Indian Journal of Chemistry Vo l. 42A, May 2004, pp., 1060- 1065

A kinetic study of the reduction of chromium(VI) by thiourea in the absence and presence of manganese(II), cerium(lV) and

ethylenediaminetetra acetic acid (EDTA)

Zaheer Khan"', Mohammad Yousuf Dar & Prab ijna S S Babu

Department of Chemistry. lamia Millia Islamia. lamia Nagar, New Delhi 11 0025. India

and

Kabir-ud-D in

Department of Chemistry. A ligarh Muslim Uni versity. A ligarh 202 002, India

Received J Septelllber 2003; revised 10 Marcl! 2004

The kinetics of reducti on of Cr(VI) by thiourea has been studied spectrophotometrically in an HClO~ medium. The reaction follows overa ll third order kineti cs. first order each in Cr(V I), thi ourea and HCIO~. The react ion is HCIO~ catalyzed. Addi tion of M n(II ) and Ce( I V) ions largely decrease the reaction rate. The reaction rate is increased by add ition of EDTA. The spectrophotometri c study indica tes that the reduction of chromi um(V I) foll ows the sequence

C1.VI R Cr lV

R Crill

where, R=reductant (thiourea). The react ion is considered to proceed through the formation of a ternary EDTA­chromium(V I)-thiourea complex wh ich decomposes giving the reduced Cr( IV)-EDTA complex. fo llowed by fast reduction with thi ourea. The reduction products of ch romium(VI) have been found to be Cr( III )- thi ourea (}I. rn", = 425 and 600 nm) and Cr(ll I)-EDTA Ol.rn", = 4 10 and 550 nm). respecti vely. in the absence and presence of EDTA.

IPC Code: Int CI7 Co l G 37/ 14

Reactions of chrom ium(V I) w ith oxygen-, nitrogen­and sulphur-containing reductants have been the subject of several investigators t-5 . In some instances the formation of oxygen-, nitrogen- and sulphur­chromium(Vl) bonded intermediates have been proposed. The complex ing agen ts (pico linic acid(" 2,i-bipyridyI7, EDTA8 and I , IO-phenanthroline\ organic acids (oxali c and a-hydroxy acids to) and trans ition metal Ions [manganese( fI )t t and cerium(lV)t2] have some effect (inhibiti on and catalys is) on the reduction rate of chromi um(VI) by di ffe rent organ ic reductants. The different mecha­nisms invo lving the one-step two- and three-electron transfer were visuali zed in the redox chem istry of chrol11 i um(V 1) on several occas ions5

. Beck and Durham8 found qualitati ve ly that EDT A is an effective catalys t for the ox idation of hydrazine by chromiull1(VI ) at pH = 1.6.

The kinetics of ac id dichromate oxidation of several organic substrates are well -documented In

literature but the use of EDTA and cerium(l V) in a similar study is rareS.'!. t3. t-l. Secondly , the dual behaviour (inhibition and catalys is) of ll1anganese(ll) prompted us to undertake the ti tle study from two viewpoints: (i) to determine whether the thiourea­chromium(V I) reaction initi ally gave chromium(lV) (one-step two-electron transfer) and (i i) to invest igate whether or not the behaviour of cerium(lV) is sim ilar to that of ll1anganese(JI). We report herein the results of the reduction of thiourea by chrom ium(V I) in the absence and presence of manganese( ll ), cerium(lV) and EDTA .

Materials and Methods Ammonium cerium(lV) nitrate (99%, Merck,

India), manganese(ll) chloride (97%. LOBA, India), K2Cr20 7 (99%, Merck, India), disod ium sal t of ethylenediaminetetra acet ic acid (98%, s.d. f ine), thiourea (99%, Merck, India) and HCIO-l (Fisher, 70% reagent grade) were used without fu rther purification.

KHAN el af. : REDUCTION OF CHROMIUM(VI ) BY THIOUREA 1061

Deioni sed, CO2-free and doubly dis till ed water was used for preparing the soluti ons.

