Kinetic parameters and mechanism of the electrochemical oxidation of L-ascorbic acid on platinum electrodes in acid solutions

J. ElectroanaL Chem., 160 (1984)159-167 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

159

KINETIC PARAMETERS AND MECHANISM OF THE ELECTROCHEMICAL OXIDATION OF L-ASCORBIC ACID ON PLATINUM ELECTRODES IN ACID SOLUTIONS *

PANTELIS KARABINAS and DIMITRIOS JANNAKOUDAKIS Laboratory of Physical Chemistry, the University of Thessaloniki, Thessaloniki (Greece)

(Received 30th December 1982; in revised form 30th May 1983)

ABSTRACT The oxidation of L-ascorbic acid has been studied in sulphuric acid solutions at a bright platinum electrode, by employing cyclic voltammetry, controlled-potential electrolysis and potential-step measurements. On the basis of kinetic parameters--Tafel slopes, reaction orders and pH effects--a possible mechanism is proposed for the overall oxidation process. There is evidence of two rapid and thermodynamically reversible charge-transfer steps which are followed by a slow non-activated desorption and rate-determining step. The kinetics are explained with the aid of the Temkin adsorption isotherm.

INTRODUCTION A l t h o u g h the electrochemical o x i d a t i o n of L-ascorbic acid ( A A ) has been thoro u g h l y investigated at m e r c u r y electrodes [1-11], o n l y a few studies have been c a r r i e d out for this substance at a Pt electrode. Brezina et al. [12] showed that A A is reversibly a d s o r b e d at a Pt electrode. T h e a d s o r b e d A A molecules cover 68% of h y d r o g e n a d s o r p t i o n sites a n d are oxidized to d e h y d r o a s c o r b i c acid ( D H A ) in the p o t e n t i a l range of surface oxide formation. A t less positive p o t e n t i a l s the A A is irreversibly oxidized w i t h o u t a n y p a r t i c i p a t i o n of a d s o r p t i o n . T h e o x i d a t i o n of A A has also b e e n studied recently on Pt electrodes m o d i f i e d b y u p d of Bi 3+, Pb 2+ a n d C o 2+ [13]. It has b e e n f o u n d that the o x i d a t i o n of A A is strongly c a t a l y s e d in the presence of Bi 3 + a n d Pb 2 +. T h e m e c h a n i s m a n d the kinetics of the a n o d i c o x i d a t i o n of A A on H g a n d A u have been given in detail b y A l d a z a n d co-workers [10,11,14,15]. However, the m e c h a n i s m on Pt has n o t been discussed, as far as we know, in a n y p u b l i s h e d work. This w o r k is an a t t e m p t to elucidate the m e c h a n i s m of the a n o d i c o x i d a t i o n of A A b y d e t e r m i n i n g the kinetic p a r a m e t e r s on Pt electrodes. T h e s t u d y was c a r r i e d o u t in H 2 S O 4 solutions using cyclic v o l t a m m e t r y in c o m b i n a t i o n with c o n t r o l l e d - p o t e n t i a l electrolysis a n d p o t e n t i a l - s t e p transients. * Dedicated to Prof. W. Vielstich on his 60th birthday. 0022-0728/84/$03.00

© 1984 Elsevier Sequoia S.A.

160 EXPERIMENTAL

Bright platinum electrodes were used in all measurements. They were activated electrochemically by periodic polarization between - 0 . 2 4 and 1.36 V. Electrode potentials are referred to the saturated calomel electrode (SCE) which was carefully separated from the working electrode to avoid contamination. The electrical circuits for i - E cyclic voltammetric and i-t curves and the cell have been described previously [16]. The experimental set-up for the rotating disc electrode consisted of a Tacussel potentiostat, a Tacussel function generator, type GSTP, and a Hewlett-Packard X - Y recorder. The working electrode compartment contained exactly 40 ml of supporting electrolyte in which the appropriate amount of AA was dissolved after activation of the electrodes. High-purity Ar was used for oxygen removal and all measurements were taken in an Ar atmosphere. Special care was taken for consistent electrode pretreatment. A continuous sweep was applied for 5 min between hydrogen and just before oxygen evolution in pure sulphuric acid solution. In order to avoid possible auto-oxidation in a stock solution, the appropriate amount of solid AA was dissolved after the activation of the electrodes. Only the first cycle was registered. Sulphuric acid and AA both "zur Analyse" (Merck, Darmstadt) and triply distilled water were used for the preparation of the solutions. After the electrolysis the final concentration of Am was determined by titration with standard iodine solution. All measurements were carried out at 25 + 0.1°C. RESULTS

