Indian Journal of ChemistryVol. 18A, November 1979, pp. 398-401

Oscillatory Characteristics in Mn2+ -catalysed Reaction BetweenMalonic Acid & Bromate Ion (Belousov-Zhabotinskii Reaction) in

Orthophosphorrc Acid MediumK. PRASAD* & M. s. PRASADt

Department of Chemistry, Ranchi University, Ranchi 834008

Received 20 November 1978; revised 14 May 1979; accepted 1 June 1979

Characteristics of MnH-catalysed oscillating reaction between malonic acid and bromate ion in orthophos-phoric acid medium have been studied. The temperature-dependence of time of initiation (tlo), time period (to) andlife-time (tL) of oscillations is given by a general equation t = A exp (CfT) where A and C are constants and T istemperature in K. The time period (to) obeys an empirical equation,

- _ [ IX[MnS04] + [3 ] [KB 0 ]-1.0 [H PO ]-2.5 (to - P [M I' id] + C r 3 3 4 exp CjT)mo aomc aCI 0where p, IX, [3, ms, Co and C are arbitrary constants. Results have been analysed in the light of the Field-Koros-Noyesmechanism for the Belousov-Zhabotinskii reacion.

CHEMICAL oscillations in Mnvt-catalysedBelousov-Zhabotinskii (B-Z) reaction havebeen observed in sulphuric-'" nitric" andorthophosphoric" acid media.' In the Ce3+jCeH-catalysed B-Z reaction with malonic acid as thereducing agent in sulphuric acid medium, Noyesand coworkers- have proposed a mechanism (Field,Karas and Noyes mechanism) involving the follow-ing reactions :

Br02 + Be + 2H+ -~ HBr02 + HOBrHBr02 + Br- + H+ -~ 2HOBrHOBr + Be + H+ --+ Br2 + H20Br2 + CH2(OOOH)2 --+BrCH(COOH)2+

Bc+H+

· . (Ra)· .(R2)· .(R1)

· .(Rs)

· . (R5)

· . (RG)

-BrOa + HBr02 + H+ --+ 2Br02 + HPBr02 + Ce3+ + H+ ----+ HBr02 + CeH

-2HBr02 -+ BrOs + HOBr + H+ .. (R4)4CeH + BrCH (COOHh + 2H20 --+ 2C02 +

5H+ + Br- + 4Ce3+ + HCOOH .. (C)6CeH + CH2(COOH)2+2H20--+ 6Ce3+

+HOOOH+2C02+6H+ .. (D)

Reaction steps (C) and (D) show a competitionbetween bromomalonic acid (BrMA) and malonicacid (MA) for CeH of which only the formergenerates Be. Reaction system becomes unstableand exhibits oscillations when the stoichiometricfactor, f, i.e. the ratio of [Br=jproduced to [CeH]consumed in the above competitive reductions liesbetween 0.5 and 1.5 (ref. 6). The induction time ofoscillation is interpreted as the time required by the

+On academic leave from Giridih College, Giridih, Bihar

398

I

system to generate sufficient BrMA so that it canfavourably compete with MA for CeH to yield thevalue of f needed for oscillations. Jwo and Noyes?have observed that an increase in [MA] decreasesthe ratio of [Be] produced to [Ce(IV)] consumed.Thus an increase in [MA] should lead to an increasein induction period. However, Noyes et al»found the inverse dependence of induction periodon [MAJ. Noyes and coworkersv"? have tried toresolve the dilemma by postulating a reactionbetween malonyl radical and BrMA, the formerbeing obtained by the oxidation of MA withCe(IV)..CH(COCH)2+ BrCH(COOH)2+ H20 ----+

CH2(COOH)2+HOC(COOH)2+H++Bc.. (E)

kE = 1.0 X 105 M-I sec"This, however, does not explain how increase in[MA] helps the reaction to attain the oscillatory statequickly. Further, the reactions between malonylradical and Ce (IV)lOor tartronic acid? respectivelyreduce the efficiency of reaction (E)..CH(COOH)2+Ce4++H20 ----+

