Dual-frequency oscillations induced by bromide ion

7 June 1996

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 255 (1996) 137-141

Dual-frequency oscillations induced by bromide ion Hexing Li, Xiaojun Huang Department of Chemistry, ShanghaiNormal University, Shanghai 200234, People's Republic of China

Received 9 February 1996

Abstract The experimental behavior of the ferroin-catalyzed Belousov-Zhabotinskii (BZ)-type reaction with 3,4-dihydroxybenzoic acid as organic substrate has been investigated. It was found that the system displays two types of temporal oscillations depending on the initial concentration of bromide. When [Br-] is very high, damped high-frequency oscillations appear. When [Br- ] is very low, low-frequency oscillations of the normal type are obtained. At moderate concentrations of bromide, both high-frequency and low-frequency oscillations can be monitored with a bromide ion selective electrode. The mechanism of the dual-frequency oscillation is discussed.

1. Introduction The most extensively investigated chemical oscillatory reaction is the B e l o u s o v - Z h a b o t i n s k i i (BZ) reaction [1]. A large number of oscillators with different organic substrates have been designed [2-8]. A detailed mechanism known as the F K N mechanism has been worked out by Noyes and coworkers [9], which was confirmed by successful numerical simulations [10,11]. The heart of the oscillating system is the autocatalytic formation of HBrO 2, which is described by BrO 3 + HBrO2 + H + BrO2 + M,,+ + H+

k2

kj

~ 2BrO2 + H 2 0

(R])

HBrO z + M(n+ 1)+

(R2)

Thus, B r - plays a crucial role in 'controlling' the oscillations, as the autocatalytic reaction is turned on or off depending on the concentration of bromide. The critical concentration of bromide is k1

[Br-]crit = -~---~2[BrO;]o. It is believed that BZ-type oscillation would be inhibited if [ B r - ] is in excess in the system. During our study of the BZ-type oscillations catalyzed by ferroin with 3,4-dihydroxybenzoic acid (H 2 A) as the organic substrate, we found that the system displayed high-, low- and dual-frequency oscillations depending on the initial concentration of bromide ion.

BrO 3 + HBrO 2 + 2M "+ + 3H + ._~ 2HBrO2 + 2M(,+ 1)+ + H20

(R'),

where M is the metal-ion catalyst. Fornlation of HBrO 2 is inhibited by B r - (reaction R3): B r - + HBrO 2 + H + ~ 2HOBr.

(R3)

2. E x p e r i m e n t a l All materials were of analytical grade and used without further purification except for KBrO 3, which was recrystallized in hot water to remove B r - and

0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PII S0009-261 4(96)003 62-4

It. Li, X. Huang / Chemical Physics Letters 255 (1996) 137-141

138

Table 1 Reactant concentrations for the oscillation (tool dm 3) [BrO 3 ]

[H 2 A]

[Fe(Phen)~ + ]

[H 2 $O 4 ]

0.027 0.027 0.027 0.022-0.032

0.016 0.016 0.011-0.018 0.016

2.50 X 1.OOX 2.50 X 2.50 X

1.44-3.60 2.52 2.52 2.52

other impurities. All solutions were prepared with doubly distilled water. Reactions were performed in a stirred 150 cm 3 reaction vessel mounted in a thermostat to keep the reaction temperature at 308 + 0.2 K. The order of addition of reagents was H20, H2SO 4, Fe(phen)~ + , HzA, Br- (if needed) and finally KBrO 3. The oscillations were followed by monitoring and recording the EMF against time generated by a bromide ion selective electrode (Br ISE) and a mercurous sulfate reference electrode. No absolute calibration of bromide ion selective ion electrode was attempted.

3. Results and discussion

3.1. Typical oscillations Oscillations in the BrO~--H2A-Fe(Phen)~ + H2SO 4 system have been obtained in the concentration ranges given in Table 1. It was found that the

10- 3 1 0 - 4 - 1 . 0 0 X 10 ~ 10 3 10 -3

system exhibited different oscillations depending on the initial concentrations of bromide ion. When [Br-] 0 < 2.3 × 10 -3 mol dm -3, only low-frequency oscillations were observed; when [Br-] > 6.0 × 1 0 - 3 mol dm -3, no oscillation appeared; when 2.9 X 1 0 - 3 tool dm -3 < [Br-] < 6.0 X 1 0 - 3 mol dm -3, highfrequency oscillations were obtained; when 2.3 × 10 .3 mol dm -3 < [Br-] < 2.9 × 10 .3 mol dm -3, dual-frequency oscillations with a transition period were observed. Typical results are given in Figs. 1-3.

