INTRODUCTION:

Oscillatory chemical systems have been the focus of research in the area of theoretical and experimental non-linear dynamics in recent years. In 1950, Belousov ^[1]was looking for an inorganic analog of the Kreb’s cycle using cerium ions instead of the protein-bound metal ions common in the living cells and first observed oscillations in the rate of [Ce³⁺/Ce⁴⁺] during the cerium catalysed oxidation of citric acid or malonic acid by acidic bromate. Zhabotinsky ^[2]demonstrated that similar oscillations are observed when malonic acid is replaced by another organic substrate with active methylenic hydrogen atom or when the cerium couple is replaced by the Mn^2+/Mn³⁺ couple or Fe(phen)₃³⁺/ Fe(phen)₃²⁺ couple. According to Noyes, ^[3] the classical bromate-driven BZ reaction is a metal-ion catalysed oxidation and bromination of an organic substrate by acidic bromate.

The cerous-catalysed BZ reaction with malonic acid has been studied most thoroughly and its detailed mechanism as elucidated by Field, Koros and Noyes (FKN) ^[4]explains the temporal oscillations as well as trigger waves.^[5,6] The mechanism contains an inorganic part mainly involving reactions of oxybromine species among themselves and with the metal ion catalyst and an organic part involving reactions of organic substrate and its derivatives with Ce⁴⁺ ion and oxybromine species. The principal features of this mechanism are strongly supported by computational modelling of Edelson et al. ^[7,8] and of Field et al. ^[9] Further, a number of modifications of the classical BZ reaction have been reported. Koros et al. ^[10] identified two different kinds of metal ion catalysts for the BZ reaction, namely, the labile Ce³⁺/Ce⁴⁺ and Mn²⁺/Mn³⁺ complexes with reduction potentials of about 1.5V and the inert Fe(phen)₃²⁺/ Fe(phen)₃³⁺ and Ru(phen)₃²⁺/ Ru(phen)₃³⁺ complexes with reduction potentials of about 1–1.3 V.

A number of other organic substrates such as citric acid,^[1,11] saccharides,^[12]ethyl acetoacetate,^[13] oxalic acid/acetone ^[14,15] and mandelic acid/ketone ^[16] have been shown to exhibit oscillations but the pyrogallol has not been worked out so much as chemical oscillator, though some preliminary data is reported on the molecule but without any catalyst.^[17]Owing to its biological significance^[18-20] the catalysed study of the molecule as chemical oscillator will serve as prototype example to understand its role in vivo. As temperature has pronounced effect on the oscillatory behaviour of the aforesaid BZ system, its dependence for a variety of catalysed and un-catalysed bromate driven oscillators have been characterised.^[21-25]An attempt was made to evaluate kinetic parameters of the pyrogallol based chemical oscillator with Ce⁴⁺ ion as catalyst.

EXPERIMENTAL:

Materials:

All reagents used were analytical grade chemicals. The reagents used were Pyrogallol 99% (SRL, AR), Potassium bromate 99.6% (Merck, AR), Manganese(II) sulphate monohydrate 98% (B.D.H, LR), Ceric sulphate 99% (S.D. fine, AR), Potassium chloride 99.5% (Merck, AR), Potassium nitrate 99% (Merck, AR) and Sulphuric acid 98% (Merck, LR).

Method:

The ion analyser-Orion 4 Star was calibrated in ORP mode with the standard solutions, using platinum and Calomel (SCE) as indicator and reference electrodes respectively. The platinum electrode was dipped in the reaction mixture while as the SCE was put in another half cell containing 2.5 × 10^-4mol L^-1 solution of potassium chloride. The two half cells were connected through salt bridge containing potassium nitrate and thermostated to desired temperature. The reaction started after the addition of 2ml solution of potassium bromate to a solution containing 2ml each of pyrogallol and metal ion as catalyst. The observations were recorded after every 5 seconds.