HOCH(COOH)2+Ce3++H+ .. (F)

kF = 1.0 X 105 M:» see:'..CH(COOH)2+HOCH(COOH)2 -~

.OCH(COOH)2+CH2(COOH)2 .. (G)

The dependence of induction time on [MAl has,therefore, helped in analysing the plausibility ofFKN mechanism. Similarly a dependence of lifetime of oscillations on sulphuric acid gives a maximumat 1.0Mn'12. This led to the reduction studies ofKBrOs by Ce3+ ion in different acid concentrationsshowing different stages of reduction-" in them. -

\

•

PRASAD & PRASAD : OSCILLATORY REACTION OF MALONIC ACID & BrO-a

Thus a study of dependence of oscillatory character-istics like time of initiation of oscillations (tin),time period of oscillations (to) and life-time of oscilla-tions (tL), on reactant concentrations can help inunderstanding the mechanism of reaction. In asystem having initially no Br-, the Be consumptionperiods are absent and so the time of induction ofreaction is the same as the time of initiation ofreaction (see experimental section for definitions of....tin, to and tL). Rastogi and coworkers'F '" re-ported the dependence of oscillatory characteristicson .reactant concentrations and on temperature inCeH- and Mn2+-catalysed B-Z reaction in sul-phuric acid medium. Prasad and Prasad-s havereported the dependence of oscillatory characteristicson concentrations of H2S04, HN03 and H3P04 andtheir normal salts and have shown the inhibitory roleof acid anions. Here we report the dependence oftin, to and ti. on reactant concentrations in Mn2+-catalysed B-Z reaction in relatively weak acidmedium of orthophosphoric acid and analyse themechanistic significance of the same.

Materials and MethodsMalonic acid (Riedel/Extrapure), manganous sul-

phate (S. Merck, GR), potassium bromate (Bakeranalysed) and orthophosphoric acid (E. Merck/G R)were used without further purification.

Procedure - The B-Z reaction shows oscillation in[Mn3+]/[Mn2+] which was followed potentiometri-cally using bright Pt wire as indicator electrodecoupled with S.C.E. as the reference electrode. Thetwo half cells were connected with the help of agar-agar/trisodium phosphate salt bridge. To a solutioncontaining malonic acid and manganese sulphate inorthophosphoric acid equilibrated at the desiredtemperature (±0.05°C) was added potassium bromatesolution in the same medium also equilibrated at thesame temperature. The bright Pt indicator electrodewas inserted in the reaction vessel and the e.m.f.of the system noted at varying time intervals usingdirect reading potentiometer (Titrierautomat,ILMLABOR, GDR). The e.m.f. of the system wasfound to decrease with time followed by its suddenrise signalling the onset of oscillatory state. The timeelapsed between mixing all the reactants and the onsetof oscillatory state is referred to as the time of ini-tiation of oscillation, tin. The time interval betweentwo minima in the plot of e.m.f, against time givesthe time period of oscillation, to. The mean of thefirst five agreeing time periods was taken. The timeinterval between the onset of oscillation and theoccurrence of last oscillation gives the life time ofoscillations, ti: . The dependence of tin, to andtt. on [KBrOa], [malonic acid], [MnS04], [H3P04]and temperature, T in degrK were studied.

Results and DiscussionDependence of tin on temperature,

in Fig. lea). Plot of log tin againstsatisfying Eq. l.

tin = Ai exp (Ci/T)where Ai = 2.69 X 10-10

T, is shownT-1 is linear

.. (1)

see and C =7008 deg.

(

0·6 B

(a)

04

(b)

oQ·2

8

.•.. 0·00>.9

o

-0·6.~---,="=-':::------::-::-'-::-----::-:i:- -'-_30·0 31·0 320 33'0 34·0

(T,OKf1xl04

Fig. 1- (a) Plot of log tin against T-1; (b) plot of Jog toagainst T-1; [[KBr03l = 0.044 M, [MAl = 0.064 M, [HaP04l

=2.67 M, [MnS04l = 0.0008 Ml

1·2r------------------_C b)

,-v..•.

