Indian Journal of ChemistryVol. 28A, December 1989, pp. 1060-1063

Kinetics of oxidation of L-ascorbic acid by ironillljin presence of2,2' -bipyridyl-Application of Marcus theory

P V Subba Rao", (Miss) G V Saradamba, K Ramakrishna, ~ Mohana Rao & K V SubbaiahDepartment of Physical & Nuclear Chemistry and Chemical Oceanography, Andhra University,

Visakhapatnam 530 003

Received 5 December 1988; revised 18 January 1989; accepted 6 March 1989

Kinetics of oxidation of ascorbic acid by iron(III) in the presence of 2,2'-bipyridyl has been investigatedunder anaerobic conditions. The reaction is first order each in [iron(III)] and [ascorbic acid]. The plots ofII k' versus 1I[BipY]3and 11k' versus [H + F are linear with positive intercepts. The kinetic results provideevidence for the formation of an intermediate complex, Feibipyj]:' which reacts with the unionised formof ascorbic acid in the rate-limiting stage. The stability constant of the complex has been evaluated fromthe kinetic data. The kinetic data are compatible with the Marcus' theory for outer sphere electron-trans-fer.

lron(III) when directly mixed with 2,2'-bipyridyl isknown to form a brown labile 1:2 complex, whichexists! in both binuclear form [(bipYkFe-O-Fe(bipy}z]4+ and mononuclear form [Fe(bipY)2P+'This is different from tris(bipyridyI)iron(III) which issubstitution-inert+'. Marcus' theory has been appli-ed to the outer sphere electron transfer reactions oftris-complex. However, the applicability of Marcus'theory to the electron-transfer reactions of the'brown complex has not been explored so far. Withthis objective, the kinetics of oxidation of L-ascorbicacid by the brown complex has now been investigat-ed and the applicability of Marcus theory has beenexamined.

Materials and MethodsAll solutions were prepared using oxygen-free

conductivity water. The course of the reaction wasfollowed under N2 atmosphere by withdrawing ali-quots of reaction mixture and measuring the absorb-ance at 510 nm, the Amax of tristbipyridyljiroufll], oneof the products of the reaction. At this wavelength,all the other reactants have negligible absorbance.All the kinetic runs were carried out at constant ion-ic strength adjusted with sodium perchlorate. Underthe experimental conditions employed, the extent ofoccurrence of the uncatalysed reaction was found tobe negligible during the time taken for kinetic runs.

Results and DiscussionStoichiometry of the reaction

Under pseudo-first order conditions, i.e.[Fe(III)] ~ [ascorbic acid] in the presence of1.6 x 10 - 2 mol dm - 3 bipyridyl and at[H +] = 8.0x 10 - 2mol dm - J, the stoichiometric studies carried

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(

out under N2 atmosphere revealed that one mole ofascorbic acid consumed two moles of iron(III).Hence overall reaction may be represented by Eq. (i)

C6Hg06 + 2Fe(bipy)~+ + Hbipy " -> 2Fe(bipy)~++ C6H606 + 3H+ (i)

Dependence of rate on [reactants]The kinetic runs were performed under the condi-

tions: [bipy], ~ [Fe(III)]o~ [H2AlOand the plots of log(Ax, - A,) versus time (where Ax, and A, are the ab-sorbances after the completion of reaction and attime t) were found to be linear for at least three half-lives showing that the reaction obeys first order kin-etics in ascorbic acid. The plots were found to beparallel to each other at different [ascorbic acid] un-der these conditions,. confirming this conclusion,The pseudo-first order rate constants (k') were de-termined at different initial [Fe(III)] and k' was foundto be proportional to [Fe(III)]o.This shows that thereaction is first order in [Fe(III)] (Table 1).

At different [bipyridyl] and fixed [H +], [Fe3 "l and[ascorbic acid], the rate increased with increase in[bipy]; The type of dependence of rate on [bipy]o isshown by the observed linear plot with a positive in-tercept between 1/ k' and I/[bipY]6 (Fig. 1). The.reaction is inhibited by H + ion and k' was deter-mined at different initial [H +] at fixed ionic strength

(Table 1).The plot of 1/ k' versus [H +Fwas found lin-ear with positive intercept.