RESULTS AND DISCUSSION:

The oscillatory parameters of the metal ion catalysed BZ system containing pyrogallol as an organic substrate show a sensitive dependence on the concentration of the reactants involved. The typical oscillatory profiles with Mn²⁺ and Ce⁴⁺ ions as catalysts are depicted in figure 1(a) and 1(b) respectively.

Figure 1(a). The typical oscillating wave of Pyrogallol system with Mn²⁺ ion catalyst

Figure 1(b). The typical oscillating wave of Pyrogallol system with Ce⁴⁺ ion catalyst

From table 1, it is observed that on changing concentration of pyrogallol from 0.003 -0.023 mol L^-1 , there is first decrease and then increase in both the induction and time periods, while as there is a steady increase and then decrease in case of amplitude and number of oscillations. The induction period is observed due to the accumulation of critical concentration of the brominated organic species prior to the commencement of oscillations. ^[4] With increase in the substrate concentration, the rate of formation of bromo-organic derivative increases and hence there is a shortening of the induction period. The increase in induction period with further increase in concentration of the pyrogallol may be because of the limiting concentration of the other reacting species.

Table 1.Variation in [Pyrogallol] having other species like [Mn²⁺] = 0.005 mol L^-1, [BrO₃ˉ] = 0.09 mol L^-1, [H₂SO₄] = 1.5 mol L^-1, Temperature = 303.15 ± 0.1K.

[Pyrogallol]	Induction period	Time period	Frequency	Amplitude	No. of Oscillations
[mol L^-1]	t_in (s)	t_p (s)	v (s^-1)	(mV)	n
0.003	*	*	*	*	1
0.005	150	140	0.0071	25	6
0.007	140	130	0.0076	35.5	12
0.009	115	95	0.01052	83.2	11
0.011	180	102	0.0098	36.0	18
0.013	220	148	0.0067	39.5	21
0.015	270	220	0.0045	20.4	17
0.017	350	240	0.0041	14.0	11
0.023	**	**	**	**	**

** no oscillations are seen

The time period is an indicator for the overall BZ reaction. It may be mentioned that the concentration of substrate at a certain measuring time may be approximated to its initial concentration and in this case the average value of the time from second to sixth oscillations was taken as the time period. For further study, the concentration of pyrogallol was choosen as 0.009 molL^-1 as the system showed optimal oscillatory characteristics at this concentration. The dependence of the reaction on the oscillatory characteristics such as induction period and time period is justified on the basis of the FKN mechanism.^[4]According to this mechanism, the overall BZ reaction may be divided into the following three processes: consumption of bromide ion (processs A), autocatalytic reaction of bromous acid with oxidation of catalyst (process B), and organic reaction with reduction of catalyst (process C).

BrO₃^-+2Br^- +3H⁺→ 3HOBr (A)

BrO₃^- + HBrO₂+ 2M_red+ 2H⁺ → 2HBrO₂ + 2M_ox + H₂O (B)

2M_ox + Substrate + Bromoderivative → fBr^- + 2M_red + other products (C)

The process A represents the region where potential increases very slowly. Process B is the region where potential increases rapidly, and process C is the region where potential decreases. ^[26]

From Table 2 it is found that with increase in [BrO₃^-]₀, the induction period undergoes a continuous decrease due to the faster accumulation of the bromoderivative of the substrate while as the time period first decreases and then increases. The number of oscillations first increase and then decrease (Figure 2). The reason being that bromate causes the bromination of the substrate first giving rise to the formation of HBrO₂ and HOBr and then Br₂ , which are responsible for generating critical bromo-substrate concentration and also have a direct role in [Mnⁿ⁺/M⁽ⁿ⁺¹⁾⁺] redox couple. However, increasing the [BrO₃^-] further from 0.160 mol L^-1 the substrate becomes limiting and hence no oscillations are seen.