Kinetics experiments were carried out in a temperature controlled (±O.I OC) water bath . The reactio n was initi ated by adding the requisite quantity of pre-equilibrated K2Cr20 7 solution to an equilibrated mi xture of HCl04 and thiourea. The progress of the reactio n was followed by measuring the absorbance o f the remaining chromium(VI) ions at

Am" = 350 nm at known inte rval s on a Spectronic 21-D spectrophotometer, using a I-cm matched quartz cell. It was observed that there is no interfe rence due to the formation of intermediate and product. The reactions were usually followed up to not less than 80% compl etio n. The thiourea was always kept in large excess and other details of kinetic measurements were the same as reported e lsewhere !5.!6.

In a typical experiment a solution of thiourea

(2x lO·2 mol dm·3) and K2CrZ0 7 (IOx IO·4 mo l dm·3) in 0.11 mol dm·3 HCI04 was allowed to react at 30°C, After the reaction proceeded to completion the mixture was passed through a Dowex SOW (l00-200 mesh) ion-exchange column and e lution was continued with increas ing amounts of NaCl04 -HC104

solution. The spectrum of the observed species in the purple colour was recorded (Fig. I) . The spectra

consist of two broad bands with Am" = 425 and 600 nm (Fig. I B). Thi s is in accord with prev ious findings of Ol atunji and McAuley!7. Thus, [(H20)5Cr(NH2hCS]3+ (PI) is confirmed to be the product under the conditions of this work. On the

o ther hand, in the presence of EDTA (2 .5x lO'z mo l

dm·3) two peaks were observed with Am"x = 400 and

550 nm (Fig. 2) . The observed Am", values are in close agreement with the literature values! 8. !9. Thus, we may sa fe ly conclude that EDT A coordin ates with chromium(VI) and Cr(IIT)-EDTA co mplex is a final product (P3) of chromium(VI) reducti o n by thi ourea in the presence of EDTA .

In order to confirm whether or not chromium(V) is formed, some experiments were also carri ed out by

measuring the absorbance at Am", =750 nm, where chromiu1l1(V) is the o nl y absorbing specics2o, but we failed to observe the appearance of thi s io n. On the other hand, in presence of 1l1anganese( lI ) the formati on of manganese(lll) was not observed at 470 nm2

!.

Attempts were made to determine the sto ichi o­metry of the overa ll redox reac tio n by spec trophoto­

metric titration us ing [thi ourea]= I x I O'~ mol dm·3

1.0 ,----------------------,

0.8

A

."" 8 0.6 \ c: 10 £ o . -...........

\ (/) 0.4 ~

B ---. ---.. \ . ---. ............ /.., ----------. / ' . ." ------. '. --"'.-"'<-: .•. 0.0 L-____ .-.-JL-___ -=:::::a ____ A_-A--..J

0.2

350 450 550 650

Wavelength ( nm )

Fig. I-Absorpti on spectra o f the reaction mi xture containing (A) chromium(V I) o nly and (B) thio urea + ch romium(V I) + HCI04 at the end o f the reac tio n. Reaction conditions: [Cr(VI )l = I Ox 10-4 mol dm'" [thiourea] = 2x I 0.2 mol dm') and [HC I04] = 0.11 mol dm·).

1.2 f.. ) .-- \ . \ 0.8 A\~\ 8

c: 10 £

/\ j. ~~ 0 (/) .Q e. \ _. ___ • __ .--....... e\ «

0.4

.---.. . ."" , .-. • "". . "'--. 0.0

350 450 550 650

Wavelength ( nm )

Fig. 2- Effec! of EDTA (= 2.5x I 0.2 mol dnf3) on the absorption

spectra o f the reaction mixture containing rCr(VI)] = 2.5x I 0.3 mol dm·3and Ithi ourea 'l = 5x 10·2 mo l dm·' . Reacti on cond itions: Time = 60 min (A). 25 h (B) and 49h (C).

and vary ing [Cr(V I)J= 1.0 to I Ox 10.3 mo l d nf 3 at [H+] = 0.11 mo l dm·3. Results obtained by thi s method indicated 3 .1 mole of thiourea consumed per mo le of chr01l1 ium(V 1).

