Indian 10urnal of Chemistry Vol. 39A, September 2000, pp.9 I 2-920

Oscillatory reaction of bromate-gallic acid: A calorimetric and potentiometric study on inhibitory effects of salts

Sudeshna Biswas, Kallol Mukherjee l, Dulal Chandra Mukherjee2 & Satya Priya Moulik'

Centre for Surface Science, Department of Chemistry 1adavpur University, Calcutta 700 032, India

Fax - 91-33-473-4266 ; e-mail ; cssju@yahoo.co.uk IDepartment of Chemistry, A.P.c. College, West Bengal

2Department of Chemistry, Calcutta University, Calcutta 700 009, India

Received 20 October /999; revised 5 July 2000

The B-Z oscillatory reaction between potassium bromate and gallic acid (in the presence of sulphuric acid and ferroin indicator) has been calorimetrically and potentiometrically studied in the presence of electrolytes, particularly chloride and nitrate salts of potassium, sodium, barium, strontium and calcium. The reaction has been studied at different concentrations of each electrolyte. The frequency of oscillations and total enthalpy of oscillation have shown dependence on the concentration of electrolytes. At concentrations below 10-7 equiv dm-3

, the electrolytes cannot inhibit the oscillatory reaction. At higher concentrations, both the frequency and enthalpy of oscillation decrease; the process is totally suppressed at electrolyte concentration> 10-4 equiv dm-3

. Among the electrolytes studied, sodium salts (both chloride and nitrate) have shown maximum inhibitory effect. The oscillatory behaviour of the reaction probed by the potentiometric method has a general agreement with the calorimetric study. The effects of the electrolytes on the damping coefficients of the oscillatory process with declining amplitudes have been calculated and compared.

The Belousov-Zhabotinsky'·2 (B-Z) reaction has been widely studied for the last two decades' or so. It is a typical oscillatory chemical reaction where concentration of the intennediates periodically rise and fall. Studies of oscillatory reactions have yielded important results to provide insights to many kinetic4

and thennodynamic5 problems. Such studies have also provided suggestive models for a number of important biological processes6

. After the fonnulation of detailed mechanism by Field, Koros and Noyes7, (FKN mechanism) quite a large number of studies have been made by various workers using different methods e.g. spectro~hotometrl,9, potentiometr/ and thenno­metr/, 0-12 detennining the periodic oscillatory behaviours in tenns of voltage / potential, colour and temperature of the systems.

Jacobs and Epstein", have proposed a mechanistic explanation for Zhabotinsky's observation2, that addition of small amount of potassium chloride can cause inhibition of oscillation in the B-Z reaction. The inhibitory effects of iodide ion l4 in the B-Z system and bromide ion 15 in Briggs - Rauscher '6 reaction have also been studied. The effects of nitrate, sulphate, and perchlorate salts have been shown in the past" not to

affect the B-Z reaction of the malonic acid/ bromatelMn(catalyst)1H2S04 . However, Jha et al. have observed inhibitory effects on the aforesaid B-Z reaction at high [KNOJl 17.

In the present study, we have undertaken a detailed investigation of the effects on the oscillatory behaviour of the chloride and the nitrate salts of K+ , Na+ , Ca2+ , Sr2+ and Ba2+ on the bromate - gallic acid - H2S04 -

ferroin B-Z system, both potentiometrically and thermometrically. As thennometric investigations of the B-Z reaction are infrequent (much less explored)12, the thennochemical understanding of the oscillatory reactions are so far insufficient. We expect the present elaborate electrochemical and thermometric studies in the presence of added electrolytes would provide insight to the thermochemical and kinetic aspects of the B-Z oscillatory process.

