Indian Journal of Chemistry Vol. 4 1 B, December 2002, pp. 26 14-2624

Co-oxidation of malic acid and manganese(II) by chromium(VI) in the presence and absence of ionic surfactants

Kabir-ud-Din*, Khaled I-Iarlani & Zaheer Khan t

Depa rtment of Chemistry, Al igarh Muslim Uni versi ty Al igarh 202 002, India tDeparl mcnt of Chemistry, Jam ia Mill ia Is lamia, New Delhi I 10025, India

Receil 'eil 20 Decell/ber 2000: accepted (rel'ised) II April 2002

The reaction of mal ic acid wit h dic hromate studi ed in presence and absence of manganesc( II ) follow s a seco nd -ordcr kinet ics with rcspect to rmali c ac idl wh ich shift s to first-order in presence of manganese( II ). The catalyti c effect of manga­nese( II ) suggests that chrom ium (IV) is not formed as an intermediate . In the first-order reaction (in pre';e nce of Illanga­ncse(I I)). Illali c aeid is ox idized to carbon diox ide and malonic acid. The effect of ca ti oni c and anionic surfaeta nts was also investi gated and it was found that ca ti oni c micellcs catalyse the reacti on while ani onic micelles have no effect. The inllu­encc of differe nt parameters sllch as rreactantl , ISll rfactanll. tcmperature and addcd sa lts was considered. For su rfactanl con­cClllrat ions we ll abol'e the cirit icalmice lle concentralion, the ratt! COIlSl,lIlt -rsllrfactantl profiles ca n be in terpreted in terms of distributi on of both the reactants between water and the micelles, lIsing the binding constants 12 1.2 and 22.5 mor ' dm~ for chrolllium(VI) 10 CPB and CTAI3, respectively. The catalytic crfec t of thi s reaction is grea tt! r for CPB than C rAB . Thi s is the resul t of greater assoc iation of chrom ium(VI). not of a greater rale constant in the micelle.

Kineti c and mechanist ic studies of ch romic ac id oxi­dation of ex-hyd roxy ac ids abound in the literature'. The reac ti ons fo llow either two- lectron or three­electron one-step ox idation of substrate by chro­mium(VI) depending on the molecul ar characteri st ic of the former. Organic substrates generall y undergo at most a two-electron ox id ati on in a single step". One­step three-electron reducti on of an oxidant in vo lves more than one molec ul e or an organ ic substrate in the acti vated com plex. Rocek el ci/. J ha ve reported that such three-electron ox idati on can take place in 1:2 chromium(V I)-substrate complex . One substrate is an organi c ac id carrying a second functi onal group and the second substrate molecule is either identical with the fi rst or it may be a compound with a single func­tional group such as an alcoho l4 . The latter case repre­sents a co-ox idati on in whi ch two different substrates are ox idi zed simu ltaneously. In the co-ox idation of oxa lic acid and arsenic(lll ) by chl orochromate in aq ueous acetic ac id , Sambrani and Raju5 found that both arsen ic( lll ) and oxali c ac id were ox idi zed con­certed ly in a three equi valent step.

Manganese (II ) ion has a peculiar effect on the re­dox reacti ons of chromium(V I) with different organic reductants. It ca n act as a catalyst6-9 or an inhi bitorl ()- t~. The catalyti c effect of manganese(ll ) is con trary to the general effect or manganese( lI) on chromi um(VI ) ox idat ions-an inhi biti on caused by

manganese( ll ) capture of the chromium(lV) interme­diate, which has been used as a tool to determine the in volvement of chromium(IV) wit h reductants. A scan through the ex isting literature revea ls lack of work on the co-oxidation in volving ex-hydroxy ac ids and an inorgani c reductant whereas large body of data are ava il able for co-ox idation of orga nic reductants .

Micellar catalys is and inhibiti on have received cons iderabl e attention in view of the analogies drawn between micell ar and enzy me catalyses' S. Mi ce ll es are fo rmed in aqueo us so lutions by iiu rfac tan ts whi ch possess water-sol ubili zing moiety and water- in soluble porti on. Micelles increase rates of bimolecul ar reac­tions by concentrating both reactan ts at their su r­faces 16. Three factors may account for the rate en­hancement of an organi c reaction in aqueous so lution when the reactants are incorporated into or onto a mi­celle: approx imati on effects, electrostatic effects and medium effects. Due to these facts, a significant amount of systemati c kinetic results have been re­ported on the effect of micelles n vari ous organic reac ti ons during the past few decades 15.1(,. The chro­mic ac id ox idation of mali c acid in rW] region of 0.0 to 0.5 mol dm-3 (HClO~) has been carried out in ani­oni c surfactant, namely, sodium dodecy l sul­phate(S DS )17 .

The purpose of th is in vesti gation is to determine the e ffects of manganese(1I ), mali c: aci d and surfactant

KAI3IR -UD-DI ('/ al. : CO-OX IDATION OF MALlC ACID AND MANGANESE( II ) BY CHROM IUM(VI) 26 15

structure on kineti c parameters in order to understand clearl y the natu re of malic ae id-chromium(VL) inter­ac ti on in the ab~e nee and presence ot' ionic suractants. Two cati onic surfactants ha vi ng di f ferent head grou ps, namel y, cety I tri meth y lammon i um bromide (CT AB) and cety lpyridinium bromide (CPS ) and ani ­onic sod ium dodecy l sulphate (SDS) have been used for the purpose.

Experimental Section

Materials. Commerciall y ava ilab le reagent grade potassium dichromate (99%, Merck), DL-malic acid (99%, s.d. f ine-chem), l1langanese(l l ) chloride tetra­hydrate (97%, LOSA), cety ltrimethy lammonium bromide(+99%, Sigma), cety lpyridinium bromide (+99%, Merck) and sodium dodecy l sulphate (+98%, Fluka) were used as rece ived. The stock solutions of the reactants were prepared in doub ly-distilled deion­ized water.

