Kinetics of oxidation of ethylenediaminetetraacetic acid (EDT …nopr.niscair.res.in/bitstream/123456789/18714/1/IJCB 43B(1) 149-156... · Indian Journal of Chemistry Vol. 438, January

Indian Journal of Chemistry Vol. 438, January 2004, pp. 149-156

Kinetics of oxidation of ethylenediaminetetraacetic acid (EDT A) by chro­mium(VI) in the presence of perchloric acid

Zaheer Khan"*, Raju' & Kabir-ud-Dinb

"Department of Chemistry, Jamia Millia Islamia, Jami a Nagar, New Delhi 110025, India. bDepartment of Chemistry, Aligarh Muslim University, Aligarh 202 002. India.

Received 4 February 2003; accepted (revised) I August 2003

The kinetics of the ox idation of EDTA by Cr(VI) in the presence of perchl oric acid has bee n followed by visible spec­trophotometry at 350 nm. The effects of the total [Cr(VJ )], [EDTA] and [HCI04] on the rate are determined. The reac ti on is first-order each in [Cr(VJ)] and [EDTA] whereas the rate-[H+] profile suggests complicated kinetic fea tures. Addition of Mn(lJ) has a large accelerating innuence. These observations pe rmit to propose detailed mechani sm for the oxidation of EDTA by Cr(VJ) in the presence and absence of Mn(IJ). The catalytic effect of Mn(lJ ) has been asc ribed to a one-step, three­e lectron process in which a termolecular complex is form ed between the ox idant , Mn(lJ) and EDTA. The ox idation has the rate expression:

Ethylenediaminetetraacetic acid (EDT A) is a multi­functional a-amino acid that behaves as a double zwitterion in diiute aqueous solution I and has a very low solubility in water2. EDTA has a wide general application in analysis because of its powerful com­plexing action and commercial availability. It is used as chelating agent in the treatment of heavy metal poi­soning, for example, when children ingest chipped paint that contains lead and also for the determination of caJcium(lI) in blood3

. Therefore, its susceptibility to biodegradation during waste-water treatment and in the aquatic environment is an important criterion for assessing its environmental impact and toxicity. Imi­nodiacetate was reported to be an intermediate in the biodegradation pathway of EDT A 4.

Decomposition of EDT A-manganese(III) complex was reported by Pribil et ai.S

.6 which was enhanced by

light, heae and manganese(ll f EDT A is capable of acting as quadridentate-, quinquedentate- , or sexi­dentate ligand~. Its di sodium salt reacts with metal ions. The reaction with cations, e.g., M2+, may be written as,

M2+ + EDTA-----t EDTA-M + 2H+

This equation indicates that there is always a com­petition2 in the solution between metal ions and· hy­drogen ions seeking the negative sites on EDTA2. The hydrogen ion concentration (or pH) of the solution has, therefore, a marked influence on the stability of a metal-EDT A complex.

The kinetics of acid dichromate oxidation of sev­eral organic substrates are well-documented in the literature lo. ls but the use of complexing agents in a ··1 d · 16-19 M · 112 I I· slim ar stu y IS rare . Itewa et. a . ·Iave ear ler

pointed out in their pioneering review that the mecha­nism of the catalytic action of EDT A, picolini c acid,

2,2'-bipyridyl, I, 10-phenanthroline and other polyva­lent acids is unclear. Beck et. al. 20 have reported the role of EDT A in the chromium(Vl) oxidation of hy­drazine but no attempt was made to determine the kinetic relationships. For lack of information on simi­lar studies using EDTA as an oxidant, we report herein the results of the title invest igation uSll1g EDTA as the substrate in acid medium both in the absence and in the presence of manganese(l I) .

Materials

Reagent grade ethylenediaminetetraacetic acid disodium salt (98%, s .d. fine , India) , potassium di­chromate (99%, Merck, India) and manganese(I l) chloride (99%, Qualigens, India) were used to prepare their stock solutions. The solution of EDT A was stored in polyethylene bottle as its solution gradually leaches metal ions from giass containers, resulting in a change in the effective concentration of EDTA. To maintain the [W] constant, perchloric acid (Fisher, 70% reagent) was used. The solution of potass ium dichromate was stored in a dark glass bottle. Double distilled (first time from alkaline KMn04), COz-free,

I SO I DIAN J. CHEM .. SEC 13. JANUARY 2004

1.0

0.8

(])