Materials and Methods

Potassium bromate, sulphuric acid and gallic acid used were of E. Merck, Gennany. Ferroin indicator was prepared in the usual wa/ 2. The electrolytes used in

BISW AS el at.: OSCILLATORY REACTION OF BROMATE - GALLIC ACro 913

A ,.of 1

lo:~

~IO~ C 0 ·5 .!! o

0.. 0 o 15 30

timefmln

~IO~ .2 C 0 .5 .!! ~

o o 15 30

time/min

10 20 30

time/min

~1 '0~tn .2 'E 0 .5

~ Q.

o o 15 30 time/min

~I '0li:= i§ 0 .5

~ o o 15 30

time/min

Fig. I A- Voltage oscillation of the bromate / gallic acid /

sulphuric acid/ferroin B-Z reaction at 303K. [KBrO,] =

0.02 mol dm" ; [G. A] = 4.4 m mol dm" ; [In] = 50 11 mol

dm" ; [H2S04] = 3.6 equiv dm,3 I. Without electrolyte; II

.NaCl=IO,6 equiv dm'} ;III . SrCI2=10-4 equiv dm'}; IV .

KNO}= 10,5 equiv dm'}; V. Ba(N0 3)2=2xlO,4 equiv dm'}

his study viz. potassium chloride, sodium chloride, barium chloride, strontium chloride, calcium chloride and nitrate salts like potassium nitrate, sodium nitrate, barium nitrate, strontium nitrate and calcium nitrate were of A.R. grade products of E. Merck, India. The purity of these salts was checked by measuring their melting point, refractive index and by conductometric precipitation (as chloride salts). For the preparation of salt bridge, ammonium nitrate/ammonium sulphate and agar-agar used were also of A.R. grade products of E. Merck, India. Doubly distilled water was used in the preparation of all solutions.

Calorimetry The reaction was studied in a TRONAC 458

(U.S.A) isoperibol titration calorimeter at 303K. The method of experiment was reported earlier' 2.

Potentiometry The oscillations of the potential during the reaction

were measured at 303K constructing a cell by coupling the experimental system (with a Pt - wire) with a reference saturated calomel electrode, through a salt bridge (NH4N03 - agar - agar, when effects of chloride

~~--------------------~

<i E

B

Il m

_ _ _ _ _ __ _ _ rHne/mln

1Y

20 25 JO

Fig. l B - Oscillatory thermograms of the bromate / gallic acid /

sulphuric acid/ferroin B-Z reaction at 303K.[KBr03] = 0.02 mol dm,3 ; [G. A] = 4.4 m mol dm'} ; [In] = 50 11 mol dm'} ;

[H2S04] = 3.6 equiv dm,3 . I. Without electrolyte ; II

.NaCI=IO,6 equiv dm'} ;III . SrCI2=1O-4 equiv dm" ; IV .

KN03= 10'5 equiv dm'} ; V. Ba(N03h =2xlO,4 equiv dm"

salts are studied, and (NH4)2 S04 - agar - agar when effects of nitrate salts are tested) , and recording the oscillation of the cell e.mJ. on a strip - chart recorder of SEKONIC, SS-IOOF, Japan .

Results and Discussion Oscillation characteristics and associated heat in presence of electrolytes

The chloride and nitrate salts have influenced the oscillatory reaction herein studied. This has been manifested in the results of both potentiometric and calorimetric measurements. Typical results of both kinds of experiment are illustrated in Fig.l A and B. The salts inhibit the process, the extent of inhibition depends on the concentration and the type of the electrolyte whose detailing is done in the subsequent sections.

The electrolytes have shown inhibitory effect on the oscillatory reaction so far as frequency and total enthalpy of oscillation are concerned. The number of oscillations (n) and the total enthalpy of oscillation !::.Hoc have been found to decrease with the increase in concentration of the electrolyte in each case up to a certain threshold value of concentration, above which there is no oscillation. These threshold concentrations are of the order of 10.4 equiv dm·3

, they are different for different electrolytes. It is reasonable that the oscillatory

914 INDIAN 1 CHEM. SEC A, SEPTEMBER 2000

Table I - Inhibitory effects of chloride salts at 303 K

[KBr03] = 0.02 mol dm·3 ; [G.A] = 4.4 m mol dm,3 ; [In] = 50 J.t mol dm" ; [H2S04] = 3.6 equiv dm"