Kinetic measurements. The reacti on was carri ed out at SO DC under pseudo-first order conditi on by keeping ten-fold excess of rmalic ac id] over Ichro­miuI11(V I)] in a three-necked reacti on fl ask fitted w ith a double-surface condenser to check evaporati on. A li ­quots were w ithdrawn at sui table time interval s and the decayin the absorbance of chromium(VI ) was measured spectrophotometri cally at 360 nm lx w ith the help of a Sausch and L omb Spectronic - 20. Rate con­stants were computed rrom the I i near plots of In (absorbance) versus time upto the complcti on of 80% of the reacti on. Dupli ca te k ineti c run s showed that the data were reproducible w ithin ± 3% error. Ohter detail s of the kineti c procedure were described elsewherelY.

cmc determination. The conducti vity measure­ments of surfactant solutions were made with conduc­ti vity bridge 305 (Systronics, Indi a) using conducti v­ity ce ll type CC-03 (cell constan t = J .02). The cmc values of CT A S and CPS in presence and absence of malic acid and chromium(V I) have been determined at the break points of nearl y two straight line porti ons of the specific conducti vity verslls concentration plots. The cmc values vvere found to be 6.0x I 0--1 mol dl1l-3 and 6.7x I 0--1mol dm-3 fo r CTA S and CPS

respecti ve ly in presence of chromium(VI) ( 1.0 x 10-5

mol dm-3) and malic acid ( 1.0 x 1O-5mol dm-\ The

corresponding va lues in aqueous solutions at SO DC are 1.5 x IO-} mol dm-3 (eT A S) and 1.03 x 10-3 mol dm-3 (CPS )20.

Product identification. In a typical experiment, solutions of malic acid (4 .0 cn/, 1.0 mol dm-·

1) and

potassium dichromate (5.0 cm" 0.0 I mol dm-.1 ) were mi xed at room temperature. A lthough visual observa­ti on indica ted the completi on of reacti on in 60-80 min , the mi xture was left for another 30 min . Forma­ti on of malonic ac id was ascertained on the bas is or spot test21 . A saturated solution of 2,4-dini tro­phenylhydrazine in 2N HCI was added and the pre­e i pi tated 2,4-d i n i tropheny I hyd razone was f i I tered, washed and dried, and was identiried as essential ly pure oxaloaceti c acid 2,4-dinitropheny lhydrazonc by melting poin't ( I S4DC) and compari son of the LR spec­

trum with that of an authentic sample (veooll = 16 10 and 1570 cm- I

, VC=N = 1640 cm- I) .

Test 1'01· free radical. The formation of free radica l (white precipi tate) was detected by adding acry loni­trile (5.0 cm ~, 11 .5 mol dm--') in a reacti on mi xture containing potassium dichromate (2.8 x 10- 1 mol dm-~ ) and malic ac id (0.6 mol dm-\ Formati on of white precipitate was al so observed in presence or M n(" ) (4.0 cm}, 0. 1 mol dm-\

Results and Discussion Reac/i () /l ill/he absell ce ofslII.fac/oll ts

To study the nature of the coloured product fo rmed in the reacti on of Cr(VI) and malic acid, a known amount of malic ac id (0.25 mol cl m-} ) was allowed to react completely w ith chromium(VI) (8.3 x I O - ~ mol dm-~) at SODC (the reacti on was over in ca. 20 min. ). The mixture was then coo led for ca. 10 min . at 1'00111

temperature and then the UV -vis spectra were re­corded. The most characteri stic part or chromi um( J II ) spectrum is the two d-d bands observab le in the 375-600 nl11 region (Figure 1) with Amax = 4 10 and 570 nm22

. Hamm e/ al.2} has reported that organic acids

form complexes w ith Cr(III ) and bands of Cr(ll l) shi rt toward shorter wavelength s. However, under our ex­perimental conditions, the two peaks of the co loured product remain in the same pos ition (Figure 1) which indicates that the free aqua Cr(l ll) ion is a reaction product of the Cr(V I) and malic ac id reacti on.

The rate of reducti on of Cr(V I) by malic ac id is first-order in rCr(Vl)l Crable I ), i.e.,

v = -rlrCr(Vl)]/dt = k obs IC r(VI )lr " . ( I )

It is observed that the rate increases with increasing [malic acid] (Table I ). The plot of k "hs versus rmalic acidl is a curve pass ing through the origin (Figure 2). Further, the slope of the plot of log k ob\ verslIs

2616 IND IAN J. CHEM ., SEC B, DECEMBER 2002

Table \- Effect of vary ing IC r(VI )], [malic acid l and [MIl (II )1 on the rate of ox idati ve degr' d.l­tion o f l11alic ac id by chromium(V I) at 50°C

104 [Cr(V I) 1 mo l dm-'

1.0

1.5

2.0

2.8

3.6

4.0

2.8

2.8

I 01 malic acid] mol dm-'

3.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

3.5

102 IMIl( II )1 11101 dill-)

0.0

0.0

0.0

10.9

11.0

9.7

lJ.7

10.5

10 .1

0 .2

1.0

2.3

3.6

5.4

8. 1

9 .7

11.9

14.7

18.3

9.7

0.2 21;.7

0.5 38.6

1.0 53.2

2.0 8 1.3

3.0 88.7

4.0 95. 1

5.0 98.0

6.0 98 .1

104

,(:<:11

S- I

O. ly:t

1.2

2 .0

3.5

5.4

7.0

9.5

12.3

14.5

19.2

+ 0.0:5

-0.2

+o.m

+0.0 1

0.0

+0.02

+O.O:~

-0.02

+0.02

-0.04

:t Calcu lated from Eq n (5) (as desc ri bcd ill the tex t).

log [mali c ac idl is approximately 2.0 which corre­spond to second-order dependence on [malic acid].

