Full text of DOI:10.1002/jccs.200200065

Journal of the Chinese Chemical Society, 2002, 49, 415-419
415
A New Redox Reaction of Ascorbic Acid at Platinum Electrodes
Bao-xian Ye* (
Bao-hui Jin (
), Lin Lin (
), Cui-hong Wang (
),
) and Ling-fang Liu (
)
Department of Chemistry, Zhengzhou University, Zhengzhou, 450052, P. R. China
A pair of new redox peaks of ascorbic acid at a platinum electrode was found and studied in detailed by
spectroelectrochemistry and electrochemistry technologies. This is a quasi-reversible redox reaction with a
one-electron transfer process. The intermediate of tertiary carbon free radical exists in this process. The ap-
pearance reaction rate constant and the diffusion coefficient were investigated. A possible reaction mecha-
nism has been proposed.
INTRODUCTION
Ascorbic acid, vitamin C (H2A), is an extremely impor-
our experiment, other redox peaks neglected by previous re-
searchers were found at Pt electrode with E0’about -0.46 V (vs
Ag/AgCl, Fig. 2, peak B and C). In this approach, we report
our investigation about this redox reaction (peak B and C) of
ascorbic acid at Pt electrode studied by electrochemistry and
spectroelectrochemistry (SEC) techniques. A possible reac-
tion mechanism is suggested.
tant biological molecule with many roles including enzyme
cofactor, reducing agent, involvement in neurotransmit-
ter-related enzymes and, of course, essential nutritional fac-
tor. During the last half century, much effort has been made to
elucidate the mechanism and the kinetics of the electrochemi-
cal oxidation of ascorbic acid on different metallic electrodes1-9
and various electrochemical techniques have been used on
this subject. Although the mechanism of this reaction at
above pH 8.0 is still disputed, a mechanism below pH 8.0 pro-
posed by Aldazet al.,11,12 who demonstrated the existence of a
radical anion as an intermediate species in the two-electron
oxidation of ascorbic acid at Pt, Ga, Au, and Hg electrodes,1,9-13
is accepted by most researchers. Fig. 1 is the scheme of the
mechanism. This mechanism involves a predissociation of a
proton to give the monoanionic species followed by a 1e-,
1H+ oxidation of the monoanionic species to form a radical
anion, which then undergoes a second irreversible 1e- oxida-
tion to dehydroascorbic acid. The latter species is rapidly
protonated and then dehydrated to yield the final product of
2,3-diketogulonic acid. The hydration rate at pH 7.2 was esti-
mated by Perone14 to be about 1.2 103 s-1. Thus, the cathodic
wave for this redox process in voltammetry is only observed
at very fast sweep rates. More recently, Azar and Nerbin7
have reported another approach for ascorbic acid oxidation at
nickel plated aluminum electrodes modified with nickel
pentacyanonitrosylferrate films. At this chemically modified
electrode, the nearly same oxidation mechanism was ob-
served as above.
For all previous investigations of ascorbic acid redox
mentioned above, the same reaction with Ep about 400 mV
(Fig. 2, peak A) at metal electrodes has been investigated. In
Fig. 1. Scheme of ascorbic acid oxidation mechanism.

