Spectroscopic parameters, electrode potentials, acid ionization constants, and electron exchange rates of the 2,2[prime]-azinobis(3-ethylbenzothiazoline-6-sulfonate) radicals and ions

Journal Article · Thu Jun 24 04:00:00 EDT 1993 · Journal of Physical Chemistry; (United States)

DOI:https://doi.org/10.1021/j100127a022· OSTI ID:6027937

Scott, S L; Chen, W J; Bakac, A; Espenson, J H ^[1]

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1. Iowa State Univ., Ames (United States)

The characteristics of the colorless ABTS[sup 2[minus]] ion [ABTS = 2,2[prime]-azinobis(3-ethylbenzothiazoline-6-sulfonate)] and of the persistent, intensely-colored radical ABTS[sup [sm bullet][minus]] have been examined since some reported values are in disagreement. The standard reduction potentials of ABTS[sup [sm bullet][minus]]/ABTS[sup 2[minus]] and ABTS[sup [sm bullet][minus]]/HABTS[sup [minus]] are 0.68 and 0.81 V vs NHE, respectively. A second wave in the CV is associated with ABTS[sup 0]/ABTS[sup [sm bullet][minus]], E[sub 1/2] = 1.09 V. The pH dependence of E[degrees] leads to pK[sub a](HABTS[sup [minus]]) = 2.2 [+-] 0.3; a spectrophotometric pH titration gives 2.08 [+-] 0.02 at [mu] = 1.0 M. The protonated radical HABTS[sup [sm bullet]] was not detected (pK[sub a] < 0), consistent with the higher acidity of the radical relative to its reduced precursor. The EPR spectrum of ABTS[sup [sm bullet][minus]] 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 ABTS[sup 2[minus]] in the presence of the radical gives an electron exchange rate constant of (4 [+-] 1) [times] 10[sup 7] L mol[sup [minus]] s[sup [minus]] for ABTS[sup 2[minus]]/ABTS[sup [sm bullet][minus]] in neutral aqueous solution. In 1.0 M perchloric acid, Fe[sup 3+] oxidizes HABTS[sup [minus]] (k = 1.30 [times] 10[sup 2] L mol[sup [minus]1] s[sup [minus]1]) and Fe[sup 2+] reduces ABTS[sup [sm bullet][minus]] (k = 7.5 [times] 10[sup 2] L mol[sup [minus]1] s[sup [minus]1]). Although the equilibrium constant K = k[sub f]/k[sub 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(H[sub 2]O)[sub 6][sup 3+] and Fe(H[sub 2]O)[sub 6][sup 2+]. 28 refs., 6 figs., 2 tabs.

• OSTI ID: 6027937
• Journal Information: Journal of Physical Chemistry; (United States), Journal Name: Journal of Physical Chemistry; (United States) Vol. 97:25; ISSN JPCHAX; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400102 -- Chemical &amp; Spectral Procedures
400201 -- Chemical &amp; Physicochemical Properties
400400* -- Electrochemistry
CHARGED PARTICLES
CHEMICAL REACTIONS
CHEMISTRY
DATA
ELECTROCHEMISTRY
ELECTRODES
ELECTRON SPIN RESONANCE
EXPERIMENTAL DATA
INFORMATION
IONS
IRON COMPOUNDS
MAGNETIC RESONANCE
NUCLEAR MAGNETIC RESONANCE
NUMERICAL DATA
RADICALS
REDOX REACTIONS
RESONANCE
SPECTROSCOPY
TITRATION
TRANSITION ELEMENT COMPOUNDS
VOLTAMETRY