3.2. Products of reactions In order to understand the mechanism of the reaction, attempts were made to identify the products obtained in the reaction system of (a) high-frequency oscillation, (b) low-frequency oscillation and (c) dual-frequency oscillation. The reaction mixtures were left to stand for 48 h to allow the reaction to go to completion. The organic species of the reaction mixture were extracted

>-

i

5

10

15

i

*

20

25

m

30

35

t/rain Fig. 1. Low-frequency oscillation. [H2A] o = 0.016 tool dm -3, [BrO3] o = 0.028 tool dm -3, [Fe(Phen) 2+ ]o = 0.0025 mol dm -3, [H2SO4] o = 2.50 mol dm 3, [Br ]o = 0, T = 308 K, V = 50 cm 3. [Br ] increases as E increases.

H. Li, X. Huang / Chemical Physics Letters 255 (1996) 137-)41 COOH

X..__

B

139 ~OOH

OH

>

COOH

OH

H

OH

OH

~2

(1) 0

2.5

5

75

10

(I1)

12.5

Vlnin

3. Both high- and low-frequency oscillations were inhibited by acrylamide, a well-known radical scavenger [14], indicating that the oscillations occurred by a free-radical mechanism [15]. 4. The color of solution changed alternately between red and blue during both the high- and lowfrequency oscillations. UV spectroscopy showed that the absorbency at h = 560 nm (the maximum absorbency for Fe(phen)32+) changed periodically, simultaneously with [Br-]. When Fe(phen)32+ was replaced by Mn 2+ or Ce 3+, which were commonly used as catalysts in BZ-type reactions, only one kind of oscillation could be observed (either high- or low-frequency oscillations). No dual-frequency oscillation occurred no matter how the reaction conditions and the initial concentrations of bromide were adjusted. 5. The effects of bromide ion on the oscillations were quite strange. It was found that both the highand low-frequency oscillations were controlled by bromide ions. Obviously, the critical concentrations

Fig. 2. High-frequency oscillation. [Br ]o = 0.0043 mol dm 3, other conditions are given in Fig. 1.

with ether. The ethanol solution was treated with hydrous sodium sulfate and filtered. In all of the above systems, two main products, BrHA (I) and Br 2 A (II) were determined by gas chromatography. Other products which may be quinines [5], were too complicated to determine. 3.3. Some effects on the oscillation

1. Effects of temperature on both the highfrequency and low-frequency oscillations were similar. The oscillation frequencies increased with increasing temperature. The activation energy falls within the range for Belousov-Zhabotinskii reactions of between 65 and 75 kJ tool -1 [13]. 2. Both high-frequency and low-frequency oscillations were inhibited by chloride ion.

0

•

|

|

1

2

7

i

12

|

17

.,.

i

22

i

27

t/rain Fig. 3. Dual-frequency oscillation. [Br-] 0 = 0.0028 mol d m - 3 , other conditions are given in Fig. 1.

140

H. Li, X. Huang / Chemical Physics Letters 255 (1996) 137-141

increasing [Br-]. The quantitative relationship was found to be r i = - 1 6 . 0 + 1.0 × 104 [ B r - ] , where the unit of r~ was min. 3.4. Mechanism discussion i

m

•

25

5

75

i

i

l0

12.5

Several dual-frequency BZ oscillating systems have been found since the first one was discovered in 1979 [4,16,17]. Ruoff and coworkers found that iodide ion could catalyze a high-frequency oscillation in the uncatalyzed BZ oscillating system with 4-[2-(methylamino)propyl] phenol as the organic substrate [4]. However, the present system is quite different as shown by the following: 1. No high-frequency oscillation was observed when bromide was replaced by iodide ion. 2. No uncatalyzed oscillation was observed no matter how the bromide ion concentration or iodide ion concentration were adjusted. Think about the reaction between B r - and BROW-:

t/min Fig. 4. High-frequency oscillation in the H 2 A - B r H A - B Z system. [BrHA] o = 0.015 mol dm -3, other conditions are given in Fig. 1.

for bromide were different. When [Br-] 0 ~< 2.3 × 10 -3 mol dm -3, only low-frequency oscillation occurred. When [Br-] 0 > 2.9 × 10 -3 tool dm -3, only high-frequency oscillation was observed, indicating that the high-frequency oscillation was induced by the bromide ion. It appeared in a very narrow range of [Br-] 0. The oscillation was inhibited completely when [Br-] 0 > 0.0060 tool dm -3. When 2.3 × 10 -3 < [ B r - ] 0 < 2 . 9 × 10 -3 m o l d m -3, the system displayed a dual-frequency oscillation with a transition period (r i) between high- and low-frequency oscillations (Fig. 3). The transition period was somewhat similar to the induction period (tin) of the lowfrequency oscillation (Fig. 1). However, the effects of Br- on 7"i and t~n were quite different. The induction period increased slightly with increasing [Br-]0, whereas the transition period decreased with

BrO 3 + 5 B r - + 6H +-~ 3Br 2 + 3H20.