1062 INDIAN J CHEM , SEC. A, MAY 2004

Results and Discussion (A) Reaction in the absence of compIexing agents

The linearity of the plots of log(absorbance) versus time implies that the reactio n is first order 111

[chro mium(YJ)] . Representati ve values of the k obs

obtained at different [thiourea], [H+] and temperatures are summari sed in Tab le I , Plo ts of k obs as a function of [thiourea] and [H+] at constant temperature were linear pass ing through the origin with s lope va lues of

1.07 (y=0,998) and 1.14 (y=0,997), respec tive ly, indicating that the kinetic order for thiourea and H+ is one each ,

On the basis of the above results, the reac tio n is considered to proceed through the formation of a complex between chromium(YI) and thi ourea which can be represented by Eqs , ( 1 )-(6) . Under the ex perimenta l conditions, protonated thio urea IS the predominant species22

. The mechanism IS therefore g iven as shown in Scheme I

HCrO; + W

A

13

k C

slow

Crl\'- thiourea + B

dimcrisation radical •

j C"'~ c" - eN

while gciilkatio ll

.. , ( I )

B ,., (2)

o NH2 II I +

Ho-Cr - S - C ~ NHo + II -o C

.. , (3)

CrIV- thiourea .. , (4)

s

faSI • CrIll _ Ihiourea + I

NH2- C ~ NH

PI rauieal

... (5)

NH2 '-- C - S - s - C ....- NH ) NII ,r ' ""' NH

1'2

. ,. (6)

Scheme 1

T able I- Values of the pseudo-first orde r rate constants wi th d ifferent va ri ables for the ox idati on of thiourea by chro mium (V I)

102[Thio urea]

(mo l dm·')

1.0

2.0

3.0

4,0

5.0

6.0

1.0

1.0

1.0

102[H+]

(mo l dm·3)

4.0

2.0

4,0

6.0

9.0

11.0

13.0

4 .0

4.0

Temperature (0C)

30

30

30

40

SO

30

1(}3 [CrVI ]

(mo l dlll ·3)

1.0

1.0

1.0

0,2

0.4

0.6

0.8

1.0

104 kobs

(S· I)

5.3

10.7

17.2

23 .6

3 1.5

40.7

2.3

5.3

9.5 12,6

15,0

17,2

5.3

8.7

10.7

5.3

5.3

5.3

5,3

5.3

The rate law consistent with Scheme I may be expressed by Eq. (7) conforming to the observed first o rder dependence in [thiourea] as we ll as [H+] .

and

=

k Ke,,' Kt Kat [H+]2 [thioureah [C1.V I iT

( Kat [W] + Kat Kt [ WI 2 )

... (7)

. .. (8)

k Kes K, [ H+] [th io ureah . .. (9)

According to Eq . (9), the plot5 of knbs versus [H+] and [thi ourealr sho uld be linear pass ing through the

KH AN el ClI .: REDUCTION OF CHROMIUM (VI ) BY THIOUREA 1063

on gill with positive slopes . Such plots have been realized in the present study and the respective values of kKcsKI = 1.1 3 and 6 .9 mol2 dm-6

S-I were calculated from the slopes . The va lues of kKcsKJ, [H+] and [thi ourea] were used to find k ca J, which is in excellent agreement with kobs . Thi s confirms the validity of rate law (9) and the proposed mechani sm (Scheme 1).

In order to evaluate the apparent acti vati on parameters, the reactio n was studied at 30 , 40 and 50°C at [thiourea] (=1.0x I0-2 mo l dm-\ [Cr(VI)] (=1.0x I0-3 mol dm-3) and [W] (= 4.0x lO-2 mol dm-3).

The va lues of k ohs are g iven in T able 1. The Arrhenius plot of log kohs versus liT was linear. The values of apparent acti vati o n parameters are E" =17 kJ mo r l

,

f1H# =1 4 kJ mo r l, f1S# = -300 J K·I mo r l and f1C# =

III kJ mOrl.

(8 ) Reaction in the presence of mangancsc( lI) and ce rium(IY)

Effect of M II (lI)

A vari ation in [Mn(Il)] (0.0 to 1O.0x 10-2 mol dm-3)

decreased the rate o f reduct io n of chromium(VI ) by thiourea fro m 5.3 to 3.6x lO--l S- I in HCIO-l (4 .65x lO-2

mol dm-3) med ium. T he va lues of kobs are g iven in Table 2 and (a lso see Fig. 3). Manganese( lI ) pl ays an important ro le (i nh ib iti on23 or cata lys is24 or no effect2s

.26

) in the redox chemi stry of chromium(VI ) in the presence of organic reductants. The decrease in the rate of chromium(VI) reducti on on additio n of manganese(lI ) has been attributed to the re moval of

chromium(IV)23 by reacti on with manganese(II )

Eq. IO).