[Electrolyte]! na 11/1 Heat released in respective MlO" b! 1 mor l

equiv dm" oscillations!l

[KCI] 10.7 4 (21) 20.8 12.47,4.4,2.5, 1.08 1023 10.6 4 (19) 20.8 10.4, 4.32, 2.74, 1.95 971 10.5 4 (15) 17.87 9.56,3.74, 2.66, 1.95 896 10.4 4 (9) 15.0 10.18,3.86,2.57,1.78 920

2xlO·4 3 (8) 11.64 9.4,3 .57, 1.73 735 4xlO-4 I (6) 12.0 5.94 297 5xlO·4 0(0) 10.0

[NaCl] 10.7 4 (10) 17.16 10.8,4, 2.88, 2.3 999 10.6 2 (9) 15.0 9.5, 4 675 10.5 I (7) 13.88 8.1 405 10.4 0(5) 13.0

[BaCI2]

10.7 5 (10) 14.0 13,4.5,2.7,2, 1.3 1175 10.6 4 (10) 17.2 10.7, 4.3,3 , 2.1 1005 10.5 3 (9) 16.4 9.7,4.2, 1.82 786 10'4 2 (7) 17.8 9.6,3.6 660

2x 10-4 1(6) 13.8 9.35 468 4xlO-4 0(0) 12.0

[SR:12]

10.7 5 (17) 22.5 15.6, 5.56, 3.16, 1.5,0.85 1334 10.6 4 (15) 25 .0 11.5,4.6,3.1, 1.4 1030 10.5 3 (13) 23.0 11 .6,4.2,2.7 925 10.4 3 (10) 26.7 14,3.2, 1.5 935

2x I 0.4 2 (3) 27.0 11.2,3.36 728 4xlO·4 0(0) 16.0

[CaCI2]

10'7 5 (19) 21.8 13.74,5.2,2.6, 1.8,0.95 1215 10.6 5 (15) 21.6 13.26,4.4,2.7, 1.63, 1.1 1155 10.5 5 (15) 19.66 10.78,3.6, 2.7, 2.1 , 1.5 1034 10-4 4 (9) 20.8 11.9,4.2,2.1,0.82 951

2xlO·4 3 (6) .18.0 12.5,4.3, 1.95 938 4xlO·4 3 (5) 14.3 9,3.57, 1.9 724

5xlO·4 2 (4) 11.5 7.2,2.2 470 7.5 x 10.4 0(0) 6.7

b, MI,,,, is expressed per mole of KBrO,

a, parentheted values are by potentiometry

BISWAS et al.: OSCILLATORY REACTION OF BROMATE- GALLIC ACID 915

Table 2 • Inhibitory effects of nitrate salts at 303K

[KBr03] = 0.02 mol dm-3 ; [G.A] = 4.4 m mol dm·3

; [In] = 50 Il mol dm-3 ; [H2S04] = 3.6 equiv dm-3

[Electrolyte ]1 n" Ih 11 Heat released in respective MI,,,, hI} mar l

equiv dm-3 oscillationsl1

[KN03]

10.7 5 (19) 19.5 11.26,3.5,2.2, 1.8, 1.25 1001 10-6 5 (17) 17.0 13.18, 3.3,2, 1.43,0.77 1034 10-5 4 (13) 18.0 12.8,3,1.7, I.3 940 10.4 3 (12) 20.6 12.5,3.6,2 905

2xlO·4 I (10) 19.0 12.0 600

4xlO-4 1(8) 13.3 8.4 420

5x10-4 1(6) 11.5 6.2 310

7.5xI0·4 0(0) 5.0

[NaN03]

10-7 3 (13) 23 .0 11.2,4.2, 2.5 895 10.6 3 (12) 22.0 12,3.7,1.5 860 10.5 I (9) 29.0 12.4 620 10.4 0(7) 21.0

[Ba(N03)2]

10-7 5 ( 15) 19.0 9.6,3.5,2.4, 1.84, 1.5 942 10-6 5 (14) 17.5 9.1,3.25,2.16,1.5,1.04 853 10-5 4 (II) 17.0 8.6, 3.2, 2, 1.4 760 10-4 3 (9) 14.0 9.2, 3.3, 1.68 709