Mechanism and rate lawSince the kinetic investigation has been carried

out in the [H+] range of 0.024-0.08 mol dm ":', theinvolvement of FeOH2 + species can be neglected

\

RAO et al.: KINETICS OF OXIDATION OF L-ASCORBIC ACID BY IRON(III)

Table I-Effect of varying [Fe3+] and [HCl04] on reaction rate[Ascorbic acidj.= 1.0 x 10-5 mol dmc3;[bipy]o= 1.6 x 10-2 moldm": !l=0.58 mol dm-3; methanol = 18% (v/v);

temp. = 30.0 ± O.l·C[H+] x 102

(mol dm-3)

4.04.04.04.04.02.44.46.47.48.4

[Fe(lII)lo x 1Q3

(mol dm-3)

0.42

0.801.201.60

2.000.40.40.40.40.4

k'x 105

(s -I)

2.704.837.669.58

11.863.272.351.521.221.02

(first hydrolysis constant of iron(III) has a value"1.58 x 10 - 3 at 25°C). Dimeric form of iron(III), as anactive oxidant species can also be neglected becauseof low iron(III) concentration employed (- 10 - 5

mol dm - 3). Hbipy + has an acid dissociation con-stant- of 0.47 x 10-4 at 25° and hence bipyridyl ex-ists in the protonated form, Hbipy + under these con-ditions. Ascorbic acid also exists in an unionisedform under these conditions (pKa = 4.25)6. Sinceplots of 11k' versus 1I[bipY]6and 11k' versus [H +Fare linear with positive intercepts, the authors be-lieve that a 1:2 Fe(III)-bipyridyl complex, Fe(bipy H".formed in an equilibrium prior to the rate-limitingstep, acts as the active oxidising species. The me-chanism shown in Scheme 1 is in conformity with theobserved kinetics.

Under the conditions [Fe(III)] ~ [H2Al the pseudo-first order rate constant (k') is given by Eq (2)

k'= kKI [Fe3+][bipy]~. [H+]2+ K, [bipy]~ ... (2)

or

1 [~+r 1k' kKI [Fe3+][bipy]~ + k [Fe3+] ... (3)

The rate-law (3) satisfactorily explains the linearplots with positive intercepts between 11k' and1I[bipY]6and between 11k' and [H +F. In conformitywith the Eq. (3) these intercepts have the same mag-nitude (Figs 1 and 2). The values of k and K, werecalculated to be 8.2 x 10-2 dm ' mol-I s - j and 10.13~t 25°C and !-l =0.5.8 mol dm - 3 from the slopes andmtercepts of these plots. This kinetic pattern isidentical with that obtained in the case of oxidationof Colbipyl]" by iron(III) in the presence of addedbipyridyl reported earlier". In this case, the electron-transfer was considered to be of outer sphere typebecause rate of electron-transfer was found to bemuch greater than the rate of aquation of Co(bi-py)~+.

In this mechanism ascorbic acid is depicted toreact in the form of the unionised form, H2A. If HA-

11·0 [ASCORBIC AClo]O a '.0'10-5mol dm-3

[F'(IIIIl, = 4·0, 10-4" "

[HClO .•) 2 8·0 X 10-2 " "/.1. =- 0·58 u ."

[METHANOL) - IS'/o(V/V)TEMP a ±O·lo C

(11 2S·C(2) 30·C

(1)

10'0

K, S'O

Fe3+ +2Hbipy+ ~ Fe(bipy)3; +2H+ ... (I)7'0

F (bi )3+ k ( 2+ . + ... (II) U; 6·0e Ipy 2 + H2A --7- Fe bipy) 2 + H2A crslow 'e

xI- S,O

F (bi )3+ H A+ fast 2+' +->:::

e lpy z + 2 ---+Fe(bipy) z HzA

f-A+2H+ ... (III)

F (bi )2+ . + fast 2+ + ... (IV)e Ipy z + Hbipy ---7 Fe(bipy) 3 + H

Scheme 1In Scheme 1, A represents dehydroascorbic acidmoiety. The mechanism shown in Scheme 1 leads tothe rate-law (1 )

_ d[HzA]_ kKj[Fe3+][H2Al[bipy]~dt - [H+]2+ «, [bipy]~

... (1)