416 J. Chin. Chem. Soc., Vol. 49, No. 3, 2002
Ye et al.
EXPERIMENTAL
RESULTS AND DISCUSSION
Apparatus and Materials
The cyclic voltammetry behavior of H2A at Pt electrodes
Cyclic voltammetry (CV) was performed in a standard
electrochemical cell with a 3-electrode system. Fig. 2 shows
the voltammograms of H2A (1 10-3 mol/L) at the Pt elec-
trode with different scan rates. 0.1 mol/L KCl solution is used
as electrolyte. Peak A is the oxidation peak of H2A and has
been widely investigated and reported previously.1,2 A new
pair of redox peaks (B and C) appears with peak potentials of
EpB = -0.422 V and EpC = -0.518 V (vs Ag/AgCl) at a scan
rate of 5 mV/s. This pair of peaks has never been reported be-
fore. Here, we are only interested in peaks B and C. We limit
the potential window of -0.1 V ~ -0.7 V for cyclic scan. Fig. 3
shows the CV course with different H2A concentrations. Ob-
viously, peak currents are proportional to H2A concentra-
tions.
The peak currents of IpB and IpC are 3.864 10-5 A and
2.548 10-5 A, with [H2A] = 1 10-3 mol/L and = 5 mV/s.
Both IpB and IpC are linearly related to the 1/2. The separa-
tion of EpB and EpC ( EP) increases with the increase of . It
is clear that this redox process of H2A is a quasi-reversible
and diffusion-driven process. By the electrochemical method
demonstrated by Laviron,15 we got dynamics parameters of
this redox reaction with cyclic voltammetry. They are Ks =
0.1054 s-1, n = 1 and = 0.866.
Model 650 A electrochemical system (CHI Instrument
Company, USA) was employed along with an IBM 586 com-
puter to control and treat data for electrochemical techniques.
In-situ spectroelectrochemical measurements were per-
formed with a spectrometer (UV-3000 Shimadzu, Japan)
coupled with a potential analyzer (Model 79-1, home-made)
using an optical transparent thin layer spectroelectrochem-
ical cell (Model JDS, home-made). A standard three-elec-
trode electrochemical cell was used for all electrochemical
experiments with Pt disk electrode as working electrode, Pt
wire as counter electrode and Ag/AgCl as reference elec-
trode.
All reagents were of analytical grade and were used as
received. Solutions were prepared daily. Doubly distilled wa-
ter was used for all preparations. N2 was employed to de-
oxygenize and all experiments were carried out at room tem-
perature.
Pretreatment of Pt Electrode
Platinum disk electrodes were polished first with emery
paper, alumina (0.1 m) and then cleaned with ultrasonic
waves in doubly distilled water. Having been rinsed with wa-
ter, the electrodes were cyclically scanned within potential
window of -0.2 ~ 1.4 V in 0.5 mol/L H2SO4 until a reproduc-
ible voltammogram was obtained.
Table 1 lists the data of EpC and IpC relationship with
pH solution. Obviously, both EpC and IpC are nearly change-
Fig. 3. Voltammograms of different H2A concentra-
tions [H2A] (from inner to outer): 0, 1.8 10-3
mol/L, 2.5 10-3 mol/L, 3.1 10-3 mol/L, Scan
rate: 50 mV/s.
Fig. 2. Voltammograms of H2A at different scan rates.
Scan rate (from inner to outside): 1 mV/s, 5
mV/s, 10 mV/s, 25 mV/s, 50 mV/s, [H2A] = 1
10-3 mol/L.

A New Redox Reaction of Ascorbic Acid at Platinum Electrodes
J. Chin. Chem. Soc., Vol. 49, No. 3, 2002 417
10-3mol/L)
vestigated widely and reported previously, here, we concen-
trated on the redox reactions of peaks B and C, especially on
peak C. 1 10-3 mol/L H2A with 0.1 mol/L KCl solution was
injected into the thin layer cell. Fig. 5 shows UV absorption
spectra recorded at potentials of 0.0 V and -0.8 V. We applied
the potentials up to 0.00 V for insuring the H2A complete oxi-
dation state and -0.80 V for insuring the H2A complete reduc-
tion state. In the wavelength range of 200~800 nm, the only
adsorption peak of H2A is at 265 nm. Fig. 6 shows the absorb-
ance changed at different potentials and recorded within a
wavelength range of 200~330 nm. Each spectrum was re-
corded after a potential was applied for 4 min, which was the
time for insuring reaction equilibrium. From Fig. 6, we know
that the absorption peak of H2A is decreased by moving the
potentials negatively. The absorption peak decreased greatly
within the potential range of -0.500 V ~ -0.600 V (vs SCE).
Table 1. The Effect of pH for Epc and ipc ([H2A]
1
pH
Epc (V)
ipc (1 10-5 A)
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
-0.537
-0.537
-0.537
-0.534
-0.531
-0.541
-0.537
-0.547
10.70
10.72
11.14
9.92
9.82
10.31
10.00
7.012
less below pH 8.0. It is means that the H+ does not participate
in this electrode process within pH 2~8. When pH is above 9,
the voltammogram metamorphoses obviously and the two
peaks decrease greatly.
Spectroelectrochemistry research
The volume of the spectroelectrochemistry cell was
calculated from the redox coulomb of ferricyanide/ferro-
cyanide couple. 5 10-4 mol/L ferricyanide in 0.5 mol/L KCl
was injected into a spectroelectrochemistry cell. Two-poten-
tial steps were applied on the cell. The first potential step was
from -0.10 V to 0.60 V and the potential was held at 0.60 V
for 4s. Then the second potential step was from 0.60 V to
-0.10 V and the potential was held at -0.1 V for 4s again. Fig.
4 shows the coulomb ~ time curves of this process. The cell
volume can be calculated by the equation: Q = nFVC (Q is the
coulomb between A and B in Fig. 4, V is the cell volume, C is
the concentration of ferricyanide). From this, we got the cell
volume of 29.5 l.
Fig. 5. The absorbance spectra of H2A at different po-
tentials. Potentials: (From upper to lower)
0.000 V, -0.800 V. [H2A] = 1 10-3 mol/L.
According to the oxidation reaction of peak A being in-
Fig. 6. The absorbance spectra of H2A at different po-
tentials. Potentials: (From upper to lower)
0.000 V, -0.500 V, -0.525 V, -0.550 V, -0.575
V, -0.600 V, -0.800 V. [H2A] = 1 10-3 mol/L.
Fig. 4. The chronocoulometry curve [K3Fe(CN)6] = 5
10-4 mol/L, Potential steps -0.1 V ~ 0.6 V.