2xlO·4 2 (6) 14.0 9.5,2.7 610

4x10-4 0(0) 12.3

[Sr(N03)2]

10-7 5 (19) 17.8 12.7,3.6,2.6,2.2, 1.0 1105 10.6 5 (17) 18.0 11,3.4,2.4, 1.5,0.44 937 10.5 4(15) 18.0 11.4,4, 2.6, 1.8 990 10-4 4 (10) 13.4 10.8,3.5,2.1,0.92 866

2x10-4 3 (8) 11.7 8.4, 3.2, 2.1 685

4xlO-4 3(7) 10.8 8.3,2.6, 1.6 625

5xlO-4 1(5) 10.2 7.8 390

7.5x 10-4 0(0) 5.7

[Ca(N03)2]

10.7 5 (20) 29.0 13 .8, 4.57,3.3,2, 1.2 1244 10-(' 5 (18) 18.0 11.3, 4, 2.7, 1.86, 1.1 1048 10.5 5 (16) 18.8 13.2, 3.7,2.3, 1.6,0.86 1083 10-4 3 ( 14) 22.7 11 .0,4.0, 2.0 850

2x10-4 3 (12) 22.6 10.0,3.76, 1.45 761 4xlO-4 2 (10) 18.7 10.4,3.6 700 5xlO-4 2 (8) 9.5 70,3.0 500

7.5xI0·4 1(4) 12.0 7.8 390 10-3 0(0) 6.0

b, Mloe is expressed per mole of KBr03 .

a, parentheted values are from potentiometry

916 INDIAN J CHEM. SEC A, SEPTEMBER 2000

4

3

c

2

20

18

16

14

12

c 10

8

6

4

2

0

A

B -7

10 10' 105 104

[el "J!equivdm-'-.-

IVY

-, 10

Fig.2 - (A)Number of oscillations (n) as a function of chloride

concentration (thermometrically) at 303K. [KBrO, ] =

0.02 mol dm,·1 ; [G.A] = 4.4 m mol dm,3 ; fin) = 50 ~ mol

dm"; [H2S04] = 3.6 equiv dm" . [ 0 KCI : II to NaCI ;

III • CaCI2 ; [V D SrCI2 ; V ® BaCl2 .

(B) Number of oscillations (n) as a function of chloride

concentration (potentiometrically) at 303K. [KBrO.1] = 0.02 mol dm" ; [G.A] = 4.4 m mol dm" ; [In] = 50 ~ 11101

dm" ; [H2S04] = 3.6 equiv dm'.1 [0 KCI ; II to NaCI ; III •

CaCl2 ; [V D SrCI2 ; V ® BaCI2 .

reactions are susceptible to ionic strength effect but as the added electrolytes affect the reaction at very low concentrations, normal salt effect (or ionic strength effect) becomes redundant. The inhibitory effects of the electrolytes towards oscillation is considered to be due to the perturbation of the transition state and thereby its destabilization 17,i)Effect of chloride salts

The effects of the chloride salts have been investigated with salts of sodium, potassium, barium, strontium and calcium. The results are shown in Table I, It is observed that the number of oscillations decrease with increase in [salt]; NaCI is the most and

CaCI2 is the least effective in this respect. For [C 1- 1 <

1400

1200

'OX)

800

600

400

200 A 7

n

'0 0 -7 E 10 ..... ~~~~~10~6---1LO~5--~10~-4'-~103

"'- --- .- - [CI]/eqiJiv dm 3 __ u 0

J: <I

1400~

·1 1:~ ~ Boot

<I 400

200 B

I 0 KNO, II DNoN03

U! eCO(N03h IY 4SdNO' )2

-:=~:::a:s;;;:~V .. 60 (N03 )2

1600~ o ---L_~7--~--~_~S--~r _~4--- J

10 10' 10 10 10

[NO;]/equivdm'