Figure 2. Potential versus time plots showing the variation of oscillatory characteristics for the system containing [Pyrogallol] = 0.009 mol L^-1 ,[Mn²⁺] = 5× 10^-3 mol L^-1and [H₂SO₄]=1.5 mol L^-1. [BrO₃^-]: (a) 0.090 mol L^-1(b) 0.10 mol L^-1(c) 0.120 mol L^-1and (d) 0.140 mol L^-1. Temperature = 303.15 ± 0.1K.

Tables 3 and 4 show the oscillatory parameters for varying [Mn^2+]₀and [Ce⁴⁺]₀which have been used as metal ion catalysts separately. It is shown that increasing the [Mn²⁺] from 0.001-0.013 mol L^-1, there is first decrease in induction and time periods and then increase, while as the number of oscillations first increase gradually and finally vanish at 0.013 mol L^-1 of the Mn²⁺ ion solution. However, in case of Ce⁴⁺ ion, both induction and time periods show a continuous increase from 0.0001-0.007 mol L^-1, with similar behavior as that in Mn²⁺ ion solution w.r.t. the number of oscillations which first increase upto 0.001 mol L^-1 and then decrease until the concentration limit approaches, i.e., 0.009 mol L^-1 after which no oscillation is observed. The unusual trend in case of [Mn²⁺]₀ may be attributed to the combined effect of process B and C. [Mn²⁺]/[Mn³⁺] depends on the autocatalytic process, giving rise to the formation of HBrO₂. However, a good number of oscillations with better amplitude were observed at [Mn²⁺]₀= 0.005 mol L^-1 and [Ce⁴⁺]₀ = 0.001 mol L^-1.

Table 2 . Variation in [BrO₃ˉ] having other species like [Pyrogallol] = 0.009 mol L^-1, [Mn²⁺] = 0.005 mol L^-1, [H₂SO₄] = 1.5 mol L^-1, Temperature = 303.15 ± 0.1K.

[BrO₃ˉ]	Induction period	Time period	Frequency	Amplitude	No. of Oscillations
[mol L^-1]	t_in (s)	t_p (s)	v (s^-1)	(mV)	n
0.040	*	*	*	*	*
0.060	360	132	0.0083	23.8	9
0.080	220	126	0.0079	27.6	14
0.090	115	95	0.01052	83.2	11
0.100	70	110	0.0090	65.6	6
0.120	50	123	0.0081	73.0	4
0.140	30	130	0.0076	77.0	2
0.160	*	*	*	*	*

** no oscillations are seen

Table 3 . Variation in [Mn²⁺] having other species like [Pyrogallol] = 0.009 mol L^-1, [BrO₃ˉ] = 0.09 mol L^-1, [H₂SO₄] = 1.5 mol L^-1, Temperature = 303.15 ± 0.1K.

[Mn²⁺]	Induction period	Time period	Frequency	Amplitude	No. of Oscillations
[mol L^-1]	t_in (s)	t_p (s)	v (s^-1)	(mV)	n
0.001	*	*	*	*	*
0.002	250	136	0.0073	8.4	9
0.004	150	84	0.0119	29.6	11
0.005	115	95	0.01052	83.2	11
0.006	135	86	0.0116	21.4	11
0.007	140	89	0.0112	39.4	8
0.009	150	92	0.0108	47.5	5
0.010	160	95	0.0105	44.0	6
0.013	*	*	*	*	*

** no oscillations are seen

Table 4 . Variation in [Ce⁴⁺] having other species like [Pyrogallol] = 0.009 mol L^-1, [BrO₃ˉ] = 0.09 mol L^-1, [H₂SO₄] = 1.5 mol L^-1, Temperature = 303.15 ± 0.1K.