The form ation of an intermedi ate complex betwecn chromium(V I) and malic aci d was co nfirmed from initial absorbance change at 350 nm. The preliminary observations indicate that the absorbance of chro­mium(VI ) (2.8 x IO-~ mol dm-3,£ = 48 10 dm' mol'l cm- ') 2~ changes with [malic acid] ;::: 10.0 x 10-3 mol dm-3

. The initial rap id reaction may be represenred as

K HerO; + malic acid ~ complex + Hp .. . (2)

At constant rH+], the value of complex formati on constant (K) was determined by using the relati on

rCr(V I)J.r rilla li c acidl lmalic acid] I --~---- = + -- ... (3)

L'l.A L'l.cJ KL'l.£ /

where [Cr(Vl)h = total metal ion concentration , M= the difference in absorbance between the complex and Cr(VI) at the wavelength (A,11'" = 360 nm ) where both the uncomplcxed and complcxed fOr/n s of chro­mium(V I) absorb, L'l.E = the difference in absorpti on coefficients, and / = the path leng th ( 1.0 cm). To test the validity of Eqn (3), the left-h and-side term was plotted against [malic ac idl wh ich was found to be fairly linear. From the slope and intercept, the va lucs of K and L'l.E were calculated and found to be 600 dm' mol" and 5594 dm) 11101'1 cm- ', re ~pec tive l y.

The effect of rMn(lI )] on the oxidative degradation of malic acid (0.35 mo l dm-') by chromi um(V I) (2 .8 x 10--l mol dm-') was studi ed at 50°C. The va lues of kohs are sumll1ari zed in Table 1. The plot of depend­ence of k "hs on rmalic ac id'l is illu :; trated in Figure 2a while the influence of [M n(I l)l is shown IJ1

KABIR-UD-DIN et (II.: CO-OXIDATION OF MALIC ACID AND MANGANESE(ll ) BY CHROMI UM(V I) 2617

.. v C o

0 .6

0 .4

0. 2 [sur fac tant ] / mol dm-3

CTAB(o)/C PB ( e ) =lOX 10-4

~ 00 f---'-----'---'-----'----'--:c---'---L--l o V> D <t

0 .4 Se t (I)

0.2

0.0 '----'-----'----'-----'----'----'------''-----' 300 350 400 450 500 550 600 650

Wavelength (nm)

Figure I-Absorption spectra of reacti on mixture aCr(VI) 1= 8.3 x 10-.1 mol dm-.1 , Imali c acid] = 0.25 mol dm- 3) ai'te r completion of reac tion at 50 °C in the absence (Set I) and presence of surf'actants (Set II ). React ion time: Sct I - 30 min (0). I hI' (e); Sct II - I hr.

Figure 2b. 1 n the latter case the reaction is rather fast as compared to the chrom ium(V I) - malic ac id case but the data are clearl y indicat ive of the combined effect of both mal ic ac id and M n(Il ) on the reaction. When the two reductants are present together, the re­act ion is faster compared to the presence of onl y one of them. A plot of kubs versus IMn(lI)] yields a con­cave curve (Figure 2b) and the double reci procal plot of kUh' and [Mn(ll)] was linear with a pos itive inter­cept and positive slope. Such a plot is indicative of Michaelis-Menten catalysis (k ineti c proof fo r com­plex forma ti on). Hence compl ex format ion between malic ac id-manganese(Il) and chromium(V l) occurs ini tial ly. The plot of log k()bs versus log[malic ac id] is linear with slope = 1.0 (r = 0.996) . Thus, the second­order dependence on [mali c acidl sh ifted to first order kinetics in the presence of rMnOT»). It is also noted that the reaction is sensitive to a quite low IMn(ll )l (2.0 x 10-3 mol dm-3). The catalyti c effect of [Mn( ll )] is considered due to one-step three-electron reducti on of chromium(V l) without passing through the forma­ti on of chrol11ium(l V) as an intermed iate9

.25

. The pre­sent results are in con tras t to the observation of Pani­grahi and Mishra l7 where they claim that the rate of reac ti on is retarded by Mn(l l) ions.

Based on the experimen tal facts , two possible mechani sms can be proposed; one in the absence of Mn(ll) ; and other in the presence of Mn(l l).

, V>

0 .0

24

20

16

V> 12 D o

co<

" o B

10 2 [Mn( II)JT

(m oldm-3)

2.0 4.0

[ Cr (VI)] = 2.BXlO- 4 moldm- 3

femp.= 500

C

a

6.0

100

80

6 a ::-, ~

V> D

40 0 -"

" 0

20

a 6.0

O ~O------'------'------'----' 0.0 2.0 4 .0

laC Malic aci d JT (mol dm - 3 )

Figure 2-Vari ation of rate constant with malic acid (a) and Mn(\l ) (Jmal ic acidl = 0.35 mol dm-·1) (b).

The first reaction in the Scheme I represents for­mation of 2: 1 mali c acid-Cr(VI) complex which ex­plains the second-order dependence in rmali c acidl. This complex is represented by an open-chain struc­ture which decomposes by one-step three-electron ox idat ion-reduction mechanism directly to chro­mium(lIl), oxaloacetic acid, 3-oxo-propanoic ac id and

a COOH free rad ical. It is well known that oxidation of aldehyd ic group (-CHO) is very fast in comparison to -OH . Therefore, 3-oxo-propanoic ac id is rapidly ox idi sed into malonic acid. On the basis of Scheme J, the rate of loss of Cr(VT) can be derived (Eq n 4).

-dICr(V I)]/dt=k,Kcs t rmalic acidf rCr(Vl)h ... (4)

whose compari son with Eqn (I) gives

kOb' = k,Kes t [malic ac idf ... (5)

A plot of kobJ rmali c acidl versus [malic acid] gave a good straight line whi ch explai ns the second-Ol'der dependence on [malic acid]. The value of k, Ke" has been ca lculated from the plot of k()bJr malic acidl ver­sus lmalic acid] and was found to be 7.0 x 10-5 mar' dm\- '. Usi ng the value of k, Kc, " the kca' can be gen­erated for vari ous kinetic runs (Table I). This is a close agreement between the k Obo and kca' which pro­vides the supporti ve ev idence for the proposed mechani sm (Scheme I ) and confirms the validity of rate expression (5).