,(

2'0

1·0

~~~0.7S--~1.70--~I~.S--72.~0--72~.S--~3L.0---3L.S--~4'01 -3 -2 6

-2X 10 .rno! dm[}3IPVJo

Fig. l-Plots of 1/ k' versus 1/[Bipy15

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[NDIAN J CHEM, SEC A, DECEMBER 1989

Hence, an attempt has been made to calculate the.value of ~ G C . The potential of Feibipy n+IFe(bipy)~ + couple has been reported in the presenceof a large excess of bipyridyl". When this value wassubstituted for E~ (0.892V) (ref. 12) along with E~(1.25V) reported for H2A +IHzA couple!', ~ G?2was calculated to be 34.54 kJ mol- I. Since Fe2+ iscompletely converted into Fetbipyj]" under these

.. conditions'? the ~G?2 value should be equal to thealso is to act as the reducing species, a different type algebraic sum of the free energy changes of steps (II)of [H +1dependence should have been observed. It is and (IV) in Scheme 1.The free energy change of stepinteresting to 'note that ascorbic acid in unionised (IV) was calculated using the stepwise stability con-form is involved in oxidation by Co(III) and the reac- stant, K3, of Fe(bipY)3+ from the data of Irving andtion is known to proceed via an outer sphere me- Mellor!" and the equation ~G= .; Rf In KaK3'chanism". Further, in the present reaction the kinetic where K; is the acid dissociation constant ofresults do not indicate the formation of a precursor Hlsipy ". The free energy change of step(IV) wascomplex between Fe(bipyH+ and ascorbic acid. All found to be -27.63 kJ mol-I. Using this value, Al2these considerations have led the authors to con- was calculated from Eq. (5). Since ~Gr2 is knownelude that the present reaction also involves an outer (20 kJ mol-I )13, ~G~ was calculated using the eqlJ.a-sphere electron-transfer. tion AI2= 2(~G~ + ~Grz) and its value was found to

be 55.78 kJ mol" '.

The accuracy of the calculated value of ~G~ hasbeen tested by calculating the activation free energy~ G izvalues of the oxidations of Coibipyj] + (ref. 15)and catechol" by this complex (These reactions areknown to be of outer sphere type). Experimental va-lues of ~ G iz f0r the oxidations of Co(bipy n+ andcatechol are 4-/. and 53.92 kJ mol-I respectively.Using the value of ~G~, the self exchange free ener-gy of Fe(bipyH+ and the values of ~Grz' the self ex-change free energies of Coibipyj]" /Cotbipyjj" (ref.17) and H2Cat+ 1H2Cat (ref. 18), and employing thesame method of calculation used for the ascorbic ac-id reaction, ~Gi2 values of oxidations of Cotbipylj "and catechol were 51.15 and 56.30 kJ mol-I re-spectively. It can be seen that the experimental va-lues are in good agreement with the calculated va-lues confirming the accuracy of the values of ~ G ~,calculated. This also justifies the. applicability ofMarcus' theory to the oxidation of ascorbic acid byFe(III) in the presence of bipyridyl. This gives further

s-o

1-0

7·0

z.o

3,0 [ASCORBIC ACltl] ,I·O.IO-Smol dm-3

[Fdml] ". 4'Ox1(i4

mol dm-3o -2-3

[B1PV]o ,HxlO mol dm

}J 05S mol dm-3

[METHANOL] ~.' IS'/.{V/V)

TeMP t~,30.0 so: °c~

2·0

1·0 -

0·0o 1·0 2·0 3,0 4·0 S,O 6,0 7·0

~ +] 3 2 -6LH X 1O,mol dm

Fig. i-Plot of 1/ k' versus [H+ F

Application of Marcus theoryThe Marcus cross relationship? for outer sphere

electron-transfer processes can be prepresented byEq.(4). .

A ( ~GO )2~G#=W +~ 1+ __ 1212 12." 4 \

« /\'12

where ~ G 0 is the activation free energy of the elec-tron-transfer step (cross reaction) which is given bythe equation kl2 = Z exp( - ~Gizl Rf) (Z= IO"mol- 1 dm ' s -I), kl2 being the corresponding rateconstant (8 x lO-z dm ' mol "! S-I at 25°C in thepresent case). ~ G?z is the overall free energy changeinvolved in the reaction.

For the reaction in which at least one of the react-ants is neutral or the reaction is carried out at appre-ciable ionic strength, W12, the work term, can be neg-lected 10 and under the conditions ~ G?z ~ Al2the Eq.(4) can be rewritten 11 as

~Giz = AI/4 + ~G?/2

... (4)

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where All = 2(~G~ + ~Grz); ~G~ and ~Grz beingthe activation free energies of the self exchanges ofthe couples Fetbipyj]" IFe(bipy)~+ and HzA+ IHzArespectively. ~G?2 is given by the equation:AGO =--nF(EO- EO) where EO and EO are theu 12 I Z' I 2

standard redox potentials of the oxidising and re-ducing couples respectively. The validity of the theo-ry can be tested if the activation free energies of re-spective self-exchanges of oxidising and reducingcouples and their redox potentials are known. But inthe present case activation free energy of self-ex-change (~G~) of the couple Fe(bipy)~+/Felbipyj]";is not known.

... (5)

RAO et al: KINETICS OF OXIDATION OF L-ASCORBIC ACID BY IRON(III)

support to the conclusioncthat all these reactionsproceed by outer sphere electron transfer.

AcknowledgementThe authors (GVS, KRK and KMR) thank the

CSIR and the UGC, New Delhi for financial assist-ance.

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