SPECTROSCOPIC PARAMETERS, ELECTRODE-POTENTIALS, ACID IONIZATION-CONSTANTS, AND ELECTRON-EXCHANGE RATES OF THE 2,2'-AZINOBIS(3-ETHYLBENZOTHIAZOLINE-6-SULFONATE) RADICALS AND IONS

The characteristics of the colorless ABTS2- ion [ABTS - 2,2-azinobis(3-ethylbenzothiazoline-6-sulfonate)] and of the persistent, intensely-colored radical ABTS- have been examined since some reported values are in disagreement. The standard reduction potentials of ABTS-/ABTS2- and ABTS.-/HABTS- are 0.68 and 0.81 V vs NHE, respectively. A second wave in the CV is associated with ABTS.-/ABTS.-, E1/2 = 1/09 V. The pH dependence of E-degrees leads to pK(a)(HABTS-) = 2.2 +/- 0.3; a spectrophotometric pH titration gives 2.08 +/- 0.02 at mu = 1.0 M. The protonated radical HABTS' was not detected (pK(a) < 0), consistent with the higher acidity of the radical relative to its reduced precursor. The EPR spectrum of ABTS.- shows a multiline spectrum centered at g = 2.0036 +/- 0.0004 from the hyperfine coupling to two sets of two equivalent nitrogens as well as six aromatic hydrogens. Line broadening of the methyl and aromatic resonances of ABTS2- in the presence of the radical gives an electron exchange rate constant of (4 +/- 1) x 10(7) L mol-1 s-1 for ABTS2-/ABTS.- in neutral aqueous solution. In 1.0 M perchloric acid, Fe3+ oxidizes HABTS- (k = 1.30 x 10(2) L mol-1 s-1) and Fe2+ reduces ABTS- (k = 7.5 x 10(2)L mol-1 s-1). Although the equilibrium constant K = k(f)/k(r) for the reaction agrees with that predicted by the measured potentials, the individual rate constants are much lower than those one calculates from the Marcus cross-relation, which is typical for reactions of Fe(H2O)63+ and Fe(H2O)62+.