The mechani sm in Scheme I differs from th at of other workers (who had proposed a cyc li c-ester as an intermed iate in presence of == 0.02 to 2.5 mol dm-3

26 18 INDIAN J. C1-IEM ., SEC 13 , DECEMBER 2002

2HCr04' W, = 98 )

2HO -CH- COOH I CH2COOH

(A)

k

HCrO ' + 4

?~ HOOC -CH-O-Cr-O-CH-COm·[

I I I HOOC -C1-[~ OJ-[ CH 2COOH

(ll)

o B

? . Cr(l ll) + HOOCCH2 -C-COOH

II • + HC -CH2COOH + COOF·I

I

COOH + Cr(VI)

fast

fast

o II

(C) (D)

. Cr(V) + CO2

A + Cr(V) HC-Cl-I2COOH + CO~ + Cr(lll )

(D)

Scheme I

HCl04, the pH was below the pK" of a-hydroxy ac ids) from the view point of formation of cycli c ester l- I

. Under the experimental conditions of th ese workers, onl y opcn chain-cster can be an intermediate as coordination of th e carboxy l group of a-hydroxy ac ids takes place on ly at the pH which is greater than I'K" of the particular a-hydrox y acid26

.

[n presence of Mn(l[) , the reaction proceeds th rough the formation of a termolecular complex be-

t ,veen HCr 0 ~ , mal ic acid and M n( II ) (Scheme II )

because the direct ox idation of Mn(lI) by I-1 Cr04-is thermodynamica ll y unfavourabl e~ 7 .

From the Scheme II mechanislll, the following rate law Illay be deduced:

... (6)

which can be written as

.. . (7)

Rate law (7) explains all the ex perimental results including the Mi chae li s-M enten kin eti c behav iour.

Reouioll ill Ill e presell ce o/"c(ltiollic SI III(lC{{[ IIIS

III order to observe th e role of surfactants on the ox idation of malic acid by chromium(V[ ), the erfects of CTAB and CPB were studi ed at 0 .35 mol dm-.1 malic acid and 2.8 x 10-1 mol dm-J Cr(Vl ). The first­order rate constan ts (k'I') are summari zed in Tab[e II. The plot of k'l' aga inst rCTAB j or rCPB] show gradual increase 01" ra tes of nearly 1.5 - fold with the increase

in surfactant concentration from (l .O to 15 .0 X 10-1 mol dlll-J at 50 °C. However, after thi s optimum con­centrat ion, the ox idation rate decreases with increase in surl"actant co ncen trations. This behav iou r (Figure 3) may be attributed to increas ing solu bili za­ti on of the reactant species with increase in surfac tant concentrati on.

The pseudo-first-order rate co nstants (kill ) for the chrom ic ac id oxida ti on of malic ac id were obtai ned ove r a range of concentrations of oxidant , mali c acid and Mn(l[ ) in prescnce of CTAB or CPB (l0.0 x 10-1 mol dm-J

) at 50°C. The results are given in Tahlc III and Figure 4. The order with respect to ox idan t ancl reductant are first and second , respectivel y. Added Mn(II) has the same dfect (cataly s i ~) as in the ab­se nce or surracta nts. These observat ions und oubted ly show that the reaction mechani sm in the presence of cationic surfac tants remains the s ~lIne as that in the homogeneous aqu eous med iulll.

In order to explain the micell ar effect on the ox ida­ti on, an attempt has been made to employ the Menger-Portnoy' ) model (Schcme III ) where sub­strate in water, Sw associates with Illicelli sed surfac­tant, D" forl11in g SD Il with an associa ti on constant , Ks and the react ion occurs in the aqu t.:o us and mice ll ar pseudophases, with first-order-co ll ." tants, k" and k,,, , respect ively. The assull1pti ons in volved in thi s l11odel, its advantages and disadvantGg s are criti cG II )' di s­cussed by Bunton's groupl 6. The rir:;, t-orcler rare co n­stan t for the overall react ion, klV is given by Eq n(8) .

KABIR - D-DIN !'I (f l.: CO-OXIDATIO or M A LI C AC ID AND MA GANfSFII I) U), CllRW,il U)\l (V I) 26 19

"I VI

7-.:.: ~ 0

16.0

12 .0

8.0

4 .0

[

l + 2HO -CH- COOH 1,"1 '> HO -C1H- COO--- M'j

I + Mn(lI ) ---77 C H1COOH CH1COOH

(A) (B1)

o - II O-Cr-O-CH-COO--- Mn+ + H 0

II I 2

o CH 2COOH

(B2)

~ HC-CH2COOH + COO + Mn(lIl )

k2 82 ----5>3> Cr(l ll ) +

(D) fa st

COO + HCrO~ --~) CO2 + Cr(V)

A +Cr(V) ~

---3>3> HC-CH2COOH + CO2 + Cr(lll) fas t

(D)

2 Mn(lll ) + 2Hp --~) Mn(l/) + Mn02

+ 4H+

Scheme II

[matic aCidJ=0.35motdm- 3

[Cr (VI) ] = 2.8 X 1O- 4 motdm-3

temp . = 50°C

f-O CTAB

O. OL-----------~----------~----------~~~----~----~----~ 0.0 2.0 4.0 6.0 8.0 12.0 16.0 200

Schelllc II I

10 3CSurfactant JT (mot dm- 3 )

Figure J---Errcct o r [surractant I on k,~ .

k = k :1' + ( II K .\ fDII J ~I I + K s IO" l

... (8)

Eqn (8) can be rea rranged to g ive Eqn. (9 ) wh ic h is

very usefu lin that it pred ic ts that a pl ot or I /(k' \\ - k'I') ve rsus 1/[0,,1 sho uld be linea r2x . Whe n the data are filled int o Eq n (9), no linearity was observed ind icat ing that the mode l is inadequate to ex plain this re ac tio n.