~ 0 .6 o .D l.. o 1/1

~ 0.4

0.2

0.0 ~----~-----~----~ __ ~~~ ____ ~ ____ ~_-JLJ 350 400 450 500 550 6 00 650

Wovel e ngt h (nm)

Figure 1- UV-vis spectra of the react ion mixture containing Ichromium(V I)] (= 5.0 x 10-' mol dm-'), l EOTA] (= 2.0 x 10-2 mol dll1--' ) and [HCI04 ] (= 1.16 mol dll1-') just aeter mixing (a). and after completi on of the react ion ( 100 min) (b) at 25 °C.

de ioni sed water was used fo r dilution and for the prepara tion of stock solu tio ns of all the reactants.

Kin etic lIleasurelllents. The reac tions were started in glass-stoppered two-necked flask fitted wi th a spi­ral double-walled conde nser to check evaporatio n. A mixture containing requ ired amounts of EDTA, I-ICl04 and H20 was therma ll y equi li brated at

25.0±0. 1 °C and to thi s was added a measured amount of chromi um(V 1) sol utio n, pre-equ i I ibra ted at the same temperature. The reaction volu me was always 50 cm3

. The course of the reac tion was mo nitored by measuring the absorbance of the remaining chro­mium(V I) at definite time in te rva ls at 350 nm aga inst blanks containing a ll the constituen ts except chro­mium(V I) on a Spectro ni c 2 1-D Spectrophotometer. The reaction was normally fol lowed up to 80% com­

pletion. The pseudo-first o rder rate constants (kubs , S- l) were computed fro m the s lopes of the pl ots of log(absorbance) versus time. The va lues of r ( linear

regression coefficient) were ~ 0.995 for all the k"bs' Other experimental detail s were the same as described e lsewhere I5

.21

•

Prodllc/ analysis. For characteri zat ion o f chro­

mium(Vl ) reduc tion produ ct, EDTA (=2.0 x 10-2 mol clm-\ HCIO~ (= 1.1 6 mo l dm-1

), and chrom ium(V I)­

(=5.0 x 10-1 mo l dm-J) were mi xed a t roo m tempera­

ture (=25°C). A ltho ugh vi sual observati o n indicated that the reac tion was over in 20 - 40 mill. the mixture

was kept for ca . 60 min and then the UV-vis spec tra were recorded (Figure 1). T he yellow-coloured reac­tion mixture (Figure la) became purp le in the pres­ence of EDT A at the e nd of the re ct ion (Figure 1 b). The most characte ri sti c part of chromi um(l ll) spec­trum is the two d-d transiti ons observabl e in the 350-600 nm region l ~. Our spectra consis ts of two broad

bands with Amax = 400 ane! 550 nm. These result s indi­cate that EDT A coordinates with ch romium(lIl ) spe­

c ies. Our Amax va lues are in good agree ment with the lite rature val ues"l. T he quinquedentate nature o f the chro mium(lll ) -EDTA complex [CrOll ) YI-I (H20 ), EDTA == Y~-J , was confirmed by l R spectroscopy wh ich showed that a non-comple ed -COOH gro up was present'l . It may be assumed th at the poten tiall y pe ntadentate ethy lenediamine triacetic acid should y ie ld a very stab le chromium( ll l) complex wi th a very similar visible spectrum to that of the EDTA complex . Direct observati on o f mixtu res of chromium( III ) ni­trate and EDT A showed th at the reac tion was slow at the usua l hydrogen ion concentralions o f SLlch mix­tu res21

. Thus, it is confirmed that EDTA must coordi­nate w it h chromate ion and be pI' sent in the inner coord inat ion sphere of chromium ion with va lency higher than th ree.

Carbon diox ide"3 and formaldehydc8 were iden ti­fied by the repo rted methods as the other reaction products .

KHAN el al.: KlNETICS OF OXIDATION OF EDTA BY CHROM IUM (VI) 151

,..... Q)

u c o .0

0 .1

0.3

0 .5

~ 0., II> .0 o ~0.2

C7> o

0.4

9

20 40 6<D 80 100 120

T-ime (min.)