. . . (10)

Therefore, the observed inhibitory e ffect is due to the o ne-step two-electron reduction of chromium(VI)

101 [Mn(lI) or Ce(IV) I (mol dm-3 )

0.0 0.5 1.0 1.5 6.-----------r-----------.----------.

4 Mn(lI)

EDTA

• • •

OL-------~--______ ~ ________ ~ ____ ~ o 2 4 6

102 [ EDTA I (mol dm·3

)

Fig. 3--E ffect of [EDTA], [M n(ll )] and [Ce( IV)] on the rate of reduct ion of chromium(V I) by thiourea at 30°e. React ion conditions: [Cr(V I)] = l Ox 10-4 mol dm-3, [thi ourea] = I x I 0-2 mol dn,-3

Table 2- Effect of complex ing agen ts on the pseudo-first order ra te constants for the ox idation of thi ourea (= 1.0x I 0-2 mol dm-3)

by ch romium (V I) (=I.Ox I0-3 mol dm-) at 30°C

102 rEDTAI " 10-lk"hs 1021Celv i h 10-lkohs 102 [Mn IIJ C 104 kohs (mo l dm-) (S- I) (mol dm-) (S-I) (mol dm-) (S-I)

0.0 0.0 5.3 0.0 5.3

0.8 0.8 1.0 3.5 2.0 4.4

1.2 1.0 2.0 3.3 4.0 4.2

1.6 1.2 4.0 2.7 6.0 4.0

2.D 1.6 6.0 2.2 8.0 3.8

2.4 1.5 8.0 1.5 10.0 3.6

3.0 1.6 10.0 1.1

4.0 1.5 12.0 D.8

5.0 2.2 14.0 0.4

6.0 2.5

7.0 2.9

" [HCIO-lJ = 2.3x I 0--' mol dm-3 ; h&crHCIO-l 1 = 46.5x I 0-) mol dm-'>.

L064 INDIAN J CHEM. SEC. A, MAY 2004

which is in conformity with the formation of chromium(lV) as an intermediate27

.

Effect ofCe(IV)

In order to gain furth er insight into the mechani stic aspects, i.e., involvement of chromium(IV), the effect of [Ce(lV)] on the reaction rate was studied over a fixed [thiourea], [ox idant] and [W] at 30ne. A variation in [Ce(IV)] (0.0 to 14.0x LO-2 mol dm-3)

decreased the k obs from 5.3 to 0.4x lO-4 sol (Table 2) . A si milar effect of cerium(IV) on the reaction rate was also observed in studi es of the ox idation of lactic acid and 2-propanol by chromium(VI)' 2.13.

From the above observations the Scheme I can be modifi ed to Scheme 2 in the presence of manga­nese(Il ) and cerium(lV).

fa SI

0.1 v - thiourea + Mn" -+ Crill -thiourea + Mn'" (11 )

fa st

0.1 v - thi ourea + Ce'v -+ Crv - thi ourea + Ce'" r 12)

Scheme 2

(C) Reaction in presence of EDT A

Effect of EDT A

To confirm the role of EDT A molecule participation in the slow step, a variation in [EDT A] (0.8 to 7 .Ox I 0-2 mol dm-)) increased the rate of the ox idation from 0.8 to 2.9x LO-4 sol (Tabl e 2) in HCI04

(=2.3x I0-:1 mol dm-3) medium. Reaction kinetics indicated first, zero and first order dependence on [EDT A] at constant [ox idant] and [reductant] (Fig. 3). On the basis of above results (formati on of chromium(l I1 )-EDT A complex as the final product and catalytic role of EDTA) it is inferred that EDTA must be present in the inner coordination sphere of a chromium ion wi th a va lency hi gher than 3, i.e., in vo lvement of EDTA somewhere before the rate­determi ni ng step.

Under the experimental conditions and in line with 'px d the arguments presented elsew here '-' , protonate

EDT A, « HOOCCH2h HN+C H2CH2N+H(CH 2COO-) (C H2COOH)), represented as Hs y + is the predominant species The mechan ism is, therefore, gi ven as in Scheme 3.