Fig.3 - (A) Enthalpy of oscillation (MI,.,) as a function of chloride

concentration at 303K. [KBrO,] = 0.02 mol dm,3 ; [G.A] = 4.4

m mol dm" ; [In] = 50 ~ mol dm" ; [H2S04] = 3.6 equi v dm'

I 0 KCI ; " D NaCI ; III • CaCI2 ; [V ... SrCI2 ; V ® BaCI2 (B) Enthalpy of oscillation (MI"C> as a function of nitrate

concentration at 303K. [KBrO,] = 0.02 mol dm" ; [G.A] = 4.4

m mol dm" ; [In) = 50 ~ mol dm" ; [H2S04) = 3.6 equiv

dm" [0 KNO,; [[ D NaN01 ; III • Ca(NO,)2 ;[V ...

Sr(NO,h;V® Ba(NO,)2

10'7 equiv dm,l, there is no observable inhibition of oscillation both in the thermometric and potentiometric probing. Of the monovalent chlorides, KCI and NaCI have shown threshold concentration of 4x I 0-4 and I x 10-4 equiv dm-1 respectively, The bivalent chlorides of Ca2

+, Sr2+ and Ba2

+ have offered the threshold va lues of 5x 10'4, 2x 10'4 and 2xlO'4 equiv dm') respectively .

The number of oscillations by potentiometry is always higher than calorimetry, It is due to higher stirring condition in the calorimetric measurements; the number of oscillations normally declines with increasing agitation of the solution, The thermodynamic analysis has been made using the calorimetric results, During the calorimetric measurements, a large quantity of exothermic heat has appeared with the addition of bromate into the gallic acid! sulphuric acid mixture in the calorimetric cell under all conditions (see column 3,

\, Tables I and 2) of measurements, The initial heat Ih has

BlSWAS etat.: OSCILLATORY REACTION OF BROMATE-GALLIC ACID 917

\ I 0 KNOl m • Ca(NOYz Ii D Sr (N03)z n6. NaN03 ·

4 D V. Ba(NOl )2

11

3 t. I:J.

c 2

V

D

A 0

167 10 6 lOS 16' 163

[NO~ / equiv dm-J-_

20 I 0 KN03 II 6. NaN03

18 III • Ca(N03)2

16 rl D Sr (N03h V • Bo(N03\z

14

12

c 10

8

6

4

2 B

0 10

7 10

6 lOS -, 10 10

3

[NO~ /equiv dm-J_

Fig.4 - (A)Number of oscillations (n) as a function of nitrate

concentration (thermometrically) at 303K. [KBr03] = 0.02 mol dm·3

; [G. A] = 4.4 m mol dm·3 ; [In] = 50 11 mol

dm·3 ; [H2S04] = 3.6 equiv dm·3 I 0 KN03 ; II ~

NaN03 ; III • Ca(N03h ; IV 0 Sr (N03h; V ® Ba(N03h(B) Number of oscillations (n) as a function of

nitrate concentration (potentiometrically) at 303K.

[KBr03] = 0.02 mol dm·3 ; [G.A] = 4.4 m mol dm·3

; [In] = 5011 mol dm·3 ; [H2S04] = 3.6 equiv dm·l I 0 KN03 ; II

~ NaN03 ; III • Ca(N03h ; IV 0 Sr (N03) 2 ; V ®

Ba(N03h

been found on the whole to decrease with increase in [salt], of course with reverse trend in several occasions. The Ih has been also found to be appreciable even at the stage of complete inhibition of the oscillatory process (n=O). The heat released per oscillation diminishes with the progress of the process ( see column 4, Table I) . The total heat released at all the salt concentrations are presented in column 5 of Table I. At the lowest [salt]

