Mechanism of dialuric acid oxidation on a mercury electrode

MECHANISM

OF DIALURIC ACID OXIDATION MERCURY ELECTRODE

ON A

E. ROLDAN and M. DOM~NGUEZ* Departamento

de Qufmica Fisica, Universidad de Sevilla, Sevilla, Spain (Received

16 November 1982)

Abstract-A s(tidy was carried out on the oxidation mechanism over dm of dialuric acid in the pH range l-12. Voltagrams of dialuric acid show the existence of one oxidation wave but no reduction wave. The twoelectron transfer takes place through a post-kineticmechanism.A schemeis proposed for the overall reaction

together with several reaction mechanismsat potentials corresponding to the foot of the wave. INTRODUCTION

Cells and electrodes

Several studies have been carried out on L-ascorbic acid oxidation[ 1, 21, o-araboascorbic acid oxidation [3, 43 and dihydroxifumaric acid oxidation[5]. However, the oxidation mechanism over dme of dialuric acid, also an ene-diol, is not clarified. Struck and Elving[6] show the oxidation polarographic wave at a single pH value and state that this wave is diffusion-controlled in all conditions. The present study is based on analysis of the i-E curves for dialuric acid at varying pH values. An analysis was also carried out on the influence of the velocity of the potential sweep on the i-E curves. The products from the oxidation process and the total number of electrons that intervene are also determined. With these results, together with the calculations of the Tafel slopes and reaction orders at varying pH values, schemes are proposed for the overall reaction of the oxidation process together with mechanisms for potentials corresponding to the foot of the wave. EXPERIMENTAL Apparatus The polarographic curves were registered automatically by means of a PO4 Radiometer where the damping circuit had been completely suppressed. The i-E curves were traced point to point with the aid of the galvanometer and the power source of the polarograph; the potential being measured, in all instances, by a Fluke voltmeter. An Amel triangular wave generator was used for the voltammetric techniques, together with an Amel potentiostat with a built-in correction for ohmic drop, an X-Y recorder from Hewlett-Packard and an oscilloscope from Amel. The capacitance-potential curves were registered automatically by means of a system based on the application of a small triangular signal superimposed on a slowly varying dc potential[7]. The spectrometer used was a Beckman DB-GT model for ultraviolet. *Present address: Dcpartamento de Quimica Universidad de Cbrdoba, Cdrboda, Spain.

Fisica,

The polarographic measurements were carried out using thermostated Amel 494 cell. A saturated calomel reference electrode was used. The working electrode was a mercury capillary with the following characteristics: m = 1.300 mg s- I, L = 6.35 s, open circuit, in our buffered solution at pH = 7 and h = 40 cm. For the voltammetric measurements the working electrode was a hanging mercury electrode (Metrohm EA 290). In the processes for obtaining the reaction products, the working electrode was a mercury pool. The auxiliary electrode was the typical platinum one. Solutions, products

and measurements

All products used were Merck p-a. except the dialuric acid which was obtained by a modification of the method proposed by Tipson and CretcherCS]. As a supporting electrolyte solution, a buffered solution was used with the following components and concentrations: 0.04 M acetic acid, 0.04 M phosphoric acid, 0.04 M ammonium nitrate and 0.2 M NaOH, which were mixed in varying proportions according to the pH desired. The ionic strength was adjusted with NaNO, to 0.2 M. Owing to the instability of dialuric acid solutions these were prepared immediately prior to each experiment and the oxygen was eliminated before adding the solid product. All measurements were taken in a nitrogen atmosphere at 25 *O.l”C. To avoid depleting the solution, those measurements for plotting the polarographic curve point to point were taken with each first drop, manually synchronizing the application of the potential with the fall of the drop. The Tafel curves were taken on the rising portion of the polarographic wave. All values of i were corrected to the residual current. RESULTS General behaviour and capacity-potential

(C-E)

curoes

The dialuric acid oxidizes on the dm producing one polarographic curve in the pH range 1.5-12. C-E curves plotted for different pH values (acid, neutral and basic] and with dialuric acid concentration

1215

E.