(I + B Kul

HOOCCH 1'-. ~H,- ?ljCHFoOH _ CH-N, 'N-CH

H' K_ , 1 •.. :'c( " o ~ c - o n 'O- C- O

o CI

C2

C2 ~ Crrv_ ROTA + P2

Crlv-EDTA .... 13 ~ Crlll - EDTA + radiCfJ

P3

P2

Schemel

with

+ 3H,O

kJ KI/ Kes J Ka2 [ H+ J [thiourealr [EDTA J

( I +Ka2 [W 1)

( 14 )

( 15 )

( 16 )

( 17 )

( 18)

... ( 19)

For EDT A, the value of K a2 = 100. Thus, under the experimental conditions([H+] =2 .3x 10-] mol dm-]), I> K a2 [H+] , and Eq. (19) reduces to

kobs J = kJ KI/ Kes J Ka2 r H+ I -[ thi oureah' [ EDTA 1

(20)

Equation (20) is in good agieement with the ex perimental results, i.e., first order with respect to EDTA at lower concentrati on. In presence of EDTA, the apparent ac ti vati on parameters (Ea. L1H#, L1S# and L1C#) were also calculated using the Arrhenius and Eyring Eqs. : E,, = 27 kJ mor ', L1H# = 25 kJ mor ', L1S# = -312 J K-' mor' and L1C ff = 119 kJ mor' .

Conclusion The reaction rates are sensiti ve to manganese(l l)

and ceri um(1 V) concentrations i 11 chromi um (V 1)­thi ourea system. A perusal of literature revealed that manganese( I1 ) has inhibitory as well as caralytic role in chromic ac id ox idation of va rious organic red uctants. Catalys is of chromium(V I) ox idat ion of organic substrates by manganese(l l) could probably be ex plained due to the formation of a complex between the reductant and manganese( I1) whi ch

KH AN et 01.: REDUCTION OF CHROMI UM(V I) BY THIOU REA 1065

Table 1--Role of manganese( ll ) in the chro miu m(V I) ox idati on of diffc rcill organic rcductants

Red uctant Role of manganese(ll ) References

Hydroxy acids" catalysis I I, IS, 29-32

Carboh ydra tes" catalysis 16 (b), 33-38

Alcohols C inh ibiti on 39

Maleic ac id inhibition 39

Formi c ac id inhi biti on 39

Formaldehyde inhi biti on 23

Thiourea inhi biti on Present work

"(tan aric , lac ti c, oxa li c. malic, citric. glyoxy li c and oxaloethanoic ac ids); \fructose, xy lose, glucose, arabinose, mannose, ribose, sorbose); C( I-propa nol, 2-propanol. I-butanol, ethanol. metha nol, bcnzy l alcohol, 2-c hl oroethanol, 2-methoxy ethanol).

decomposes in a one-step three-electron ox idation reduction mechani sm directl y to manganese(lll ) and chromium(lIl ) (Table 3). One of the electrons transferred is donated by the manganese atom and the other two by the organi c substrate. The observed catalyti c effect rul es out the poss ibility of chro llliulll(lV) formati on in the rate-determining step. The inhibition effect is due to the capture of chrolllium(JV) by lllanganese( II ). Thus, we may conclude that the organi c redu ctants whi ch are not able to form complexes with manganese( lI ) act as two-equi valent reducing agents. In the inhibit ion effect , the manganese( lI ) is involved after the rate­determining step in the chromi c ac id oxidation of organic reductants. These studies can be used to resolve the differences of ex isting opinions regarding the ro le/participati on of manganese( lI ) in the redox chemi stry of chromium(V I).

Acknowledgement One of the authors (Prabijna S S Babu , JR F) is

thankful to the CS IR, New Delhi fo r financial ass istance.

References I Hasan F & Rocek J, J Alii Chelll Soc, 94 ( 1972) 3 lS I, S946. 2 Sa la L F. Pa lopoli C, Alba V & Signorell a S, Polvhedmll . 12

( 1993) 2227. 3 Milewa M & 8 0ntchev P R, Coord Chelll ReI'. 6 1 ( 1985)

24 1. 4 Khan Z & Kabir-lid-Di n. lil t J Chelll Kill et, 3 1 ( 1999) 409. 5 Khan Z. Kabir-lid-Din & Ak ram M, J Chelll Res (S) ( 1998)

460.

