Anal. Chem. 1997, 69, 650-658
Square Wave Voltammetry for Two-Step Surface
Reductions
John J. O’Dea and Janet G. Osteryoung*
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
In this work, we examine the effect of a second, irreversible
reaction on the normal redox cycling of a surface. Such reactions
are observed or expected under sufficiently harsh conditions for
a wide variety of systems, including redox-modified surfaces,
conducting polymer films, sensors, and “self-assembled” micro-
structures populated with redox sites. Modeling the surface
reactions as first-order processes provides a basis for quantitative
interpretation of square wave voltammograms. With a suitable
model, the COOL algorithm4 also permits the refinement of
electrochemical parameters from experimental data. The resulting
rate constants characterize the complex behavior of the system.
Here we develop a mathematical description of a two-step surface
reaction and demonstrate its application to the redox behavior of
an adsorbed dye. We restrict ourselves to the first-order model,
which is widely applicable to pseudo-first-order reduction of
organic compounds in buffered aqueous solution.
Strongly adsorbed species on an electrode surface are
used to create a stable, redox-modified surface. Square
wave voltammetry is then used to degrade the surface
electrochemically, as evidenced by the resulting voltam-
metric response. This process can be mathematically
modeled as a quasi-reversible surface reaction coupled
with a first-order irreversible surface reaction of the
product. This is the simplest possible model that can
explain a two-step surface reduction. Exemplary calcula-
tions for square wave voltammetry show a wide variety of
peak shapes depending on rate constants and square wave
amplitude. The reduction of Dimethyl Yellow (4 -(dimeth-
ylamino)azobenzene) adsorbed on mercury is accurately
described by this model. Characteristic parameters of the
overall surface process are obtained from voltammograms
by using the two-step model with nonlinear least-squares
analysis (COOL). For Dimethyl Yellow in Britton-Rob-
inson buffer (pH 6 .0 0 ) at a surface concentration of 1 7 .3
pmol cm-2 , these parameters are as follows: standard
Electrochemical data concerning the surface electron transfer
reactions of azo dyes are sparse, in spite of the widespread use
of such reactions in textiles and newly found applications in
nonlinear optics5,6 and optoelectronics.7,8 Electrochemical kinetics
and energetics yield information about molecular energy levels
and may be useful for predicting the photochemical reactivity9 of
these types of materials. Here we demonstrate the application of
a two-step model using the surface reduction of the azo dye
Dimethyl Yellow (4-(dimethylamino)azobenzene) as an example.
Adsorptive accumulation combined with square wave voltammetry
has already been used for the trace level determination of
azobenzene.10 In this case, the reversible turnover of the azo/
hydrazo moiety is exploited for increased analytical sensitivity.
Dimethyl Yellow also undergoes such redox cycling on the
electrode surface. This cycling constitutes the first step of a two-
step reduction model. In mildly acid solutions, the hydrazo
derivative produced by the first step gradually degrades to amines
by reductive cleavage of the nitrogen-nitrogen bond. Cleavage
to amines destroys the hydrazo group and any possibility for
0
potential, E1 ) -0 .3 9 7 ( 0 .0 0 1 V vs SCE; transfer
coefficient for the first step, r1 ) 0 .4 3 ( 0 .0 2 ; rate
0
constant for the first step, k1 ) 1 0 3 ( 8 s-1 ; transfer
coefficient for the second step, r2 ) 0 .1 1 ( 0 .0 4 ; and
0
0
rate constant for the second step, k2 (referenced to E1
) 1 1 .1 ( 1 .7 s-1
Uncertainties are 9 5 % confidence
intervals derived from a pool of 1 1 voltammograms
)
.
collected at different square wave amplitudes (Esw
0 -1 0 0 mV).
)
In previous work, we have treated charge transfer reactions
of adsorbates using as examples the totally irreversible reduction
of midazolam1 and the quasi-reversible reduction of azobenzene.2
Many adsorbed species, however, do not undergo a single simple
surface reaction. The cathodic reduction of an organic adsorbate
usually activates an intermediate that reacts either cathodically
again or with the surrounding environment. Such secondary
reactions are easily detected using square wave voltammetry,3
which repetitively drives the potential of the electrode to values
selected to cause oxidization and reduction. Chemical complexity
of a surface process may be revealed or concealed by the shape
of the voltammetric response.
(4) O’Dea, J. J.; Osteryoung, J. G.; Lane, T. J. Phys. Chem. 1 9 8 6 , 90, 2761.
(5) Ross, D. L.; Blanc, J. Photochromism by cis-trans Isomerization. In
Techniques in Chemistry, Vol. 3, Photochromism; Brown, G. H., Ed.; Wiley-
Interscience: New York, 1975; pp 471-556.
(6) Rau, H. Azo Compounds. In Photochromism. Molecules and Systems; Durr,
H., Bouas-Laurent, H., Eds.; Elsevier: New York, 1990; pp 165-192.
(7) Yabe, A.; Kawbata, Y.; Niino, H.; et al. Thin Solid Films 1 9 8 8 , 160, 33.
(8) Yariv, E.; Reisfeld, R.; Weiss, A. Proceedings of the SPIE, Optoelectronics
and Applications in Industry and Medicine, Vol. 1972, 8th Meeting on Optical
Engineering in Israel, 1992; pp 46-54.
(1) O’Dea, J. J.; Ribes, A.; Osteryoung, J. G. J. Electroanal. Chem. 1 9 9 3 , 345,
287.
(2) O’Dea, J. J.; Osteryoung, J. G. Anal. Chem. 1 9 9 3 , 65, 3090.
(3) Osteryoung, J. G.; O’Dea, J. J. Square Wave Voltammetry. In Electroanalytical
Chemistry, Vol. 14; Bard, A. J., Ed.; Marcel Dekker: New York, 1986; pp
209-308.
(9) Rifi, M. R.; Covitz, F. H. Introduction to Organic Electrochemistry; Marcel
Dekker: New York, 1974; p 7.
(10) Xu, G.; O’Dea, J. J.; Mahoney, L. A.; Osteryoung, J. G. Anal. Chem. 1 9 9 4 ,
66, 808.
650 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
S0003-2700(96)00647-6 CCC: $14.00 © 1997 American Chemical Society