1216

ROLDAN

AND

M.

of 10e3 M show that this substance is not noticeably adsorbed over a mercury electrode. Determination of electrons number The number of electrons taking part in the reaction was calculated from the decrease of the limiting current following to the application of a potentiostatic impulse to an electrode of fixed area. The total number of electrons was obtained from the slope of the straight line i USt - W, in agreement with the equation proposed for semi-infinite diffusion at spherical electrodesC9j. The average value obtained was found to be 2.

DOMiNGUEZ

slope of 0.5; temperature coefficient 1 T0 per “C. The limiting current is proportional to the concentration of the dialuric acid in the bulk of the solution. E,, is independent of the dialuric acid concentration, but varies in a linear way with log td with a slope of - 14 mV. The maximum current at potentials corresponding to that zone where the condition (i,,/i,) 4 0.02 is satisfied, is independent of the height of the mercury column whilst at potentials corresponding to that zone where (i,,,/i,) > 0.8 is proportional to hlR Tafel curves and reacrion orders

Poiarographic

behaviour

The limiting current is independent of the pH, whilst EInshifts to more negative potentials with an increase in pH (Fig. 1). The values of the slopes of the linear Order with respect to H+ ion

pH = 1.5-2.5 - 2.0

segments are close to - 60 mV, - 30 mV, -60 mV and - 90 mV per pH unit, respectively. Every logarithmic analysis of the polarographic wave gives straight line with a slope of 30 mV dec- ‘, being practically independent of the pH. The limiting current is controlled by the diffusion, ie the log i, vs log h plot is linear with an approximate

-a2

(a) Influence ofpH. The Tafel slope is independent of the pH with a value of 30 mV dec- ’ (Fig. 2). The reaction orders with respect to the H+ concentration are: pH = 4-5 - 1.0

(b) Influence of dialuric acid concentration. The reaction order with respect to the concentration of dialuric acid is 1 and independent of the pH and of the potential at which it is measured.

-

-

Fig.

1.

Variation

pH = 11-12 - 2.9

The orders are independent of the potential in the area where Tafel’s law is obeyed, as shown in Fig. 3.

-x3-

-44

pH = 7.5-8.5 - 2.0

of

E,,,

with

pH.

1217

Mechanism of dialuric acid oxidation

log i(pA

c

-0.

- 1.

- 3.

Fig. 2. Representation of Tafel’s law. (A) PH = 1.41, (0) PH = 1.62. (A) PH = 1.78, (a) PH = 2.0% (0) pH = 2.28.

I

-

a-

-1

-

-2

-

I -2.0

-1,s

Fig. 3. Reaction

orders.

(A) E =

-

10 mV,

(0) E = 0 mV,

--a.slog[Hq IO) E = 10 mV.

E. ROLDAN

I218

Influence of potential

sweep

AND

velocity

At normal conditions ofdialuric acid concentrations no reduction voltammetric wave is observed even with the maximum sweep velocity reached with our system (4clVs~‘). The oxidation wave displaces towards more anodic potential values when the sweep velocity is increased. Figure 4 gives the plot of E, USlog v. The i us ulf* plot is linear in all conditions. Electrolysis and ident@cation of compounds. The oxidation product of dialuric acid was identified by uv spectroscopy of a dialuric acid solution at pH 7 after electrolysis at a controlled potential. The oxidation product was identified as alloxan because of its A,,,,, characteristic (245 nm). The spectra of dialuric acid, alloxan and the electrolysis product are shown in Fig. 5. The characteristic hand of dialuric acid (approx. 280 nm) and the weak band of alloxan can be observed.

