Oscillating behaviour of gallic acid – methyl ketone system ... - NOPR

1 downloads 0 Views 51KB Size Report
potentiometrically using the ion analyzer ELICO;. LI-126 in ORP mode coupled with platinum and calomel electrodes as indicator and reference electrodes ...
Indian Journal of Chemistry Vol 47A, May 2008, pp. 705-707

Oscillating behaviour of gallic acid – methyl ketone system catalysed by metal ions Mushtaq A Lone, Masood A Nath, Nadeem B Ganaie & Mustafa G Peerzada* Department of Chemistry, University of Kashmir, Srinagar 190006, J&K, India Email: [email protected] Received 12 November 2007; revised 21 April 2008 Studies on oscillatory behaviour of gallic acid with potassium bromate in presence of methyl ketones as co-substrates and various metal-ions as catalysts in aqueous acid medium are reported here. The investigations establish the order of reactivity of single and mixed organic substrate systems and also explore the relative efficiency of metal ions as catalysts in the reaction systems. The Mn2+/Fe2+ couple has been examined for co-catalyst effect on both single and mixed organic substrate systems.

The study of oscillating chemical reactions has been dominated by Belousov-Zhabotinskii (BZ) reaction1, which behaves like a chemical clock. In these reactions, which are far from equilibrium, the concentrations of some reaction intermediates in the reaction system exhibit a regular change with time, which is a kind of self-regulating function similar to what happens in an organism. Amongst the various classes of BZ reactions, the metal-ion catalyzed reactions are extensively studied2-14. The present investigation has been made on single and mixed substrate systems, viz. Gallic acid, Gallic acid + acetone, Gallic acid + butanone and Gallic acid + pentanone with Ce3+, Ce4+, Mn2+ and Fe2+ ions in order to judge the relative efficiency of these metal ions as catalysts on the oscillatory behavior of both single and mixed organic substrate systems. Since the nature of oscillations observed with Mn2+ion as catalyst were found to be far better in both single and mixed substrate systems, different concentrations of Mn2+ion solutions were added to the reaction systems under investigation in order to establish upper and lower levels at which the metal ion retains better efficiency as catalyst. Further, different concentrations of ferroin (Fe2+ion) were mixed with the reaction systems containing fixed concentration of Mn2+ion as catalyst to have the

co-catalyst effect (ferroin) on the oscillatory behavior of Gallic acid with and without methyl ketones as co-substrates. Experimental All the chemicals used were of analytical grade. Thermostat bath (Siskin Julabo) having a precision of ±0.1°C was used. The reactions were followed potentiometrically using the ion analyzer ELICO; LI-126 in ORP mode coupled with platinum and calomel electrodes as indicator and reference electrodes, respectively. These electrodes were connected through salt bridge filled with potassium nitrate in agar agar. The electrodes were activated properly and the ion-analyzer was calibrated at ORP mode with standard solutions. The solutions of the required reagents with desired molar concentrations were prepared in 1.5 M sulfuric acid solution. Solutions (5%) of acetone, butanone and pentanone were also prepared in 1.5 M sulfuric acid as co-substrates. These solutions were mixed separately to gallic acid solution (GA) to have three mixed organic substrate systems: gallic acid + acetone (GAA), gallic acid + butanone (GAB) and gallic acid + pentanone (GAP). The oscillatory parameters of these systems, viz. time of initiation (tin), time period (tp) and life time (tl), were observed at 35±0.1°C with inorganic bromate in presence of Ce3+, Ce4+, Mn2+ and Fe2+ (ferroin) ions as catalysts. Different concentrations of ferroin were prepared in 1.5 M sulfuric acid and mixed separately to single and mixed organic substrate systems containing fixed concentration of Mn2+ ion. The experiments were performed under identical conditions. Results and discussion Figure 1 depicts one of the typical potential time plots of our reaction system with Mn2+ ion as catalyst. The data reported in Table 1 show the effect of different metal ions as catalysts on oscillatory parameters of gallic acid with and without methyl ketones as co-substrates. The concentrations of these metal ions were plotted against the observed oscillatory parameters, which shows that tin, tp and tl values of pure substrate system (GA) are lower than the mixed organic substrate systems

INDIAN J CHEM SEC A, MAY 2008

706

Table 1 — Effect of [Mn+] catalyst on oscillatory parameters of the system containing [GA] =0.0059 M; [BrO3-]=0.0333 M; [H2SO4]=1.0999 M; [acetone]=[butanone]=[pentanone]=5% each (v/v) as co-substrates (temp.=35+0.1°C) Metalion Conc. (M) ×10-4 II

Mn CeIII CeIV FeII

tin (min)

3.0 5.0 8.0 2.5

tp (min)

tl (min)

GA

GAA

GAB

GAP

GA

GAA

GAB

GAP

GA

GAA

GAB

GAP

3.25 2.84 2.50 2.50

3.53 2.99 2.80 2.91

3.98 3.44 3.37 3.54

4.50 3.90 3.73 4.24

2.10 2.09 1.90 1.60

2.44 2.36 2.22 1.95

2.90 2.71 2.64 2.41

3.40 3.06 2.92 2.92

60.00 60.90 58.00 61.00

68.65 68.00 64.50 66.00

79.00 78.69 71.80 74.30

86.00 85.80 85.00 83.00

Table 2 — Effect of [MnII] catalyst on the oscillatory parameters of the system containing [GA]=0.0059 M, [BrO3- ]=0.0333 M, [H2SO4]= 1.0999 M. [acetone] = [butanone] = [pentanone] = 5% (v/v) as co-substrates (temp.=35+0.1°C) [MnII] (M) × 10-4

tin (min)