2620

x ... o

18

t1

o

INDIAN J. CHEM., SEC 13 , DECEMBER 2002

Table II- Effect of vary ing [CTAI3I. ICPI:3I or [SDS I on the rate of ox idative degradati on of malic aci d (0. 35 mol dm-' ) by chromium(V I) (2 .8 x 10-4) 11101 dm-.l at 50°C

1041su rfactanti 1 04k,~/ - I

mol dm -.l C r AB CPI3

0.0 9.7 9.7

1.0 10.3 9.9

2.0 11 .0 I 1.1

3.0 11 .6 13.2

5.0 12.6 16.0

6.5 14.0 16.6

R.O 13.8 16.8

10.0 14.2 16.R

12.0

15.0 14.2 17.0

20.0 13.2 15.9

25.0 13.0 14.4

30.0 13.0 12.0

35.0 12.9 10.R

50.0 12.7 10.0

60.0 9.0

65.0 12.0

80.0 11.6

120.0

150.0

200.0

"Calcu lated frolll Eqn ( 10) (as descri bed in the tex t).

[Cr tVI) J; 1.8X 10-I'mo l dm- 3

[sur fo ctan t ] :;10X lO - L. mol dm- 3

temp; 50°C • CPS

100

80

x

I 04k IllC" I/,-1

SDS CTAB CPB

9.7

13.5" 16.5"

9.8 14.3 16.4

14.6 16.8

13.2 15.6

12.8 14.2

13.0 12.0

12.9 10.7

10.7 12.4 9.8

8.9

lUI

10.0 11 .5

9.5

9.8

10.0

I I ------ = + . - - --(/' ;' - k1j/ ) « . - (J (k". -k", )Ks rDn l

. . . (9)

Inspecti on of data in Table II ind icates th at cataly­sis of the ox idation first reaches maximum and then falls with increase in [CTAS] or [CPS]. For an obser­vation of thi s type it is likely th at both the reactants may be incorporated/associated (S in the mi cell ar phase. One of the reactants, chromic ac id , ex ists as HCr04-and Cr20 l- in our ex perimental condi ti ons (pH == 2.0) due to eq uilibrium

1? 2HCrO ~ ~ Cr20 ~ - + H20 1,0

10

~~ ____ ~ ________ L-. ______ -L ____ ~O

10C Mal;c ac ;d Jr (mOl dm- 3)

Figure 4--Val'ialion of rate constant with lIlalic acid and Mn( lJ )

(I malic acid l = 0.35 mol dm-') in the presence of surfactants.

Out of these two spec ies, HO'O j - is more reacti ve th an Cr20 72-. Due to the electrostatic attrac tion be­tween HCr04- and positive head groups of CTAS/CPB micelles, the HCr04-may get incorpo­rated/associated into the Stern layer (most of the ionic micelle med iated reac ti ons are believed to occur in thi s region), which is a water ric h regiOl? ). Two

KASIR-U D-DIN ef al.: CO-OX IDATION OF MALIC ACID AND MANGANESE(II ) BY CHROM IUM(VI) 262 1

Table IlI- Effect of varying ICr(V I)J, I malic ac id I and [Mn(ll ) I on the rate of oxidati ve degradation of malic acid by chromium(V I) at 50 °C in presence of CTAS or CPS ( 10.0 x 10-1mol dm-')

104 [Cr(V I)l 10[malic acid] mol dm-) mol dm-'

1.0 3.5

1.4

2.0

2.8

3.6

2.8 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

2.8 3.5

important factors, namely, hyd rophobic and electro­static play important role in the associa­tion/incorporation of organic molecules into mi celles. Hydrophobicity of malic ac id is diminished due to the presence of two hydrophili c -COOH goups. On the other hand , the fo llowing ac id-base equi libria ex ist in malic ac id :

From the K,\ va lucs, one ca n ascertain that under the experimental conditi ons of pH = 2.0, the union­ized species A is the major ex isting and reac ti ve spe­cies which is responsible for the formation of an es ter­like spcc ics with HCrO.)- (see Scheme I). The electro­stati c effect (due to wh ich HCr04- is in the micellar pseudo-phase) will thus help in the association of A in the Stern layer.

As both the rcactants are considered to be incorpo­rated in the micell ar phase, Eqn (8) can be mod ified as Eq . (10):

102 [Mn( lI ) I 04 k\~/s- I mol dm-) [CTABI CPS

0.0 14.2 16.8

14.0 17.0

14.1 16.9

14.4 17.1

13.9 16.7

0.0 0.4 0.6

1.5 2.0

3.0 5.0

5.6 7.8

8.0 10.0

10.3 14.0

12.4 15.3

16.0 17.6

17.6 18.6

19.8 21.0

22.0 24.3

24.5 28.0

0.0 14.2 16.8

0.1 26.0 22.0

0.2 28.0 25 .1

0.5 35 .8 33.8

1.0 44.S 39.0

2.0 70.0 60.0

3.0 84.2 75.0

4.0 100.0 9 1.6

5.0 104.0 98.5

6.0 105.0 100.0

k = kill [A] T + (K s k 11/ - k II ' ) M ~\ [D" ]

IV 1 + K s r D,,] ... ( 10)

where kw = k'w / rAw] and kill = k ~n MAS; kw and kill are the second-order rate constants and MAS (being thc mole rati o of malic acid bound to the mice llar head groups) is given as

.. . ( I I )

It is difficult to ascertain the exact location of malic acid at the surface of the micell e wi th any degree of reli ab ility. The micellar reacti ons in volving an ionic and a neutral reactant as well as ionic mi celles are beli eved to take placc ei ther inside the Stern layer or at the interface between the micell ar surface and bulk so lvent water. The positive catalytic effect of CTAB or CPB micelles on the ox idati on rate of mali c ac id by chromium(V I) reveals that this reaction probably

:U:2 2 INDIAN J. CHEM , SEC n, I)I :CEMBER 2002

dccu rs :1t th l: interfacial r :giOl~ or the Gouy-Chapman .llld Stern byers . Vcry fe\1' reports have so rar appcc't\.d whi"h rc cal :hc ncc Ll rrenu.: or such rcacti o il ~ ,!1 Ihe inlu f:lc: i;.i i"l-giU il or Stern alld Gouy-C I \'1 ktprnall ayers" .