=10.7 equiv dm" , the total heat released follows the order SrCh > CaCh > BaCh> KCI > NaCL At the respective threshold concentration (n=O) , the order becomes srCh > CaCh > BaCl2 > NaCI > KCL The order of the alkali salts is reversed compared to the alkaline earth salts. The number of oscillations (n) as a function of the concentration of the electrolytes are plotted in Fig. 2A (thermometry) and Fig. 2B (potentiometry) . Enthalpies of oscillation are plotted against the concentration of the salts in Fig 3A. The curves resulting from the potentiometric measurements [ Fig 2(B)] are less sharp than those from calorimetry [ Fig 2(A»). The difference is due to the efficient agitation in the calorimetry experiments. The enthalpy-[ Cl-] profiles [ Fig. 3(A)] are all nicely linear. In the overall comparison, the enthalpy of oscillation in the presence of salts ( judged from the slopes of the lines) follows the order NaCI > BaCh> SrCh > CaCh > KCL This distinctly differs from the orders of the activities of the salts at their lowest and threshold concentrations.

The inhibitory effect of KCI on both the malonic acid / bromate, and gallic acid / bromate systems has been reported in the past13

•17

. Chloride-ion-induced oscillation has been also reported at high malonic acid / bromate ratio in the cerium catalysed B-Z system. Interestingly Koros et at lOa have reported that the uncatalysed malonic acid / bromate oscillation increases substantially in the presence of 10" mol dm" Cl-; the amplitude and the period, however, decrease considerably. The [CI- ] required for the termination of the oscillation depends on the composition of the B-Z system. According to Jacobs and Epstein J3

, Cl­lengthens the induction period of the cerium catalysed malonic acid / bromate system and the inhibition is a transient phenomenon. It has been observed that the inhibition by Cl- depends on the stage of its addition .

The Cl- competes with Br and BrO,- for the bromous acid HBr02.

KI cr + W + HBr02 <=> HOBr + HOCI .... ( I )

K2 Br- + W + HBr02 <=> 2HOBr

.. .. (2) K3

Br03- + W + HBr02 <=> 2Br02 + H20 .... (3)

The bromous acid will be consumed by the Cl- so long as

918 INDIAN J CHEM. SEC A, SEPTEMBER 2000

Table 3 - Effect of electrolytes on the damping of the oscillatory where [CI- ]i is the initial concentration of the CI- in the reaction mixture and [CI-]erit is its critical concentration below which consumption of bromous acid does not occurl3. Reaction(4) is also considered feasible. I 06[Electrolyte J/equiv

dm·3

[KClJ

0.1

1.0

100

200

[NaClJ

0.1

1.0

10.0

[BaCI2J

0.1

1.0

10.0

100

[SrCI2J

0.1

1.0

100

[CaCI2J

1.0

100

[KN03J

0.1

100

400

500

[NaNO)J

0.1

10

200

[Ba (NO)hJ

0.1

10.0

200

[Sr (N03)2J

1.0

100

400

[Ca(NO)2J

1.0

10.0

200

500

process . 103almin· t

Oxidation Reduction

99 85

103 92

42 38

28 38

76 62

38 40

37 34

56 58

36 33

38 45

26 48

88 100

40 34

20 40

3 1 40

25 40

42 45

76 68

23 21

II 94

68 50

53 51

78 78

120 116

62 63

43 37

120 120

30 29

10 10

97 87

99 95

78 67

23 27

Computer evaluation considering the reaction given in Eq.(l) has shown complete inhibition of the cerium catalysed malonic acidibromate system at [CI-]= I O·3M, if it is added initiallyl8. The substituted phenols used as the organic substrates, have also shown inhibition by the [CJ-]. The quenched system can be triggered to oscillation by increasing the temperature l9

•

In the present system with gallic acid, the CI­competes with Br and Br03- for HBr02 as shown above whereby it is converted into the non-reacting CI03- (CI03- does not affect the oscillating process) and/or the organic form with the formation of the C-CI bond.