Figure 2 - Plols of log(absorban ce) \ '('I"SIIS lime for Ihe ox idal ion of EDTA by chromium(V I) at 25 °C. ReaClioll COilililiollS: Set A­IHCIO.d = 0. 11 ( I ). 0.46 (2). 0.69 Cl). 0.9:1 (4), 1.:19 (5), 1.62(6), 1.86 (7). and 2.32 mol dm-' (8) ; [chromium(VI)l = 1.0 x 10-' : IEDTA I = 2.0 x 10- 2 mol dm-.1 : Set B-IEDTAl = 2.0 x 10-2 (9), and 5.0 x 10-2 mol dm-' ( 10); Ic hromiulll(V I)l = 1.0 x 10-' mol dm-'; rHCI041 =

1.16 mol dm-' .

Results and Discussion Sloicliiolll e'uy 1I1NISllrelll ellls. The compositi on of

the purple coloured complex was determined by Job' s method o f cont inuous vari ations . Fo r this purpose, so luti ons of chromium(VI) and EDTA were mixed, kept in a thermos tat at 25 °C for 90 min and then their absorbance were recorded at 550 nm. The co mposi­tion was found to be 4:5 , i. e., ca. one mole of chro­mium(VI) react w ith 1.25 moles of EDTA to give chromium(Il I)- EDT A complex as the final product. Due to the co mpl ex kineti c behaviour exact stio­chiometri c eq uation can not be predicted.

Reaclioll -lilll e' CII 1"1 'e. T he log (absorbance) verslls time pl ots indi ate clearly that the redox process has induct ion period (no reac ti on) fo llowed by autoaccel­eral i( n2

').:" (Fij.',un; 2) It wa~ observed that the ex tent cl i' the i nducli()n p ri od depended on the reaction con­ditions, I.e., !HCIO.)J (Figure 2. Set A) and lEOTA] (Figure 2, Set B). It was ()b.~crved tha t the plots o f log (abso rbance) \'f!rSIIS time had the general shape shown in Figu re 2 which illdicates apparent complexi ty of

the EDTA ox idation by chromium(VI), The absorb­ance of the chromium(Vl ) remains constant for some time, after which there is sharp decrease. The time for commencement of the change in absorbance depends upo n the react ion condi tions (Table I) , i .e., [HCIO.) l and [EDTAJ.

Rale depelldence 011 [HCIO.J}. The effect of [H+] o n the rate of EDT A decomposition (second step, auto­acceleration) was studied as a function of [HClO.)]

between 0.11 and 2 ,32 mo l dm-3 at constant [EDT A] (2.0 x 10-2 mol dm- \ [Cr(Vl)l ( 1.0 x 10-:1 mol dm-3

)

and temperature (25 °C), The rate increased wi th the increase in [HCI041 (Table I),

Rale depelldence all [EDTA}. The order with re­spect to [EDT A] was deduced from the experiments performed at several [EDT A 1 with fixed chro­mium(Vl) and HelO.) concentr [ion'>. The ;EDT A 1 was varied in the range (1.0 - 6 .0) x LO-~ mol dm - .1

([Cr(VI)]= 1.0 x 10-4 rnol dm-J, lHCI04] = 1. 16 mo l

dm-'\ 25 °C) . The res ults (Table I) indicate that the order with respect to E DTA is one.

152 INDIAN J. C HEM., SEC B, JAN UARY 2004

Table I - Effect o f [HCI04] , [Cr(VI)], and [EDT A] on the k obs for the ox idat ion of EDTA by chrom ium(VI ) at 25°C.

[HCI04] I03[CrVI ] 1110 1 dm- mol dm-·1

A

3

0.0

0. 11

0.23

0.69

1.1 6

1.39

1.62

1.86

2.32

1.1 6

1.1 6

1.0

1.0

2.0

3.0

4.0

5.0

1.0

I02[EDTA] mo l dm-3

2.0

2.0

1.0

2.0

3.0

4.0

5.0

6.0

Time of 104

Inducti on kohs

period" (mi n) (S- I)

No change in 0.0 absorbance

90 1.1

70 1.4

50 2.3

30 3.8

25 5.7

20 7.6

15 7.6

10 8.0

30 3.8

30 3.8

30 3.7

30 3.9

30 3.8

30 2. 1

25 3.8

20 5.7

15 7.2

IO 9.2

8 11.1

No change in absorbance was observed du ring thi s peri od.

Rate dependence on /ChrollliulIl(VI)} . The in vari­ance of rate constants over a variation in the initi al [Cr(VI)h (1.0 x 10-3 to 7.0 X 10-3 mol dm-3

) at fi xed [EDTA] = 2.0 x 10-2 mol dm-3

, [HC I04] = 1.16 mol

dm-3 and temperature = 25°C is indicati ve of first­order dependence of the reaction in [Cr(VI )h (Table I). Thus, the reaction fo llows second order kinetics (first-order each in oxidant and EDTA). These results enable us to write the rate equation at constant acidity as:

- d[Crv'hldt = kobs [Crv'h [EDTAh

where 'T' denotes total concentration.