The k~, - Isufactant] profi les show max imum which is charac teristi c or a bimolccular reaction. Thnefore. thc associati on of malic acid in micell ar and aqueous pscudophascs arc gi vcn in thc followi ng eq uation:

Kn (A)", + DI/ ~ (A)1ll

and

K = r(A)", ]

" r(A)". I(1 D,, ]- I(A)", l )

which reduces to

Kll l (A)Ill((KIl I DIlJ+K,JAh+ / )1 (A)Ill)] +KIl IDIl ]rAh=O

where [A IT = I (A) w] + [(A )11 ,1.

... ( 12)

.. . ( 13)

In order to ca lculate the values of Ks, KIl and kill by using Eqn ( 10)-( 13), exact va lucs of cmc (under kinetic conditions) o f CTAS and CPB are required fo r ca lculati on. Sroxton" method was used to determine the cmc of the work ing CTAS or CPS solu tions. In thi s method, the point of intersection or two linear

parts of k\jJ versus [e TASl or rCPSl drawn just below and above the cmc gi ves the va lue o f kineti c cmc ( Fi gun~ 5) which are found lO bc 7.5 x 10 -1 (CTA S)

and 7.3 x 10- ' mol dm-3(CPS). The va lues of Ks, KII and kill were ca lcu lated by using these cmc values with the help of a computer prog rammeJ~ . These values are given in Table IV . The value of Ks was

al so determined by spectroscopic method. A Alllax = 370 nm was chosen at which the changes in

absorbance were max imum. A t rHCrO ~ 1 =5.0 X 10-5

mol dm-" the rCTASl or ICPS] was vari ed upto 9.9x I O· ~ mol dm-3

. The value of Ks was fou nd to be 14 11.7 mOr l dm] which is higher in compari son to Ks va lue determined under the kinetic conditions. Though the values differ significant ly, the kineti ca lly determined Ks values are pertinent for the reason that thcy belong to the ac tual experi mental conditions. M oreover, fitting o f the ca lculated data (Ks , KII and kill) to Eqn ( 10) is evident fro 1 the ca lculated values

o f the rate constants, k\jJcaI shown in Table II. In the mice llar mediated reacti ons, micell es are act­

ing by taking up the reactants or merely affecting the properti es of the solvent. To decide the micellar ef-

Tahle IV - Errcct or tC l l1pe r~illl rL: on the r<ltc or oxi(\,ni l'(: dq 'radati on or mal ic ac id ((U5 mol dill ') hy cil ro ll1i um(V I)

(l.X x I O-lmol dm-.1) <I t 50 °C in prL:scncc and a h~c l ll'c of' sur­

ract <l nts ( I n.D x 10- ' mol UIll -.1) :tllt1 va luL:s or ( nK. Ks. An. A",. ,,2n •. k" / "2,,,. Ca. ""I I' . e.S~ and e.C!!

10"" "h, or kljl /S- I

TL: mpcralun:r C Aqucow, CTAB CPH

·W 5.7 turb iJ ily turb idit y

45 7. t W.7 12.5

5D 9.7 14.2 1 6.~

55 I 1.1 I Xl{ 19.6

(i0 13.6 23.5 2·J.(i

65 26.3 32 .3

Parul1lctcrs

cme (mol dm-.1)" 7.5 x 1(;-' 7.3 x to -' ((i .D x 10 ((i.7 x lO-

.') -l)

Ks Imor-1dll1') 22.5 12 1.2

K" (1ll0rl dill" ) 3.0 2.0

10\" (S- I) 4.7 -u 10.11;2", ( rnor- I drn\-I)" (-5 (i.O

k" / 1\ "2 ./ 0 4 1 0.45

f a ( id mor ' ) H2 41.0

1:111' (kJ rnor ' ) 35.5 ~8.3

1:15-11 (J K-1mol- l) - I 'XU - 180.2

I:1c ff (k Jrnol- I),I 98.0 96.5

"Obtained undcr the kincli c condi tions by using Broxton's mel hod (3 1): conductomclri call y determi ned values are given in parenthescs. hSecond-order ralc conSlanls (1; 2",) arc based on lhe relati on: ( 2111 = kill ' VIII 'J" = 2.7 x 10-' rnor l dm' s- ' is the second ·ordcr ra lC conSlanl in abscnce or surraclant. dCa lculalcd ror 50°e.

fect, the ox idation reaction was carried out in pres­ence of inorganic electroly tes at fixed [CTAS] or [CPS] ( 10.0 x 10-1 mol dm-\ [ HCrO~-] (2.8 x 10-1 11101 dm-\ Imalic acid] (0.35 mol dm-J) and tempera­

ture (50 °C). The variation of k\jJ with concentrati ons of NaCl , NaSr, NaN01 and Na~SO~ are shown in Figure 6. Added sa lts show inhi biti on effect which could be caused either by exclusion of the reactants frol11 the micelle or by bringing changes in micellar structure. The exclusion of reactant s from the reacti on site is generall y applied to salt effects upon micell ar catal ys is and inhibitionJ3, but changes in micellar structure are important in decarboxy lations34

. I n the present study , the anions of added salts (Cr , Br-, NO.l-, and SO.t ) may try to assoc iate with the posi-

KAB IR-U D-DIN el 01. : CO-OXIDATION OF MALIC ACID AND M ANGA ESE(II ) BY CHROM IUM(V I) 2623

16.0

17 .0

)- 8.0

4.0

[ mat ic ac id ] : 0 35 m ol dm-3

[Cr ( VI ) ] : 2.8 XlO- 4 m o ld m- 3

te mp. : 50°C

• (--'----- C P B • ~CTAB

0.0 '---- -----'---- - ----'-\f--L---'--'---- -' 00 5.0 10. 0 15.020.0

104CSurtactant J (mol dm - 3)

Figure 5--Determinat ion or r lllc under the experi mental ki neti c; conditions by Broxton Illethod.