(R4)

The bromine produced in R 4 would react with H2A by monobromination or dibromination: Br 2 + H2A ~ BrHA + H + + B r - ,

(Rs)

Br 2 + BrHA ~ BrzA + H + + Br-.

(Rr)

The more initial bromide is added, the more BrHA accumulates. It is possible that BrHA plays an important role in the high-frequency oscillation.

>

i

0

1

2

,

7

i

12

i

17

22

27

I/rain Fig. 5. Dual-frequency oscillation in the H 2 A-BrHA-BZ

system. [BrHA] = 0.0092 tool din- 3, oU~cr conditions are givcn in Fig. I.

H. Li, X. Huang / Chemical Physics Letters 255 (1996) 137-141

In order to prove this assumption, BrHA was prepared by R 5 and the Br- produced in R 5 could be removed by fresh AgOH: Br- + AgOH --* AgBr + O H - .

(R7)

The precipitation (excess AgOH and AgBr produced) was removed by filtration. Because both AgOH and AgBr are insoluble in aqueous solution ([Br-] and [Ag +] are less than 10 - 7 mol dm -3 according to Ksp), the effects of Br- and Ag + remaining in the BrHA solution on the oscillation could be neglected, which was confirmed because no significant effects on the BZ oscillation with H 2 A as organic substrate were found when 10 -7 mol dm -3 Ag ÷ or Br- was introduced in the system. The following results were obtained for the H 2A - B r H A - B r O 3 -Fe(phen)~ + - H 2SO4 system by changing the concentration of BrHA in the system. 1. Only the low-frequency oscillation was observed if [BrHA] was less than 0.0072 mol dm -3. 2. The high-frequency oscillation could be observed when [BrHA] was more than 0.011 mol dm -3 (Fig. 4). 3. Dual-frequency oscillations were obtained when [BrHA] was in the range from 0.0072 to 0.011 mol dm -3 (Fig. 5). It is clear that the dual-frequency oscillation were produced by the different systems as follows: (a) The low-frequency oscillation was given by H 2A - B r O 3 -Fe(phen)32+ - H 2SO4. (b) The high-frequency oscillation was given by BrHA-BrO 3 -Fe(phen)32+ - H 2SO4The dual-frequency oscillations were induced by bromide ion because adding Br- initially would change the contents of BrHA in the system. But there are still some problems about the mechanism, such as what is the role of the induction period for the low-frequency oscillations and the transition pc-

141

riod for the dual-frequency oscillations and the relationship between them. Further investigation concerning the above questions and detailed mechanisms as well as the model simulations is considered.

Acknowledgement We are grateful to the Nature Science Funds of Shanghai Academic Science of China for providing financial support for this work.

References [1] R.J. Field and M. Burger, Oscillating and traveling waves in chemical systems (Willey-lnterscience, New York, 1985). [2] G.J. Kasperek and T.C. Bruice, Inorg. Chem. 10 (1971) 382. [3] Z. Noszticzius, P. Stirling and M. Wittmann, J. Phys. Chem. 89 (1985) 4914. [4] P. Ruoff, M. Varga and E. Koros, J. Phys. Chem. 91 (1987) 5332. [5] L. Kuhnert and H. Krug, J. Phys. Chem. 94 (1990) 678. [6] I. Gonda and G.A. Rodley, J. Phys. Chem. 94 (1990) 1516. [7] H.X. Li, Acta Chim. Sinica 48 (1990) 478. [8] H.X. Li and H.H. Xu, Acta Chim. Sinica 49 (1991) 454. [9] R.J. Field, E. Koros and R.M. Noyes, J. Am. Chem. Soc. 94 (1972) 8649. [10] D.E. Edelson, R.J. Field and R.M. Noyes, Int. J. Chem. Kinet. 7 (1975) 417. [11] R.J. Field and R.M. Noyes, J. Chem. Phys. 60 (1974) 1877. [12] M. Orban, E. Koros and R.M. Noyes, J. Phys. Chem. 83 (1979) 3086. [13] E. Koros, Nature 251 (1974) 703. [14] K. Showalter and R.M. Noyes, J. Am. Chem. Soc. 100 (1978) 1042. [15] Z. Noszticzius and J. Bodiss, J. Am. Chem. Soc. 101 (1979) 3177. [16] E.J. Heilweil and M.J. Henchman, J. Am. Chem. Soc. 101 (1979) 3698. [17] H.X. Li, Nature (China) 11 (1988) 77.