Mechanism

It has been established that chromate ox idations generally fall into two categories 12:

(i ) Initial reduction of chromium(VI) to chro­mium(lV) by two equivalent substrates .

(i i) Initi al reduction of chromium(VI) to chro­mium(III) by three-equi valent substrates.

Chromium(VI) + EDTA

1l Chromiwn(VI) - EDTA

p/ Cr(IV)

Scheme I

pa th II

~

Cr(IlI)

Accordingly, the fo llowing Scheme I can be pro­posed .

Before attempting to propose a mechani sm, it is necessary to di scuss on the species o f EDT A ex isting in the HCI04 medium. It has been reported26 that EDTA behaves six coordinated in aqueous solutions and participates in the fo ll owing ac id-base equilibri a:

+

+

+

+

+

+ where,

K a3 = 1.02 x 10-2 ; K ,4 = 2 .1 4 x 10-3 ; Kus = 6.92 X 10-7

and Ka6 = 5.50 X 10-". The first .two speci es (H6 y 2+ and Hs y +) are rela­

ti vely strong ac ids and normally are not of importance in evaluati on of dissociation constan ts. Therefore, E DTA has only four values of dissoc iation constants. Under the experimental conditions used in this work ([HCI04] = 0.11 - 2.32 mol dm-3

), H4 Y species exis ts in signi ficant concentration and this species is reactive

KHAN et al.: KlNETICS OF OXIDATION OF EDTA BY CHROMIUM (VI)

HCrO';- + W .. . (1)

-OOCCH2 + + CH2COO II~N-CH2-CH2-N~H

-OOCCH2 + + CH2COO _ 1I~N-CH2-CH2-N~H

HOOCCH2 CH2COOH OOCCH2 CH2COOH

(H4Y) (H3Y )

+ H+ ... (2)

CH - CH 0 HOOCCH 2"'" I 2 I ,7CH FOOH II K an CH - N+ +N - CH

H4Y + HO - Cr - OH I 2 /'c( I 2 + 2 H2O ... (3) II 0 O = C - O II "'u - C= O

0

(C l )

/ C""" + o 6 ~2 / CH2- OH2

H2O I NJ H2 + Cl

+ k, CO2 ... (4) + H ~ Cr H2

o,,~~H2 C II 0

(C2)

C2 fast + HCHO .. . (5)

Scheme II

towards complexation. The fractions of various forms (a) of EDTA in solution were calculated by using the relation27

:

a3 = Ka3Ka4[H+f/B; ~ = Ka3Ka4 Kas [H+]/B;

as = Ka3Ka4KaS KadH+]/B where B = [H+t + KdH+]3 + Ka3Ka4[H+]2

+ Ka3Ka4 Kas [H+] + Ka3Ka4 Kas Ka6

153

154 INDIAN.I. CI-IEM .. SEC B. JANUARY 200-+

At pH 1.0, the values of a i, a2, aJ, a.j and a s were found to be 0.90, 0.092, 1.86 x 1O-J, 1.3 X 10-8 and 7.54 x 10- 18

, respectively. 011 the other hand, chromic acid also participates in the ac id-base equilibria28 and the on ly species possibly controlling the rate of oxida­tion seems to be H20·0.j . Hence, the steps of the oxi­dation can be written as shown in Scheme II .

In Scheme II , the H.j Y reacts with chromic ac id to form an anhydride species (C I ) th rough its -COOI-I groupS2(J . The above mechanism is in close agreement with the resu lts of various au thors5

.s on decomposition

of EDT A by manganese(IJ). Except, the rate acce lera­ti on effect in the presence of HCIO.j, we were un ab le to obtain any spectrophotometric evidence for a ch romium(VI) complex with EDTA. The rate limit ing step is a one step, th ree-e lectron ox idat ion-reduct ion taki ng pl ace in C1. The presence of electron­withdraw ing group wi ll increase the tendency of the chromium species C 1 to accept electrons from the Teducing agent EDTA. Therefore, the escaping ten­dency of -COOH group ult imate ly increased. On the basis of this mechanism the fo llowing ki netic Eq. 6 can be derived .