The interference of Cl- to the oscillatory process by way of interaction with HBr02 (Eq.1) and Br02 as

Br02 + 3cr + 3W = HOBr +(3/2) Cl2 + H20 .... (5)

is thermodynamically possible in view of the EO values

Br02 + 3W + 3e HOBr + H20 (E" = 1.74v) . . .. (6)

HOBr+ H+ +e 1/2 (Br2) + H20 (E" = 1.57v) ... . (7)

HBr02 + 2H+ + 2c HOBr + H20 (E" = 1.74v) . ... (8)

1/2 CI2 + e cr (E" = 1.358v) .... (9)

1/2 Br2 + e Br- (E" = 1.07v) ( 10)

The one electron process being faster than the two­electron process, the interaction with HOBr is more probable. Nevertheless, the reduction potential to form Cl- being greater than that of Br, the Cl- becomes more active at the intermediate stage of the FKN mechanism (when B( falls short of the critical value) and further lowers the[Br ] which is required for the process to return to the initial stage to complete the cycle for the maintenance of continued oscillation20

.

The counter cations of the chloride salts are responsible for their different inhibitory activities on the oscillatory process at comparable concentrations. The order of the total enthalpy change at 10-7 equiv dm-3 of the electrolytes for the process follows the sequence of their sizes except for Ba2+ and Sr2+.The same trend nearly follows also at the threshold condition i.e. at n= I.

,

BISW AS et al.: OSCILLATORY REACfION OF BROMATE - GALLIC ACID 919

This is only a tentative rationalization. Other factors may have appreciable contributions in the complex ionic environment of the oscillatory process.

ii) Effect of nitrate salts

Five nitrate salts of sodium, potassium, calcium, strontium and barium are chosen to study the effect of nitrates. Table 2 shows that the minimum concentration below which there is no inhibition is also 10.7 equiv dm" for each nitrate salts as in the case of the chlorides. The threshold concentrations above which there is no oscillation for sodium and potassium nitrates are 10,5 and 5x 10-4 equiv dm" respectively. Of the bivalent salts, calcium, strontium and barium nitrates have 7.5 x 10'4 , 5x 10-4 and 2x 10-4 equiv dm" respectively as the threshold values. The order of efficiency is Na + > K+ ,

d B 2+ S 2+ C 2+ In 11 . an a > r > a . overa companson, thermometrically obtained oscillations decline more sharply (Fig. 4A) than those obtained potentiometrically(Fig. 4B). KNO" Sr(NO,h and Ca(NO,h are distinct in this respect. KNO, and Sr(NO,h are also distinct with respect to enthalpy values (Fig. 3B); the courses are curvelinear whereas the other salts NaNO" Ca(N03h and Ba(N03)2 have produced linear dependence of enthalpy values on [NO',]. The chloride and nitrate salts have produced nearly comparable inhibitory effects.

Working on the oscillatory system of malonic acid/bromate/cerium system, Jacobs and Epstein i3 have observed no inhibitory effect of NO'3 ion. The range of [NO', ] used by them has been low, Jha et al.1? have supported oscillation inhibition for the system malonic acid/bromatelMn (catalyst)1H2S04 by using KNO, in the range of 0 ,05-1.5 mol dm". With such high [N03-], they have considered the inhibition to be due to the primary salt effect. The observations made in- the present study are entirely different from the work in the past. The low effective concentration of NO,- suggests specificity to inhibit the oscillatory reaction of gallic. acidibromate/ferroin(catalyst)1H2S04 like CI-. While there is a probable mechanism for the action of cr (as described in the previous section), a plausible mechanism for the action of N03- is not at hand . The inhibitory effect of NO,' and the different effects of the nitrate salts need rationalization. How the nitrate salts can hinder the growth of [Br] after the intermediate stage of the FKN mechanism7,2o is intriguing.

iii) Electrolyte induced damping of oscillation The oscillations of potentials studied

electrochemically are presented in Fig.l A. The voltage

periodically oscillates and the peak height decreases with time.The studied reaction js thus a damped oscillatory process. Since the oscillatory reaction is basically a multi-step process, evaluation of the kinetics of the rate process may not be meaningful. We herein, therefore discuss the damped oscillatory process in terms of the damping coefficient according to the

I . 21 re atlOn ,

In [A(t)/ A(t+ I) = a T . .. (11)

where A(t) and A(t+I) are the amplitudes of two successive oscillations, T is the time period and a is the damping coefficient.