DlSCUSSION Overall reaction The data collected demonstrate that the oxidation wave of dialuric acid is produced by a polarographically reversible two-electron transfer, in which two protons intervene in the pH 1.5-2.7 interval, one between pH 2.7-6.5. two between pH 6.5-9.8 and three between pH 9.8-12. However, the peculiar variation of i,,, US ht/2 confirms that the process under study is a post-kinetic one, with a rapid chemical reaction. This fact is confirmed by the variation of E ,,zwith log t,; the independence of E ,,z with the concentration implies a first order or pseudo first order reaction with respect to the electrodic product for the kinetic process.

M. DOM~NGLIEZ Our voltammetric data confirm this hypothesis. The fact that the peak potential shifts towards more anodic values with an increase in the sweep velocity is in agreement with the conclusions of Nicholson and Shain[lO]. A differentiation can be made of three distinct overall reactions. Thus, the reaction in the pH 156.5 interval would be AH, + H,O

-i BH, + 2H+ + 2e.

At pH greater than the polarographic pK, (approx. 3) the AH, form is substituted by AH (pK, > 12). For the pH 6.5-9.8 interval, the reaction would be AH-

+H,O-+BH-

+2H+

And for the pH 9.8-12 interval,

+2e.

the reaction

would

be

AH-+H,O+B’-+3H++2e.

Oxidation

mechanism

at the

foot of

the wave

Acid media (pH 1.56.5). With the aforementioned conclusions, the mechanism can be represented by the reaction kinetic pathways shown in Fig. 6, where reaction (1) represents the dissociation of H+ from carbon 4, and reactions (2) and (3) represent the oneelectron transfers with the presumably formation of a free radical. Reaction (4) represents the hydration of the carbonyl group. Assuming reaction (4) as the rds and applying the approximation of the equilibrium state, the value of the anodic current intensity on the rising portion ofthe wave can be expressed (taking the electrokinetic potential to be negligible) as

i=

ZFK,K,K,k,

C2

,’ l-l

C I

expI2FEIRTI, H

where C, is the total concentration

Fig. 4. Variation of E, with potential sweep velocity, pH = 4.98.

of dialuric acid in

1219

Mechanism of dialuric acid oxidation

the bulk of the solution; C, is the concentration of the H+ ions; K,, K, and K, are the equilibrium constants of reactions (l), (2) and (3) and k, is the rate constant of the rate controlling step. The comparison between the theoretical values deduced in the aforementioned sequences and the experimental results is shown in Table 1. Basic medium (pH 7.5-X.5). In the pH 758.5 interval, the mechanism can be represented by the following reaction kinetic pathways.as shown in Fig. 7, where reactions (1) and (2) represent the one-electron transfers, reaction (3) represents the dissociation of alloxan, pK, 6.5 approx.[l I], and reaction (4) represents the hydration of the carbonyl group. After pH 6, the alloxan experiments a benzilic acid type rearrangement to give alloxanic acidcl 11. In this case the function i = E (E) would be i = ZFK,K,K,k,$

expl2FEjRTl. ”

The comparison perimental results

between is shown

both theoretical in Table 2.

Under these conditions Basic medium (pH 1 l-12). the proposed scheme would be that shown in Fig. 8, where reaction (3) represents the total dissociation of alloxan, pK, = 10 approximately.

,m

Fig. 5. Ulrraviolet spectra. (---) Dialuric acid, (-. - .) Alloxan, I-) electrolysis product; pH = 7.

“N’,NH

1)

I

k-1

I

_ + H+

-=,/O AH

0 t HN’

2)

o=c,

I

- k2

‘NH 1 ~C’o-

7

H.‘,,,L +e

I

+H+

=,J0

c bH

8

1

8 k3

3)

HN/=\NH

z=== k-3

%_., ! o=c ,,,L HN

4)

Hz0 +

8

and ex-

-

%

r.d.s.

H&H

Fig. 6. Reaction kinetic pathway of dialuric acid. Acid media.