[Ce⁴⁺]	Induction period	Time period	Frequency	Amplitude	No. of Oscillations
[mol L^-1]	t_in(s)	t_p(s)	V (s^-1)	(mV)	n
0.00008	*	*	*	*	*
0.00010	95	87	0.01150	47.4	6
0.00030	115	124	0.00806	52.0	9
0.00100	170	283	0.00353	45.0	12
0.00300	640	576	0.00173	37.6	6
0.00500	1170	1193	0.00083	29.3	4
0.00700	2605	1805	0.00055	18.0	2
0.00900	*	*	*	*	*
** no oscillations are seen

Table 5 . Variation in [H₂SO₄] having other species like [Mn²⁺] = 0.005 mol L^-1, [Pyrogallol] = 0.009 mol L^-1, [BrO₃‾] = 0.09 mol L^-1, Temperature = 303.15 ± 0.1K.

[H₂SO₄]	Induction period	Time period	Frequency	Amplitude	No. of Oscillations
[mol L^-1]	t_in (s)	t_p (s)	v (s^-1)	(mV)	n
1.0	**	**	**	**	**
1.25	260	124	0.008	39.4	10
1.5	115	95	0.01052	83.2	11
1.75	110	66	0.0151	59	6
2.0	80	87	0.0114	38.7	5
2.25	**	**	**	**	**

** no oscillations are seen

Table 6. Variation in [H₂SO₄] having other species like [Ce⁴⁺] = 0.001 mol L^-1, [Pyrogallol] = 0.009 mol L^-1, [BrO₃‾] = 0.09 mol L^-1, Temperature = 303.15 ± 0.1K.

[H₂SO₄]	Induction period	Time period	Frequency	Amplitude	No. of Oscillations
[mol L^-1]	t_in (s)	t_p (s)	v (s^-1)	(mV)	n
0.50	*	*	*	*	*
0.75	400	192	0.00520	30.4	6
1.00	360	290	0.00345	63.0	10
1.25	250	217	0.00460	71.5	11
1.5	170	283	0.00353	45.0	>13
1.75	160	320	0.00312	62.0	12
2.00	245	308	0.00325	36.2	>14
2.25	270	211	0.00474	38.3	8
2.50	*	*	*	*	*

** no oscillations are seen

The data depicted in Tables 5 and 6 pertains to the effect of [H₂SO₄] as medium on the oscillatory behaviour of the present system in presence of Mn²⁺and Ce⁴⁺ion based systems separately. It can be seen that Ce⁴⁺ has a wide oscillatory window over a range of concentrations of [H₂SO₄] from 0.50M-2.25 mol L^-1 as compared to 1.00-2.25 mol L^-1 concentration range with respect to Mn²⁺ ion based system. In case of Ce⁴⁺ ion based system, the induction period first showed a decrease and then increase but in Mn²⁺ion system there is a continuous decrease with

increase in [H₂SO₄]. However, the maximum number of oscillations with good amplitude was observed at 1.5 mol L^-1 [H₂SO₄]. The H⁺ ion gives the protonation of the pyrogallol and this reactive protonated intermediate acts as a good nucleophile for bromide ion to form the bromopyrogallol.

Further from the above data the optimal concentrations of the reagents were selected for the investigation of kinetic behaviour of the present system with Ce⁴⁺ ion as catalyst at different temperatures. The data obtained to this effect is recorded as Table 7.

From the above, it is inferred that frequency of oscillations increases or else time period decreases with the increase in temperature, thereby reducing the oscillatory characteristics such as induction time (t_in) and time period (t_p). The decrease in induction period is due to the increase in the rate of formation of critical amount of bromoderivative of substrate^[4] with increase in temperature. The decrease in induction period shows a linear relationship with temperature (figure 4)

Figure 3. Potential(mV) verses time(s) plot showing variation of oscillatory characteristics of the system containing [Pyrogallol]= 0.009 mol L^-1, [BrO₃^-]= 0.09 mol L^-1, [Ce⁴⁺]= 0.001 mol L^-1, and [H₂SO₄]= 1.5 mol L^-1, at temperatures (a) 293.15 K (b) 303.15 K (c) 313.15 K and (d) 323.15 K.