[ m atica c id ] =0.35mot dm- 3 IXl

[ Cr(V I) ] =2 .8 XlO- 4motdm- 3 ~ [surfac lanl]= 10X l0-4motdm- 3 -

o 15 l emp. = 50 C

iD 14 ;;:-

« "' ..... T U

"" 14

-.r 12 S'

CPB 10

~ x

12 10

CTAB 8

o 3 5 9

102[5011J (m o l dm- 3 )

Figure 6-EtTect of NaCI (0), aBr (- ), NaNO] (t.) and NacS04 (.6. ) on kill '

tive head groups of micelles. As a resul t, malic ac id may be pushed furth er away from the mi celle assoc i­ated HCr04 -. On the other hand, the reacti on proceeds through the decomposition/decarboxy lat ion of mali c ac id. Therefore, we can safely conclude th at the above two effects i.e. exclusion and change in micell ar SLructure work together on the additi on of salt.

The temperature effect on the ox idati on rate was also studi ed in the absence and presence of thc CTAB and CPB 0 0.0 x 10-1 mol dm-3

) , Acti vation parame­ters have been computed from rate constants obtained at 40, 45, 50, 55, 60 and 65 °C using the Arrhenius plot and their values are sUll1mari zed in Table IV.

The observati on is consistent WiLh the accepted view that a slow reaction would require hi gher energy of ac ti vat ion .

The second-order rate constant (kill in S-I)* and k\\' (in mor l dm] S- I) can not be compared direc tly as both have di fferent dimensions. In order to circumve nL the di fficulty, Bunton'S suggested that the co mpari son can be made by considering a volume element of reacti on in the micellar pseudo-phase and then estimating the molarity of the reactant in that volume element. An apparent second-order rate constant (em) was then defined as

k 2 - k V

III - 1/1 0 11/ '" ( 14)

where Vm = molar volume of the reacti ve region at the mi cell ar surface (range of Vm for ionic mice lles: 0.14

O ') - d 3 116 '9 36 C 'd ' I f' d to ._,} m per mo '- " . onsl enng t le 0 L quote va lue I 6

.37.:1R as 0 .1 4 mor l dm" the calcul ated va lues of

em are given in Table IV . The second-order rate constants (em and kw, both

in mor l dm3s- l) (Table IV) are consistentl y similar in

. d G II I . k? 3'J 4U b I .. magnl tu es. enera y, !\.w > ' -m" ut t lere are many exa mples of bimolecul ar reacti ons in whi ch second­order rate constants in micell ar and aqueous pseudo­phases are similar in magnitude41

. It is due to the con­centrati ons of both reactants in the same volume of the mi cell ar pseudo-phase. Thus, we may safe ly con­clude that the associated HCr0 4 -forms an ester-li ke species with the reductant in the same mi celle. This is the main cause for the over all rate enhancement (ca­talys is) of the reac tion.

We note that km (Table IV) is nearly the same for both the surfac tants, while the Ks valuc is higher for CPB as ex pected for strong associati on with HCr04-' The kinetically observed value of the binding constant (K Il ) for the mali c ac id with CTA B and CPB are 3 and 2 mor l dm3

, respec ti vely. Therefore, thi s is attribu ted to the probable locati on of the mi celli zed malic ac id molecules being at the junctural regIon of the Stern and Gouy-Chapman layers.

i?eac/ioll in/he presence o(anionic slI/joe/am To see the effec t of ani oni c surfactant , the reaction

of malic ac id (0.35 mol dm-3) and Cr(VI) (2.8 x 10-1

mol dm-J) was also in vestigated at 50 °C in presence

of varying amounts of SDS. The va lue of kljl was

• T he second-order rate constants, kll\ is defined as k", = k' lI\ M " s, where k'ill is the first-order rate eonSlanl and M AS (= IA", l / [D ,, \) is lhe mole ral io of the second reactant bound to micellar head groups. Thererore, the unit of kill is same as /.;' 11\ i .e. , S- I.

2624 iNDIAN J. CI-IEM .. SEC B, DECFMB ER 2002

fo und to be nearly constant with increasing [SOSJ (Figure 3 and Table II). 1t is not surpri sing because there is an electrostatic repulsion between HCrO~ - and the negative head groups of SOS micelles. As a result HCr04-ex ists in the bulk aq ueous medium and the rate remains unaffected.

References I Milewa M & Bonlehev P, Coord Cltelll ReI', 61, 1985. 24 1.

and Ihe references ciled therein . 2 Ilasan r & Rocek J. J Alii Cltelll Soc. 94, 1972.9073. 3 Ip D & Rocek J, J Org Clteill . 44, 1979,3 12: Srini vasan KG

& Rocek J. J Alii Cltelll Soc. 100, 1978, 27!N. 4 Hasan F & Rocek J. J 11 111 Cltelll Soc, 94. 1972. 31 X I : 96,

1974,534; 97.1975,3762. 5 Saillbrani M I & Raj u J R, ' lldiall J Cit 1'111. 30A, 1991 , 243. 6 Kemp T J & Walers W A, J Cltem Soc, 19M, 3 193. 7 I-Iuber C F & Haight G P Jr, J Alii Cltelil SIIC, 98. 1976. 4 12!S. 8 Oswa l S L & Bakore G Y, IlIdiulI J Cltelll . 19A, 1980.2 11. 9 Khan Z, Akralll M & Kabir-ud-Din. Oxid COli III 1/111 . 24, 2001.

257. 10 Challelji A C & Mukherjee S K. J Alii Cltelll Soc, RO. 1958.