': 0'1

G)o

o (a )o

o0·0 0:':'On---,O;-"3:;-----;::0-';-.6----0:::-'.7

9----1,.'-.2----..1

[M;!!onic Acid 1, M

-1

Fig. 2 - (a) Plot of (In against [MAl ; (b) plot of to-1 against[MA] ([KBrOal = 0.044 M, [MnSOJ = 0.0008 M, [HaPO,]

= 2.67 M, Temperature = 40°C ± 0.05]

The relation between tin and malonic acid con-centration, [MA], is shown in Fig. 2(a). The plot

-1

of tin against [MA] is linear obeying Eq. 2.tin = {mi[MA] + Ci}-l .. (2)

where mi = 0.13 sec? mole-! and C, = 0.022 sec!The dependence of tin on [MA] in sulphuric acidmedium has been shown by Prasad-a to follow Eq. 3.

tin = A[MA]-l + B .. (3)where A and B are arbitrary constants. Qualitativelythe two results are similar and also agree with thedependence of induction period on [MA] observedby Noyes et al», Thus malonic acid seems to hasten

399

INDIAN J. CHEM .. VOL. 18A, NOVEMBER 1979

the onset of oscillatory state. This can be explainedon the basis of a modified FKN mechanism as shownin Scheme 1,

(n+1)+ 0- _/o=-:c, n+ • /coo +

M" CHZ _M +zc +H ---- -(F!),,=..-c/ 'c-oo ]

""0-Scheme 1

followed by reaction steps Ro' R5; R4, R1, Rs, C andD of FKN mechanism. [M(H20)]n+ stands for

COO-hydrated catalyst ions, Ce3+ or Mn2+. ZCH/

"COO-may be malonate or bromomalonate ion and Zmay be either H or Br. Evidence for reaction stepPI is furnished by the possibility of reduction ofBrO-3 to Br(IV)13 by Ce3+ ion. The possibilitiesof reacti on steps P2 and P3 have iHsobeen accepted 7•

Variation in (KBr03], [MnS04] \and [H3PO~] leadto a minimum tin in each case, Consequently thedependence of'r.,' (whichmaybe taken as a measureInof average velocity, v, of the reaction leading to theonset of oscillations) on [H3P04] shows a maximumat a critical acid concentration. This dependence isincidentally similar to those for enzymatic reactions",

Dependence of to on temperature, TOn "Kfollows Eq. 4 (Fig. 1b),

to = Ao exp (ColT) .. (4)where Ao = 1.66 X 10-9 see and Co = 6142 K~

Dependence of to on [MA] is also similar to thatof tin on [MA] and is shown in Fig. 2(b),

to = {mo [MA] + CO}-l •• (5)where mo = 0.039 sec-1 mole'? and Co = 0.0082

sec=.3-2,-----------,----------,

o

o

oo·

os o

O-O~--+_,__-__::'_::_--__,_l;~-__:;;-';,-----'..."._I-2-4 -2-0 -16 -1-2 -O-S -0'4

logCKBr03J

Fig. 3 - Plot of log to against log [KBr031 {[MAl = 0.064 M,{MnSO,l = 0.0008 M, [H3PO,1 = 2.67 M, Temperature =

= 40°C ± 0.051

400

..

(

320-0,-------------------,

240-0

160-0

e 0'"°-to 0

80-00

O-O'-::-::-:,__--=-'-:-:c:-----'----:-'-------.J'-----'0-000 0-008 0-016 0-024 0-032 0-040

[Mn 504] M

Fig. 4 - Dependence of to on [MnSO.] {[MAl = 0.064 M,[KBr031 = 0.044 M, [H3PO.] = 2.67 M, Temperature = 40°C

± 0.05]

Fig. 3 shows linear dependence of log to on log[KBr03] satisfying Eq. 6,

to = Kl [KBr03]-1.1 ::::;K1[KBr03J-1.o ' ... (6)where K, = 0.728 see mole.

Dependence of to on ([MnS04] is linear (Fig. 4)obeying Eq. 7,to = cx[MnS04] + ~ ..(7)where IX = 6111 sec mole ? and ~ =25.0 sec.Rastogi and coworkers-v-! found that in the B-Zreaction in sulphuric acid medium the time period(and hence the frequency as well) was independentof concentration of catalysts like Ce3+ICe4+ orMn 2:-IMn 3+ and was dependent only on the concen-trations of frequency controlling species like Br03-

-MA and H2SO4' It seems, in orthophosphoric acidmedium, the catalyst, Mn2+, interacts some whatdifferently to control the frequency and time periodof oscillations.