Figure 4 . log t_in(s) vs 1/T (K) plot

The maximum numbers of oscillations were observed at 30±0.1˚C. However, at lower temperature oscillations occur for a longer period of time compared to higher temperatures where oscillations stop after some time owing to increase in the rate of the reaction.

Since chemical oscillation is a monomolecular process, the reciprocal of time period at initial concentrations was taken as first order rate constant ^[27] and accordingly the activation parameters were calculated from the logarithmic form of Eyring’s equation i.e.,

(1)

Table 8 . Enthalpy and Entropy of activation for the BZ System containing [Pyrogallol] = 0.009 mol L^-1, [BrO₃^–] = 0.09 mol L^-1, [Ce⁴⁺]= 1 × 10^-3 mol L^-1 and [H₂SO₄] = 1.5 mol L^-1.

Activation Parameters	Oscillation	Parameters
	t_in (s)	t_p (s)
ΔH^#(kJ/mol)	77.540	56.308
ΔS^#(J/mol K)	-31.636	-105.892

The analysis of activation parameters from apparent rate constant provided information about the extent to which temperature influences the oscillatory behaviour of the Pyrogallol based BZ system.

CONCLUSIONS:

Pyrogallol shows ideal oscillatory behaviour in presence of Mn²⁺ and Ce⁴⁺ ions as catalysts. The optimal concentration of the reagents has been established at 30 ± 0.1 ºC. The temperature influences the oscillatory behaviour of the present system. The values of enthalpy and entropy of activation are apparent values that don’t correspond to any elementary reaction step. It is not possible to calculate the actual overall rate constant of the BZ system owing to its complex nature. The variation of induction period with temperature reveals almost a linear dependence, indicating that the present system could be used as temperature probe within its own limits.

ACKNOWLEDGEMENTS:

The authors are highly thankful to UGC, New Delhi for providing financial support in the form of a major research project which facilitated our requirements for undertaking the present investigation.

REFERENCES:

1. Pechenkin, A. B P Belousov and his reaction. J Biosci. 34; 2009: 365–371.

2. Zhabotinsky, A M. Periodical oxidation of malonic acid in solution (a study of the Belousov reaction kinetics). Biofizika. 9; 1964:306-311.

3. Noyes, R M. Chemical oscillations and instabilities. A generalized mechanism for bromate-driven oscillators controlled by bromide. J Am Chem Soc. 102; 1980: 4644–4649

4. Field RJ, Koros E and Noyes RM. Oscillations in chemical systems. II. Thorough analysis of temporal oscillation in the bromate-cerium-malonic acid system. J Am Chem Soc. 94; 1972: 8649–8664.

5. Zaikin AN and Zhabotinsky AM. Concentration Wave Propagation in Two-dimensional Liquid-phase Self-oscillating System. Nature. 225; 1970: 535 – 537.

6. Epstein IR and Pojman JA. An Introduction to Nonlinear Chemical Dynamics: Oscillations, Waves, Patterns and Chaos. Oxford University Press, New York. 1998.

7. Edelson D, Field RJ and Noyes, RM. Mechanistic details of the Belousov–Zhabotinskii oscillations. Int J Chem Kinet. 7; 1975: 417–432.

8. Edelson D, Noyes RM and Field RJ. Mechanistic details of the Belousov–Zhabotinsky oscillations. II. The organic reaction subset. Int J Chem Kinet. 11; 1979: 155–164.

9. Gyorgyi L, Turany T and Field RJ. Mechanistic details of the oscillatory Belousov-Zhabotinskii reaction. J Phy Chem. 94;1990: 7162–7170.

10. Koros E, Burger M, Friedrich V, et al. Chemistry of Belousov-type oscillating reactions. Faraday Symp Chem Soc. 9;1974: 28-37.

11. Kasperek GT and Bruice TC. Observations on an Oscillating Reaction. The Reaction of Potassium Bromate, Ceric Sulfate and a Dicarboxylic Acid. Inorg Chem. 10; 1971: 382-386.