Potential-step transients and rotating disc electrode The i-t curves that are obtained using potential-step transients, yield the net diffusion current for extended periods of time [17]. From the slope of the J o - 1 / t 1/2 plot, the diffusion coefficient of AA was found to be 6,5 x 1 0 - 6 c m 2 s - 1 . This value agrees reasonably well with other published values [12,18]. Figure 1 shows the behaviour of Am at a rotating Pt disc electrode. By means of the Levich-Riddiford equation [19,20], the diffusion coefficient was determined as 6.6 x 10 -6 cm 2 s-1, while the number of electrons consumed in the overall electrode reaction was two, as estimated by controlled-potential electrolysis. From the slope of the log[J/(Ja - J ) ] vL E plot the value of 0.45 for an was obtained. Current-potential diagrams In Fig. 2 the cyclic voltammogram of Am (5 x 10 -3 M in 1 M H2SO4) is given as an example. This voltammogram was taken on an activated electrode as described

161

-

4000

-

8 ID 7 B O IDRmO

I 7too '1 m O O 'lmao lOOO

4

7.o

i •-~

~.

t

o ~

° I o,o

I 0,5

10

-1 I

dh/(~r°~';')V"

20

i 1,o

E/v

Fig. 1. Current density-potential curves for AA (5 × 10 -3 M ) oxidation on a Pt rotating disc electrode at different rpm in 1 M H 25104; d E / d t = 100 mV s - ] . Inset; limiting diffusion current density JD vs. ~1/2 (~o = 2 , ~ / , / =

rpm/60).

1

la

E

0 ~...,..,..........,.,..,....~."; ....... '"...,."'Y..................... " o.o

I o,5

E/V

I 1,o

Fig. 2. Cyclic voltammogram of AA (5 × 10 -3 M ) on Pt in 1 M H2SO 4. Scan rate 100 mV s -1. ( . . . . . Without AA.

)

162

above. It can be seen that the adsorption of AA suppresses the hydrogen adsorption-desorption peaks, hindering complete activation of the electrode. The oxidation of AA in acid solutions yields a current peak in the double-layer (dl) region. At low potential sweep speeds (v < 0.6 V s -1) the curves show the lasual behaviour of an irreversible oxidation controlled by diffusion [21]. The peak current density at these speeds is directly proportional to v 1/2. Here jp is also directly proportional to the concentration of AA, since the slope of the logjp-log c diagram, at the same sweep speed, is equal to unity. Under quasi-stationary conditions, i.e. low sweep speeds (2 mV s 1), the i - E curves give Tafel lines by log analysis at the foot of the wave. The Tafel lines shift to less positive potential values with increasing AA concentration (Fig. 3). In all cases studied, the slope of the linear segments is 60-65 mV/decade. From Fig. 3, at constant positive potential, the l o g j vs. log e diagram of Fig. 4 is drawn. The slope of this diagram gives a reaction order of AA equal to 0.95. With increasing sulphuric acid concentration at constant ionic strength the polarization curves of AA and consequently the Tafel lines shift to more positive potentials, as Fig. 5 illustrates. The ionic strength was adjusted by addition of the proper amount of K z S O 4 t o the solution of low H2SO 4 concentration. In all cases the Tafel slope remains the same, i.e. 62.5 _+ 2.5 mV/decade.

300 RO re "1o

200

100 80

~

5o

"'~

4.0

5o

I

30 20

10

I 0,25

o3o

I o35

I o,4o

E/V Fig. 3. Tafel plots of different concentrations (in r a M ) of AA on Pt in 1 M H2SO 4.

163

--

2,0

"~ 1,5 o

1,0

I

I

I

-2,5

I

-2,0 log CAA

Fig. 4. Plot of logj vs. log CAA for AA oxidation on Pt in 1 M H2SO4. Positive potential 325 mV.

T h e c o r r e s p o n d i n g r e a c t i o n o r d e r w i t h r e s p e c t to t h e h y d r o g e n i o n s was f o u n d to b e e q u a l to - 1 . 9 0 (Fig. 6).

200 0'751

I,S

2 4

® (

/

10o

80

~'~ 80 ~ 5o °-'~ 40

/

30

/

20

10 I

I

I

0,25

0,30

0,35

I ....

0,4O

E/V Fig. 5. Tafel plots of AA (5 × 10-3 M) in different n 2 s o 4 concentrations (in normalities) on Pt.

164

o', o

2~0

--

1,5

--

1,0

I o,o

I o,5 log CH+

Fig. 6. Plot of logj vs. log CH+ for Am (5)< 10 -3 M ) oxidation on Pt. Positive potential 325 mV.