Dependence of time period, to on concentrationof orthophosphoric acid 14 is given by Eq. 8,

to =,K2[H3PO 4]-2.5 •• (8)where K2 = 347.0 see mole2.5

Using Eqs (4) to (8), one can obtain a generalisedempirical equation, for to (Eq. 9),

= {cx[MnS04] + ~} [KB 0 ]-1.0to P mo [MA] + Co r 3

[H3P04J-2.5 exp (C/T) .. (9)where P, IX, ~, mo, Co and C are arbitrary constants.

Fig. 5 shows the dependence of life time of oscilla-tion, tL on temperature, T as

ti: = AL exp (eL IT) .. (10)where AL = 4.87 X 10-9 min and CL = 7485 "K,

Dependence of tL on [KBr03], [MA] and [H3P04]shows a maximum in each case. The depen-dence of ti. on [H3P04]14 shows that oscillation isbest favoured at a critical acid concentration. Thisis supported by the reduction studies of KBr03 byCe3+ in different acid concentrations-". At higher

PRASAD & PRASAD: OSCILLATORY REACTION OF MALONIC ACID & BrO-• 3

2·7

eno

o

2·3o

o

1·9o

1·5L.- ...L.. ----L ~ __130·0 31-0 32-0 33·0 34·0

-1 4(T~K)X 10

Fig. 5 - Plot of log tt: against T-1 {[KBrOsl = 0.044 M,[MA] = 0.064 M, [MnSO.] = 0.0008 M, [HaPO.] = 2.67 M]

concentration of acid, KBr03 can be reduced toBr(IV) and Br(IIl) stages and not Be. At lowacid concentration, Br(IV) and Br(IlI) are notformed. For oscillations to occur Br(IV), Br(IlI)and Be must exist. This is best facilitated at anoptimum acid concentration.

Fig. 6 shows the dependence of tL on [MnSO 4](Eq. 11), ,

tL = {m[MnS04] + ctl .. (11)where m = 0.714 min-I mole-:' and c=S.6 X 10-3min-I.The dependence shows deviations at very low(MnSO 4]'

Dependence of tin, to and ti: on T is given by ageneralised equation

t = A exp(CfT) .. (12)This is understandable in view of these factors beingkinetically controlled and can therefore be relatedto Arrhenius equation. 'Thus, t, = A exp (C/T) = A exp (E/RT) .. (13)where E can be related to the activation energies ofthe processes concerned and R is the gas constant.Thus froni Eqs (1), (4) and (10) the calculated valuesof Ei, Eo and EL are 13.9, 12.2 and 14.S7 kcalrespectively. In the Mn 2-f'-catalysed reaction bet-ween malonic acid and bromate ion in sulphuricacid medium, Prasad+ has obtained a similar depen-dence of tin on temperature (Eq. 14), ,

tin = K exp (C1/T) .. (14)whete K=3.50S X 10-8 min a~d C1=5.64 X 103 deg.

Equating C. with Ei/R one obtains the value ofEl as 11.22 kcal. Rastogi and coworkers'> have

I

80·0r---------....:--.:....---!..----,

e

0·008 0,016 0,024

[MnS 04J,M0·032 0'040

-1Fig. 6 - Plot of tt: against [MnSO.] [[MA] = 0.064M,[KBrOa] = 0.044 M, [HaPO.] = 2.67 M, Temperature =

40°C ± 0.05]

obtained Eo = 13.0 kcal. Under somewhat differentinitial reactant concentrations, Koros-? has foundEo = 16.1 kcal.

AcknowledgementAuthors thank Prof. H. C. Mishra, Head of the

Chemistry Department, Ranchi University for faci-lities and Dr. G. S. P. Verma for helpful discussions.One of them (M. S. P.) is also thankful to the UGC,New Delhi and Ranchi University for allowing himacademic leave for the period of investigation.

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