Indian Journal of ChemistryVol. 31A, March 1992, pp. 182-183

Belousov-Zhabotinskii reaction withacetone alone as organic substrate in

the phosphoric acid medium

S M Pastapur & V R Kulkarni"Department of Chemistry, Gulbarga University,

Gulbarga 585 106Received 6 May 1991; revised and accepted 3 December 1991

Oscillations in the Belousov-Zhabotinskii system areobserved using acetone alone as the organic substrate in thephosphoric acid medium with ferroin as catalyst at 31°C. Ithas been found that the tin (time of initiation) and tp (timeperiod) of oscillations decrease with the increase in theconcentration of acetone.

Oscillations in the Belousov-Zhabotinskii reactionscould be generated either by using a single or mixedorganic substrates! - 5. In general, acetone can formone of the constituents of the mixed organicsubstrates+">. However, efforts have been made inthe past, to produce oscil.ations with acetone alone asit can act both as a bromine scavenger and areductanr':". Stroot and Janjic reported oscillations+with 5 molar acetone; these oscillations however,could not confirmed by Noszticzius '. Rastogi andMisra 5 have recently reported oscillations in the BZsystem containing acetone alone as an organicsubstrate at a much higher temperature, namely 50°C.Here we are reporting oscillations in the BZ systemwith acetone alone as organic substrate (1.5-4.7 M) ata relatively lower temperature of 31°C (±0.1) in asystem containing phosphoric acid as a medium andferroin as the catalyst.

ExperimentalKBr03 (Baker, AR), H3P04 (Sisco) and acetone

(BOH) used were of good purity. Acetone wasinvariably distilled and all the solutions wereprepared in doubly distilled water. The ferroinsolution was prepared by using I, IO-phenanthrolinemonohydrate and ferrous ammonium sulphate". Thetotal volume of the reaction mixture was 80 ml.

The reaction was followed potentiometricallyusing bright Pt electrode as indicator electrode andcalomel electrode as reference electrode. Theexperimental set-up is shown in Fig. I. Agar-agarcontaining 1% NH4N03 was used as the salt bridge.

Notes

rii I

ir,.- 7

oFig. I-Experimental setup [I Platinum electrode, 2 calomelelectrode, 3 salt bridge, 4 reaction cell, 5 to thermostat, 6 magnetic

bead, 7 magnetic stirrer)

Table I--Oscillatory characteristics for different acetoneconcentrations

{Composition: [bromate) = 0.1M, [H3P04) = 3 M and [ferroin)= 0.0015 M}

[Acetone] Onset Time period(M) time lin Ip (see)

(see)

Amplitude ofoscillation (mY)

Min Maxl.Ot --- No oscillations ---1.52 28320 360 20 302.03 19290 300 70 1002.36 15720 270 70 1002.70 13050 216 70 1003.04 12600 168 60 903.38 10800 102 50 903.71 9660 72 50 804.05 9330 42 50 804.39 9120 36 20 404.73 6900 30 20 304.89 --- No oscillations ---

Its use was necessary to avoid the possibility ofchloride from calomel electrode influencing thecourse of the reaction 7. The reaction mixture wasthoroughly stirred with the help of a magnetic stirrer.The initial temperature of the reaction mixture,except K Br03, was maintained at 31°C (± 0.1) bypassing preconditioned water. The oscillations weregenerated by quick addition of requisite volume of

(a)

I 100 mV

pooo 13300 13600 13900

(b)

I100 mV

10700 11000 11300 11600

(el

I100 my

9JOO 9600 9900

TIME (sod10200

Fig. 2---Oscillations in redox potentials composition: {[KBrOJ)= 0.1 M, [H)P04) = 3 M, [ferroin) = D.OOI5M and [acetone) = (a)2.70 M; (b) 3.38 M and (c) 4.05 M, temperature = 31·C (± D.I)}

KBr03 thermostated at the same temperature. Theoscillations were recorded using an x-t strip chartrecorder (ECIL, Hyderabad).

Experiments were performed at differentconcentrations of acetone, while the concentrationsof the other constituents were kept constant. Theinitial composition of the mixture (other thanacetone) was [KBr03] == 0.1 M, [ferroin] = 0.0015 Mand [H3P04] = 3M.

Results and discussionTable 1 shows the behaviour of the system with

respect to tin, tp and amplitude of oscillations as theacetone concentration is varied. During the course ofreaction the period of oscillation is not uniform andhas a tendency to increase. The time for the secondoscillation was considered as fp. Fig. 2 showsoscillations in the redox potentials at 2.70 M, 3.38 Mand 4.05 M concentrations of acetone.

NOTES 183

Oscillations are not found both at lower and upperlimits of acetone concentrations. Onset time (tin) andtime period (tp) decrease with increase in theconcentration of acetone. Fig. 2 shows that the timeperiod and the amplitude of oscillations increase asthe reaction progresses.

The generation of oscillations depends on theproper coordination of rates of bromine removal andthe reduction of the oxidised metal catalyst species'.The inability of acetone alone as an organic substrateto give oscillations in other BZ systems was thought tobe due to low rate of oxidation ofacetone/bromoacetone by the oxidised form of themetal catalyst species'. Rastogi et al." have tried togenerate oscillations using higher temperatures atwhich these rates would be increased. The resultspresented here indicate that another method ofchanging the rate. by changing the environment andthe nature of the metal catalyst species is alsosuccessful in generative oscillations. This has beendone by using phosphoric acid as medium and ferroinas the catalyst. The decrease in tin and tp with increasein the acetone concentration could be rationalised bythe fact that increase in the concentration of acetoneincreases both the rates of the removal of bromine aswell as the reduction of oxidised metal species.

AcknowledgementThe authors are thankful to Prof. S P Hiremath,

Chairman, Department of Chemistry, GulbargaUnversity, Gulbarga for facilities. One of the authors(S M P) is grateful to the UGC, New Delhi for theaward of a Teacher Fellowship.

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