418 J. Chin. Chem. Soc., Vol. 49, No. 3, 2002
Ye et al.
We have recorded the absorbance at potentials of 0 V, -0.500
V, -0.525 V, -0.550 V, -0.575 V, -0.600 V, and -0.800 V. The
ratio of [H2A]/[H4A] (H4A: the reduced form of H2A) is con-
trolled by the applied potentials as defined by the Nernst
equation:
RT
nF
[O]
[R]
RT
nF
[H2 A]
[H4 A]
E
E0' 2.303
Log
E0' 2.303
Log
For each value of Eapplied, the corresponding ratio of
[H2A]/[H4A] can be calculated from the absorbance at 265
nm by the equation of:6
Firstly, H2A gets one electron from a working electrode
and transforms into the intermediate of tertiary carbon free
radical. Secondly, 2 free radicals turn into H2A and H4A by
disproportionate reaction.
[H2 A]
A AR
Using the SEC technique is a good means to study the
appearance reaction rate constant ks.8 Applying a single po-
tential step from 0.00 V to -0.80 V to H2A solution (1 10-3
mol/L) can result in the diffusion controlled reduction of
H2A. The absorbance at 265 nm was recorded every 30 sec-
onds as soon as potential was exerted. The absorbance is
changeless after 210s. The ks of this first order reaction was
calculated by the equation of Ln (AO-A) = kst + Ln (AR-AO).
From plotting the lg(AO-A)~t curve, we got kS = 0.104 s-1. It
corresponds with the results obtained by cyclic voltammetry.
[H4 A] AO
A
Here: AO is the absorbance of H2A and is measured at a
potential of 0.00 V.
AR is the absorbance of H4A and is measured at a poten-
tial of -0.80 V.
A is the absorbance of the two states co-existing and is
measured at different potentials between 0.00 V and -0.80 V.
So:
RT
nF
A
AR
A
E
E0' 2.303
Lg
The diffusion coefficient of H2A studied by
chronoamperometry
A0
The diffusion coefficient D0 has been investigated us-
ing the chronoamperometry technique. A fixed potential-step
(0.0 V ~ -0.8 V) exerted on a electrode that is immersed in dif-
ferent H2A concentration solutions and the i~t courses were
recorded simultanously. For the first-order electrode reac-
tion, the diffusion current at a disk electrode was given by the
cottrell equation:
The curve of E~lg[(A-AR)/(A0-A)] should be a linear
relationship with the intercept = E0’ and the slope = 2.303
RT/nF. By the data read from Fig. 6, we got the electron
transfer number n = 0.925 and E0’ = -0.504 V (vs. SCE). This
result means that the H2A reduction is a single electron trans-
fer process. It corresponds with the result of cyclic vol-
tammetry.
H4A is the reduced product of H2A. It seems that it is a
2-electron transfer process from H2A to H4A. But it is found
by our investigation that the electro-reduction reaction is a
one-electron transfer process. From the point of thermody-
namics, the electro-reduction of the double-bond conjugate
system ( CH = CH ) located in H2A is irreversible. That is, it
is impossible to get its oxidation peak at same time. So this
electro-reaction does not react on the double-bond conjugate
system. Analysis of the changes in H2A spectra during reduc-
tion shows that the electro-active center of H2A is located in
the carbonyl group. Upon reduction, this group is destroyed
and the absorption peak at 265 nm in the ultraviolet region
decreases. We consider the redox mechanism of H2A as fol-
lows:
nFAD01/2CO*
i
1/2t1/2
Here n is the electron transfer number; F is the Faraday
constant; A is the electrode geometric area (7.302 mm2), C0
*
is the bulk concentration of species O. The equation is valid
for a diffusion-driven electrode process. The linear relation-
ships i ~ t-1/2 were obtained with a slope of:
nFAD01/ 2CO*
k
1/ 2
*
*
A different k with a different C0 . k ~ C0 relationship

A New Redox Reaction of Ascorbic Acid at Platinum Electrodes
J. Chin. Chem. Soc., Vol. 49, No. 3, 2002 419
should be a straight line again. From a k ~ C0* straight line, we
can get the diffusion coefficient accurately. We got D0 =
2.635 10-5 cm2/s.
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CONCLUSIONS
A new redox reaction of H2A was found and has been
investigated by electrochemical and spectroelectrochemical
methods. A possible reaction mechanism is proposed. The re-
action of H2A is a one-electron process with E0’ = -0.504 V
(vs SCE). The electrode reaction kinetics parameters ks and
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ACKNOWLEDGMENTS
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The support of The Chinese Education Committee and
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Key Words
Electrochemistry; Spectroelectrochemistry; Ascor-
bic acid; Mechanism.