Mechanism proposed and discussion Taking into account the experimental results of this work, the oxidation mechanism proposed must be consistent with the following: (1) Reaction order with respect to AA (a l o g j / a log CAA)E,c,+ = 0.95. (2) Reaction order of H + (a l o g j / a log CH+)E,c^^= --1.90. (3) Tafel slope of 62.5 inV. The Tafel slope can be considered equal to RT/F. On the other hand, the reaction orders are very close to the theoretical values 1 and - 2 , which correspond to the following reaction: HO

0

HO

~ CHOH

I

CH2OH

0

0 0I T

~-2H*

+o

(I)

H CH20

ods

A suggested sequence of reactions leading to the adsorbed radical and finally to D H A in solution is as follows:

165 HO

O

HO

HO

"1 HOH

o. o / o-i-

/O

[ CHOH CH20H

/

o

-I;

o

__,0- o I

CHeOHI

O

-] -

O

CH2OH .o

0

~-

O

CH2OH

ods

O

ads

IHOH /

CHOH[

CH2OH[ ads

CH2OH

ob° ]

+H+ ÷e

+e

(Ia)

(Ib)

(II)

(III) CHOH I CH2OH

fost + H20

HOH CH20H

H÷

rds b

~HOH CH2OH ods

0 0 O~'~O

/

+

t.

OH 0 HoH~H O O H H H H

(IV)

Adsorption of AA on Pt electrodes was first confirmed by Brezina et al. [12]. T h e y pointed out that the adsorption proceeds reversibly and no adsorption occurs for the oxidation products. The large overvoltage on Pt is probably due to the adsorption of AA itself. The anionic radical formed in step (Ib) was detected by EPR spectra [22]. The same radical was previously observed in other types of oxidation processes such as enzymatic [23] oxidation and radiolysis [24]. The formula of the final oxidation product given in step (IV), as a dimer of DHA, was confirmed by 13C-NMR spectra during the oxidation of AA by iodine [13,25] or by p-benzoquinone [26]. If we assume that steps (Ia) and (Ib) are in rapid equilibrium, then combination of these steps leads to eqn. (I). The corresponding rates for the forward and backward partial reactions of (I), applying the Ternkin isotherm for intermediate values of 0, are

U(I)=/~I( 1 -

0T) CAAexp(aFE/RT) e x p ( - f l r O / R T )

(1)

166 ~I) =/~IOAc2+ e x p [ - (1 - a ) F E / R T ] exp[(1 - fl)rO/RT]

(2)

From eqns. (1) and (2) we have for the equilibrium: 0A(1 - 0 T) -1 e x p ( r 0 / R T ) = KlCAAC~3 e x p ( F E / R T )

(3)

where the subscript A denotes the adsorbed anionic radical. If reaction (III) is the rate-determining step and reaction (II) is under quasi-equilibrium conditions, we finally have for reaction (II) 0A/0B

=

glI exp( - F E / R T )

(4)

and the rate of the reaction for a non-activated adsorption will be

UIn = kinO B e x p ( r 0 / R T )

(5)

and for activated adsorption UIII

=

kili0 B e x p ( f l r 0 / R T )

(6)

where B is the adsorbed DHA. If 0 s > 0A, 0 B = 0T, substituting for exp(rO/RT) the values from eqn. (3) and neglecting pre-exponential terms in 0, the reaction rate for non-activated adsorption will be Vli I

=

klli0 B exp(rO/RT) = klIIOBKICAACH 2 e x p ( F E / R T )

(7)

and for activated adsorption UIII

=

klli0 B e x p ( f l r O / R T ) = kIIIOBKIC~AACH2+ '8 e x p ( f l r E / R T )

(8)

The case presumed here, 0, > 0A, is the one most often encountered in electrochemical oxidations [27]. The same assumption has also been made in the case of 1,1-dimethylhydrazine [28]. If the adsorption of species B is a non-activated process and its desorption the rds, a Tafel slope of 60 mV and reaction orders of 1 and - 2 will be obtained. This process fits the experimental results reasonably well. If the adsorption is an activated one, a Tafel slope of 60/fl and fractional reaction orders should be expected. In this case the experimental results fit only if /3 = 0.95. Such a value of fl seems not to be reasonable [27]. A Tafel slope of 60 mV would also be obtained if the rds were an electrochemical step, as has been already suggested in the cases of Hg [10,11], Au [14,15] and Pt [29], though controlled by the desorption of the DHA. In that case the reaction order and the pH dependence would be equal to/3 and - 2 / 3 respectively. Small deviations in experimental values of T a M slopes and reactions orders must be attributed to the influence of the supporting electrolyte, mainly to KzSO 4. Other suggested mechanisms and kinetic processes do not fit with the experimental results. REFERENCES 1 E. Kodicek and K. Wenig, Nature, 142 (1938) 35. 2 C. Cattaneo and G. Sartori, Gazz. Claim. Ital., 72 (1942) 351.

167 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

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