- d[Crv l h kl[H+IJKmKdCI·Vlh[EDTAh

dt (I + K,IH +])( t + K aJ IH+ J) .. (6)

and, for the first-order rate constants:

kohs = klKIil K,[H+f [EDTAh/(l/ [H+]+ Ka3 + KI + KI Kd W]) ... (7)

Equation (7) is in agreement with the experimental results for the reaction, since kineti c order in [EDT A] is unity.

Effect afadded mal/ganese (1/). In order to confirm and get more insight into the reaction mechani sms shown in Schemes I and II, the effect of added man­ganese(II) was also studied because manganese(lI) plays an important role in chromic acid oxidations and is used as a tool to determine the in volvement of chromium(IV) as an intermediate3o.J' . The effect of [Mn(Il)] on the reaction rate was studied in the pres­ence of HCl04 (1.15 mol dm-3

) over a fixed [EDTA] (2.0x1O-2 mol dm-3

) and [Cr(VI)] (1.0x lO-2 mol dm-3) at 25 °C. As can be seen (Table II) , the reac­tion is sensitive even to small concentrations of Mn(II) . The effect that addition of Mn(IJ) to the solu­tion resulted in a notable increase of the reaction rate is contrary to the general effect of Mn(IJ) on chromate oxidations where an inhibition is caused due to cap­ture of chromjum(IV) intermediate30.3 I. The positive

Table II - Effect of [Mn"] on Ihe knbs for the ox i d~tion of EDT A (2.0 x 10.2 mo l dill" ) by chromiulll(VI ) ( 1.0 x I (r' 111 01

dm" ) ~t 25 °C ~nd IHCI0 41 = 1.16mol dill '"

102[Mn"] T ime of inducti on period 104

k"I" 11101 dlll-' (Illi n) (S-I)

0.0 30 3.8 0.4 22 8.1 1.0 18 18.5 1.5 16 -+0.1 2.0 15 68.2 3.4 8 Very fa, t 4.0 6 Very fast ,

cataly tic effect is In conformity wi th the reduction Cr(Vl) ----7 Cr(lll) (one-step, three electron ox idat ion) without passing through formatio n of chromium(l V) as an intermediate. Therefore, the reaction mainly occurs through path II of Scheme I.

In the presence of manganese(II), the Scheme II mechanism can be modified as Scheme III.

The catalyt ic role of manganese(II) suggests its involvement in the rate-determining step. Thus, it forms an anhydride C4. The di rect ox idation of manganese( II) ion by chromiu m(V[) is thermo­dynamically unfavourab leJo. The decomposition of EDTA-manganese(lI)-chromate complex (C4) is due to the intramoiecul ar electron transfer from the chelated EDT A to chromium(VI) and manganese(ll), respectively. Time required for the intern al redox reaction also decreased In the presence of manganese(IJ) (Table II).

Rate Eq uation ( [1 ) has been deri ed on the basis of Scheme III mechanism which shows a first-order dependence of the reaction on [Mn(Il)] and [W] , re­spectively.

kobsl= k' / K'an [H+] [C3] .. , (11 )

Conclusion The most interesting feature of thi s study is the

ox idation of EDT A by chromium(VI). We are unaware of a precedence in the redox chemi stry of EDT A. The constancy in the plots of log (absorbance) vs time in the EDT A oxidation is unique in the sense that the induction period has no reaction rate. Again , the reducing nature of EDTA may be explained by the innersphere complexation between EDT A and chromium(VI) (Scheme II); this type of behaviour is rare in the redox reactions involving intramolecular electron transfer from the chelated red uctant to the oxidant. It is well established that complex fo rmation results in considerable increase in the oxidation

KHAN et 01.: KINETICS OF OXIDATION OF EDTA BY CHROMIUM (VI) 155

/o~ / 0 , O= C Mn ' C= O

I CH2- CH2 I H2C" 1 + +1 / CH2 fast

H-__ N N - H

1 1

H4 Y + Mn(U) ... (8)

HOOC CH2 CH2COOH

(C3)

K'an ... (9)

C4 + +

H P + HCHO + CQ + Mn(III) ... (10)

Scheme III

potential of a metal ion32. This is good reason to

believe that the influence of -COOH groups in the oxidation potential of chromium(VI) may have a profound effect on the oxidizing tendency of chromium(VI) in aqueous solutions . Secondly, the positive charge on complexes Cl, and C4 may help to destabilize the higher valence state of chromium, i e., six.

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