The values of a calculated for the studied system are presented in Table 3.The a values for the oxidation and reduction steps on the whole agree with each other with only minor instances of variation. It is observed from Table 3 that for each electrolyte, a decreases with increase in [salt] both for the oxidation and the reduction steps. At comparable [salt], a follows the order Sr(N03h> Ba(NO,)2 >Ca(NO,h>KCi>SrC\z> NaCI>NaNO,>BaCI2>KNO,. Among the chloride salts, th d . K+ S 2+ N + B 2+ C 2+ e or er IS > r > a > a > a whereas among the nitrates, the order is Sr2+>Ca2+>Ba2+>Na+>K+. The nitrate salts are in general more efficient damping agents than the chloride salts. Rationalization of the above sequence in the light of the physical properties of the ions is worthwhile and is kept pending for a future study.

Conclusions (i)The chloride and nitrate salts have comparable

inhibitory effects on the oscillatory process and the threshold concentrations are of the order of 10-4 equiv dm"; (ii) the number of oscillation obtained by potentiometry is always greater than calorimetry because of the higher stirring condition of the reaction mixture in the latter method; (iii) a probable mechanism of the action of CI- is given, the effect of NO,- remains unexplained; (iv) the overall enthalpy of oscillation (~Hoc) declines with increasing [salt]; and (v) the salts have specific damping effect on the oscillatory process and the nitrate salts damp the reaction more than the chloride salts.

Acknowledgement

S B thanks Jadavpur University for laboratory facilities to pursue the work in the Centre for Surface Science, Department of Chemistry. We thank Dr. R,S, Banerjee of the Department of Chemistry, Calcutta University for useful informations.

920 INDIAN J CHEM. SEC A, SEPTEMBER 2000

References 1 Belousov B P, Sb Ref Radial Med, 1958 (1959) 145.

2 Zhabotinsky A M, Biojizika, 9 (1964) 306.

3 Noyes R M & Field RJ, Ann ReI' phys Chem, 25 (1974) 95.

4 Field R J & Noyes R M, Acc chem Res, 10 (1977) 214.

5 Glansdoff J P & Prigogine I, Thermodynamic theory of structure

stability and jluctuatiolls,( Wiley- Interscience, New York ),

1971 .

6 Chance B, Pye E K, Ghosh A K & Hess B, Biological alld

biochemical oscillators, (Academic Press, New York), 1973.

7 Field R J, Koros E & Noyes R M, J Am chel11 Soc, 94 (1972)

8649. 8 Bowers P G, Caldwell K E & Prendergast D F, J phys Chem , 76

(1972) 2185.

9 Kasperek G J & Bruice T C, Inorg Chem., I O( (971) 382.

lOa) Koros E, Orban M & Nagy Z, Nature phys Sci, 233 (1971)

137; 242 (1973) 30;

b) J plzys Chelll, 77 (1973) 3122.

II Rastogi R P, Yadava K D S & Kumar A, Indian J Chelll, 12 (1974) 1282.

12 Mukherjee K, Moulik S P & Mukherjee D C, lilt J clzelll Killel,

27 (1995) 561.

13 Jacobs S S & Epstein I R, JAm chem Soc, 98(7) (1976) 1721.

14 Kaner R J & Epstein I R, J Alii chel11. Soc, 100 (1978) 4073.

15 Carvellati R & Mongiorgi B, Int J chem Killet, 30(9) (1998) 641.

16 Briggs T S & Rauscher W C, J chem Educ, 50 (1973) 496.

17 Jha P N, Prasad B N & Prasad R K, J Indian chem Soc, 65

(1988) 177.

18 Rastogi R P & Misra G P, J p/tys Cllel11, 96 (1992) 4426.

19 Kurin-Csorgei K, Nagy G & Koros E, Chell! ph)'s Letts,

271 (1997) 67.

20 Scott S K, Oscillations, waves and chaos ill chemical killelics,(

Oxford Univ. press, NY),(1995).

21 Yarosky B & Detlaf A, Handbook of physics, (M.I.R Publishers,

Moscow) Ch 6, (1997) pp143.