactionbutinpresenceofS_2O_8^2-thedicationwasstableformorethanthreedays.IntheabsenceofexcessS_2O_8^2-,ABTS^2+wentbacktoABTS^·+,possiblyviaoxidationofwater.TheradicalcationwasconvertedbacktotheparentcompoundABTSbyreducingagentssuchasdithioniteandsulfiteions.