3600. II Haight G P Jr. Huang T J & Shakhashiri B Z, J IlIorE{ Nllci

Chelll. 33. 1971. 2169. 12 Perez- Benito J F. Arias C & La lllrhari D. J Citc/II Suc, Cltelll

COIIIIIII/Il . 1992.472. 13 Perez-Benito J F & Arias C, Can ./ Cit 1'111. 7 1. 1993.649. 14 Khan Z & Kabir-ud-Din, III dian J Cltelll, 39A , 2000, 522. 15 Bun ton C A. Fendler E J & Fendler J 1-1 , .f Alii Cltelll Soc, 89 .

1967, 122 ; Menger F M & Port noy C E . ./ Alii Cltelll Soc, 89, 1967.4698: l3unlon C A. Fendl ::: r E J, Sepul veda L & Yan!! K-U, J Alii Cltelll Soc, 90. 1968,55 12. -

16 Bunion C A & Savell i G. 11£1\ ' Ph."s Org Cltelli. 22, 1986, 2 13; BuniOn C A. Nome F. ROlllsted L S & Quina F 1-1 . Ace Cltelll Re.l', 24. 1991. 357; Bunion C A.'/ Mo/ Liq. 72, 1997. 23 1, and references ciled therein.

17 Pan igrahi G P & Mishra S K. Indian J Cit 1'111 , 32A. 1993. 956. 18 Sa la L F. Palopoli C. Alba Y & Signore lla S. Polrltedron, 12.

1993,2227. 19 Khan Z, Rafiquee M Z A & Kahir-ud-Din, Trans M ew/

Cltelll. 22, 1997, 350; Khan, Z, Kabir-ud-Din & Akram M , J ChI'lli Res (S), 1998,460.

20 Mukerjee P & M ysels K J. Crilica/ Micelle CcJIICelll ralirlll.l' of

Aqlleolls SllI jaclw ll.I' SY.I'lelllS (NSRDS- NBS) 197.1 ,36. 2 1 Feigl F, SpOI Tesl in Orgall ic IIlIa/ysis, ed ited by R E Oesper.

(E lsev ier, Amsterdam). 1960.

22 Signorelia S. Ri zzotto M , Daier Y . FrnscDroli M L. Palopol i C, Manino D. Bousseksou A & Sa la L. F . ./ ChI'lli Soc. Da/lon Trail S. 1996. 1607 .

23 Hamm R E, Johnson R L. Perkins R H &. Davis R E, J AI!1 ChI'lli Soc. 80, 1951), 44()CJ.

24 Connett PH & Wellcrhahn K E . ./ 11 111 Cltelll Soc. 107, I'.IS5. 42!l2, and Ihe references cited Iherei .

25 Sri ni vas~ n C, Che l!::Jman i A & Rajagr'pa l S, J Org Chelll, 50. 1985.1 20 1.

26 Khan Z, Hashmi A A. Ahmad L & Haq !VI M . 1111 J Chelll K i-111'1, 30. 1998,335 .

27 Beall ie J K & Haight G P Jr. Illorgollic i? l'ocl ioll Mechlill/SIII. Part II, ediled by J 0 Euwards. (Wiley. 'cw York). 1972. 93.

28 BUIllOIl C A, Rob ill ~ :J n L & Sepulvcd:, L , J Org Ch(,llI. 35. 1970. \08.

29 A I-Lohedall II A , 1 Cho ll Soc Perkin TraIlS 2, 1995, 1707, and references ciledlhereill.

30 BUlli on C A, Rom:i ted L S & Savcli i E. .f Alii ChI'lli Soc. 101. 1979, 1253; BUllion C A . SO/lIlioll Che'lI/ isl ry of'SI /l j{IClWII.I. Vol 2, edited by K L Milla\. (Plellum PI·e's. Nc:w YIJI' ~ ) .

1982, 5 19; /-l crn es D G, Bi shop W & R I ('hard~ . F M, J l ' iI )'s

Choll. nR. 196.:1, 1842: Khan !VI ' . .! Colloid IlIlerlllU' ··~·i . 170, 1995, 59X: Kabir-ud -Din. Han Il ; K & Khan Z, Colloid,' SlIr/'.lI. 193. 2001 , I.

3 1 Broxton T J, Chri slie J R & Dol i A J, ./ Phvs Org Cheill . 7. 1994,437.

32 Kabir-ud-Din. Salem .I K J. Kumar S ( ' Khan Z, J Colloid 111-ler(ace Sci. 9. 1999. 2 15: Colloids Surr A, 16X. 2000. 24 1; Kabir-ud-Din. Sa lem J K J. Rafiq lle,; M Z A. Kumar S &. Khan Z . ./ Colloid IlIl l'ljllce Sci. 2 13, 1999, 20: Kabi r-ud-Din , Akram M, Rafiquec M Z A & Khan Z . fill . ./. Chelll . Kill/' I. 31, 1999. 47.113. 729:ColloidsSllrlll. 178, 20111 . 167 .

33 Bunion C A. Prog Solid S/(i/C Chelll , 8. 1973. 239. 34 Bunton C A. Mineh M J. Hidalgo J & Sepul veda L. J ;\111

ChI'lli Soc, \/5. 1973. 3262. 35 BUllion C A, Cm Rell-Sci Ellg 20, J 979, I. 36 Fendler J fl. M elllIJralle Milllelic C ,elll/slr\', (Wiley. New

York) 1983. .

37 Buntoll C A, Clrrasco N, I-luang S K, Pdik C H & Romstcd L S . ./ ; \ 111 ClICIII Soc, 100, 1978. 5420.

38 BUllton C A , SIIIj{ICIl/ II IS ill SO/lIlion, edited by K L Mill:t\ [) o Shah, (Plellum Pre"s, New York) I I , 1991 , 17.

39 Bunion C A, Rivera F & Sepulveda L. '/ Org Chelll, -13, 1978, 1166.

40 Bu ist G J, Bunton C A , Robinson L. SepLt\ ved3 L & Slam M, ,/ 11 111 ChI'lli Soc. 92, 1970. 4072.

4 1 Bunion C A, Gan Leong-Hua!. Hamed F H & Mofralt J R. J Ph), Chelll. 87, J983, 336, and the refere nces eitedlhcrcin.