E. ROLD~N AND M. DOM~NGIJU

I220

A,,

HN I o=c ,/*-

1)

HN’

kl

G==

I

8

HN’

k2

+

H++c

8

‘NH

I I+c o=c, ,c=D F

I

o=c

-

d

HNRCANH

I

‘NH

o&J_0

k-1

OH

2)

8

k-2

0

ek

HN’

3)

I

0-q

P HN/%JI

k3

NH

I

e

,c=o

I+H+

k-3

F 0

r. d.s.

4)

9

9 "N----=---NI I o=c , ,c=o

5)

k-5

C HdbH

Fig. 7. Reaction kinetic

Table

HNlcLNH

k5 z===

pathway

of dialuric

acid. Basic medium

Value

Tafel slope Order with respect to diiuric acid concentration -2 Order with respect to H+ ion -1

i = FKIK~K,k,

Experimental

30 mV

1 for K, $C, forK,gC”

alloxanlc acid

) narrangcmcnt

(pH 7.5-8.5).

The function i = E(E) would be

1

Theoretical

Parameter

benrilic

3OmV

-2 -1

2

H

expIZFE/RTj.

The comparison between the theoretical values deduced in the aforementioned sequences and the experimental results is shown in Table 3.

1 (pH 1.5-2.5) (pH4-5)

Table

3 Value

Parameter Table

Tafel slope Order with respect to diiluric acid concentration Order with respect to H+ ion

Experimental

2 Value

Parameter

Theoretical

Theoretical

Experimental

30 mV

30 mV

1

I

- 2.0

- 2.0

Tafel slope Order with respect to dialuric acid concentration Order with respect to H+ ion

30 mV

30 mV

I

1

-3.0

- 2.9

Acknowledgement-This study was supported by the Comisi6n Asesora de Investigacidn Cientffica y T&mica. Madrid.

Mechanism

8 HN’

1)

I o=c \

of dialuric

acid oxidation L

9

‘NH

I=-.

HN

kl

I

/O-

o=c

k-1

F

1221

btn

I

LC.&“.

I C-O

+ H++e

&

OH

2)

‘,,,bo +=

o=c

k-2

6 9 _N----CyN_

k3 k

I

I

+ 2n+

-3 -‘cc=o

-N 4)

H20’

0

8

B

XN_

_N--+ I

k4

I

o=c,c/L=o

ac \fJ=O

5)

0.x

r.d.s

HOOH

d

-N

N-

P N-

HN'

P’

o=LyLo

NH

benzilic rearrangement

alloxanlc acid

-db-

Fig. 8. Reaction

kinetic

pathway

of dialuric

REFERENCES 1. S. Ono, M. Takagi 356 (1958).

and T. Wasa, Bull. them. Sot. Japan 31,

2. J. J. Ruiz, A. Aldaz and M. Domittguez, Can. .I. Chem. 55. 2799 (1977); 56, 1533 (1978). 3. M. Dom’inguez, A. Aldaz and F. Sanchez-Burgos, J. electtwnnal. Chem. 68, 345 (1976). 4. M. Domfnguez, A. Aldaz and F. %inchez-Burgos, An. Quim. 74, 199 (1978). 5. M. Dommguez and E. Valera, Electrochim. Acta 25, 833 (1980).

l?ti*:9-”

acid.

Basic medium

(pH

11

12).

6. W. A. Struck and P. J. Elving, J. Am. them. SK. 86, 1229 (1964). 7. E. Roiddn, D. GonztUez-Arjona An. and M. Dommguez. Quim. 78, 158 (1982). 8. R. S. Tipson and L. H. Cretcher, J. org. Chem. 15, 1091 (1951). 9. P. Delahay, New lnstrutnental Methods in Electrochemistry, p. 59. Wiley-Interscience, New York (1954). and I. Shain, Analyt. Chem. 37, 191 10. R. S. Nicholson (1965); 37, 179 (1965). 11. H. Kwart and 1. M. Sarasohn, 1. Am. rhem. Sot. t33,2579 (1961).