6 Rocek J & Peng T Y, J 1\ 111 Chelll Soc, 99 (1977) 7622. 7 Venkatasubramani an N & Sundaram S, J Illorg Nllci Chelll,

3 1 ( 1969) 176 1. 8 Beck M T & Durham D A, J Ill org Nllci Chelll , 33 ( 197 1)

46 1. 9 Kh an Z & Kabir-ud-D in . TrailS Met Chelll , 27 (2002) 832. 10 Hasan F & Rocek J, J Alii Chelll Soc, 95 ( 1973) 542 1. . I I Kabir-ud-Din , Hartan i K & Khan Z, Co/loids SIIIj" A:

Physicochelll Ellg Aspects. 193 (200 I) I. 12 Khan Z, Masan S, Raju & Kabir-ud-Din , TrailS Met Chelll ,

28 (2003) in press. 13 Doy le M, Swedo R J & Rocek J, J Alii Chelll Soc, 25 ( 1973)

8352. 14 Ab id M & Khan Z. TrailS Met Chelll , 28 (2003) 79. IS Kabir-ud- Din . Hartani K & Kh an Z, TrailS Met Chelll . 25

(2000) 478. 16 (a) Khan Z & Kabir-ud-D in, TrailS Met Chelll , 26 (200 1)

48 1; (b) Kabir-ud-D in , Morshed A M & Khan Z. Carbohydr Res, 337 (2002) 1573.

17 Olatunji M A & McA uley A, J Chelll Soc. Daltoll TrailS, ( 1975) 682.

18 Hamm R E, J Alii Chelll Soc, 75 ( 1953) 5670. 19 Thorneley R N F, Sykes A G & Gans P, J Chelll Soc (A),

( 197 1) 1494. 20 Krumpolc M & Rocek J, J Alii Chelll Soc, 10 1 ( 1979) 3206. 2 1 Macartney D H & Sutin N. Ill org Chelll , 24 ( 19S5) 3403. 22 Wa lter J F, Ryan J A & Lane T J, J Alii Chelll Soc, 78 ( 1956)

5560. 23 Perez-Bcnito J F, Arias C & Lamrhari D, J Chelll Soc Chelll

COIIIIIIIIII , ( 1992) 472. 24 Huber C F & Haight Jr. G P, J Alii Chelll Soc, 98 ( 1976)

4 128. 25 OI:llunji M A & Ayoko G A. Polyhedr(JII, 7 ( 1988) II. 26 Gasw ick D C & Krueger J H, .I Cltelll Soc. 9 1 ( 1969) 2240. 27 Srini vasan C, Chell amani A & Rajagopa l S, J Org Chelll . SO

( 1985) 120 I. 28 Underwood A L, Qllw ltitatil'e Allalysis. ed ited by R A Day

(Prenti ce-H all of India, New Delhi), 61h Edn. , p. 202 , 1993. 29 Kabir-ud- Din , Hanani K & Khan Z, Orrlll COII I II IIIII. 24

(200 1) 99. 30 Kabir-ud- Din, Hanan i K & Khan Z. lilt J Chelll Kill e! . 33

(200 1) 377. 3 1 Kab ir-ud-D in . Hanani K & Khan Z. TrailS Met Chelll. 27

(2002) 6 17. 32 Kabir-ud-Din , Hanan i K & Kh an Z. Illdimi J Chelll . 4 1 B

(2002) 26 14. 33 Kabir-ud- Din. Azmal Morshed A M & Khan Z, Illorg React

Mech, 3 (2002) 255. 34 Kabir-ud- Di n. Azmal Morshed A M & Khan Z, Ordll

COIIIIIIllII , 26 (2003) 59. 35 Kabir-ud-Din . Azmal Morshcd A M & Khan Z, lilt .I Chelll

Kill et, 35 (2003) 543. 36 Kabir-ud -Din . Azmal Morshed A M & Khan Z, .I Carbohvdr

Chelll , 22 (2003) 835. 37 Kabir-ud-Din, Azma l Morshecl A M & Khan Z, IlIdiwl J

Chelll, 43 B (2004) in press. 38 Percz-Benito J F. Ari as C, Rodriguez R M & Ros M, NeIll J

Cltelll . ( 1998) 1445. 39 Perez-Benito J F & Ari as C, Call J Chelll. 7 1 ( 1993) 649.