12. Ou CC and Jwo JJ. Kinetic Study of Some Reactions Related to the Mn(III)-Catalyzed Bromide –Saccharide Oscillating Reactions. Int J Chem Kinet. 23; 1991: 137-249.

13. Salter LF and Sheppard JP. A dual-frequency Belousov Zhabotinskii oscillating reaction with ethyl acetoacetate as organic substrate. Int J Chem Kinet. 14; 1982: 815-821.

14. Krisztina P, Maria W, Klara L, et al. Mechanistic Investigations of the BZ Reaction with Oxalic Acid Substrate. I. The Oscillatory Parameter Region and Rate Constants Measured for the Reactions of HOBr, HBrO₂, and Acidic BrO₃^- with Oxalic Acid. J Phys Chem A. 108; 2004: 5377–5385.

15. Basavaraja C, Kulkarni VR, Mohan S, et al. Experimental studies on BZ reaction system containing oxalic acid and acetone as mixed organic substrate in a CSTR. Indian Journal of Chemistry : A. 43; 2004: 739-742.

16. Lee SS and Jwo, JJ. Kinetic study of the Ce(III)- or Mn(II)-catalyzed Belousov–Zhabotinsky reactions with mixed organic acid/ketone substrates. Int J Chem Kinet. 30; 1998: 595-604.

17. Sridevi V and Ramaswamy, R. Potential response in the uncatalyzed bromate oscillatory system. React Kinet Catal Lett. 65; 1998: 93-100.

18. Yang CJ, Wang CS, Hung J Y, et al. Pyrogallol induces G2-M arrest in human lung cancer cells and inhibits tumor growth in an animal model. Lung Cancer. 66; 2009: 162-168.

19. Saeki K, Hayakawa S, Isemura M, et al. Importance of a pyrogallol-type structure in catechin compounds for apoptosis-inducing activity. Phytochemistry. 53; 2000: 391-394.

20. Joharapurkar AA, Wanjari MM, Dixit PV, et al. Pyrogallol: A novel tool for screening immunomodulators. Indian J Pharmacol. 36; 2004: 355-359.

21. Koros E. Monomolecular treatment of chemical oscillation. Nature. 251; 1974: 703-704.

22. Blandamer MJ and Roberts DL. Analysis of the dependence on temperature of the frequency of oscillation of the Belousov-Zhabotinskii reaction. J Chem Soc Faraday Trans 1. 73;1977: 1056-1065.

23. Yoshikawa, K. Distinct Activation Energies for Temporal and Spatial Oscillations in the Belousov-Zhabotinskii Reaction. Bull Chem Soc Japan. 55; 1982: 2042-2045.

24. Fujieda S and Kawahito J. Caloric behaviour in the induction period of Belousov- Zhabotinskii reactions monitored by heat exchange calorimetry of batch type and the simultaneous estimation of the potentiometric aspect. Thermochim Acta. 1; 1992: 210-213.

25. Rouff P. Antagonistic Balance in the Oregonator: About the Possibility of Temperature-Compensation in the Belousov-Zhabotinsky Reaction. Physica D. 84; 1995: 204-211.

26. Ganaie NB and Peerzada G M. Effect of initial reagent concentrations on the oscillatory behavior of the BZ reaction in a batch reactor. Int J Chem Kinet. 41; 2009: 650-657.

27. Nath MA and Ganaie NB, Rastogi RP, et al. Effect of temperature on oscillatory behaviour of the system containing isomers of hydroxybenzoic acid in batch reactor. E J Chem. 5; 2008: 832-837.

Received on 29.11.2012 Modified on 14.01.2013

Asian J. Research Chem. 6(3): March 2013; Page 189-194

Temperature	Induction period	Time period	Frequency	No. of Oscillations
(K)	t_in, (s)	t_p, (s)	v, (s^-1)	n
293.15	445	576	0.00173	>9
303.15	170	283	0.00353	>12
313.15	70	134	0.00746	12
323.15	20	61	0.01639	9