1.70 3.00 7.00 13.00

tp (min)

GA

GAA

GAB

GAP

GA

GAA

GAB

GAP

GA

GAA

GAB

GAP

3.90 3.25 2.35 1.50

4.18 3.53 3.34 2.56

4.56 3.98 3.78 3.10

5.08 4.50 4.15 3.48

2.47 2.10 1.60 0.80

2.84 2.44 2.00 1.30

3.15 2.90 2.34 1.59

3.67 3.40 2.76 2.11

64.60 60.00 53.90 46.00

75.00 68.65 62.10 52.30

90.00 79.00 76.70 67.30

94.20 86.00 83.60 75.30

900

850

Time ( min )

800

750

700

650

0

10

tl (min)

20

30

40

50

60

Potential ( mv )

Fig. 1 — Potential time plot of one of the reaction systems comprising: [GA]=0.0059 M; [BrO3]=0.0333 M; [H2SO4]=1.0999 M, acetone=5% (v/v) as co-substrate. Temperature=35 + 0.1°C.

(GAA, GAB and GAP). Thus, the following increasing order of the reaction systems with respect to oscillatory parameters in presence of the four metal ions (Mn2+, Fe2+, Ce3+, Ce4+) is reported: GA < GAA < GAB < GAP The methyl ketones act both as bromine scavengers and reductants2; but at lower temperatures the rate of oxidation of gallic acid is higher than methyl ketones. Therefore, the oscillatory parameters of gallic acid are lower as compared to mixed substrate systems. Amongst the mixed substrate systems (GAA, GAB,

GAP), it is observed that the tl value in GAA initially increases with increase in molar concentration of acetone, but at much higher concentration of acetone (4.25 M), the life span of oscillations shows a decreasing trend. This is because in the lower concentration range, acetone behaves as bromine scavenger, but at higher concentration, it is also effectively contributing to the overall oscillatory phenomenon in the system through bromination. However, we could not observe this trend in GAB and GAP because of practical difficulty in preparing the higher molar solutions of butanone and pentanone in the given aqueous acid medium. This may be due to the less solubility of these co-substrates at much higher molar concentrations owing to increase in the length of carbon chain. All the four metal ions promote oscillations in both single and mixed organic substrate systems. However, the reaction systems have shown more reactivity with Mn2+ ion catalyst and thus the nature of oscillations are much more pronounced as is depicted in Fig. 1. In order to establish the concentration effect (upper and lower levels), the oscillations were studied with different concentrations of Mn2+ ion as catalyst and the oscillatory parameters obtained are reported in Table 2. It is observed that increase in the concentration of Mn2+ ion in both types of reaction systems increases the rate of reaction and thus leads to lowering of onset time of oscillations (tin). Further it also increases the frequencies of oscillations owing to which both tp and tl values of the oscillations get reduced.

NOTES

707

Table 3 — Effect of [Ferroin] as co-catalyst on the oscillatory parameters of the system containing [GA]=0.0059 M; [BrO3 ] =0.0333 M; [MnII]=0.003 M; [H2SO4]=1.0999 M. [acetone] = [butanone] = [pentanone]=5% each (v/v) as co-substrates. [temp.=35+0.1°C] [Ferroin] (M) × 10-4

tin (min)

tp (min)

tl (min)

GA

GAA

GAB

GAP

GA

GAA

GAB

GAP

GA

GAA

GAB

GAP

2.00

2.28

2.87

3.50

4.19

0.78

1.62

2.45

3.00

58.00

85.00

139.00

150.00

5.00

1.90

2.45

3.09

3.76

0.55

1.30

2.05

2.52

47.70

76.00

128.00

138.00

10.00

1.47

2.13

2.62

3.36

0.32

0.92

1.52

1.98

39.00

68.50

116.00

133.20

20.00

0.75

1.40

1.75

2.56

0.04

0.15

0.55

1.10

33.20

56.00

101.00

120.00

In order to study the co-catalyst effect, the oscillations were observed in both the types of systems with different concentrations of ferroin (co-catalyst) at a fixed concentration of Mn2+ ion and the oscillatory parameters obtained are reported in Table 3. From the least square fit curves drawn by plotting tin, tp and tl values against different concentrations of ferroin, it is seen that co-catalyst (ferroin) increases the reactivity of both single and mixed substrate systems owing to which the oscillatory parameters decrease with increase in concentration of ferroin. This signifies the positive role of co-catalyst. It is a testimony to the study made by Paul and Banerjee2 according to which ferroin shows only few oscillations in absence of Mn2+ion solution in gallic acid and acetone as mixed organic substrate system but a large number of oscillations occur when both Mn2+ and Fe2+ion solutions are added to the systems.

References 1 Field R J & Burger M, Oscillations and Traveling Waves in Chemical Systems (John Wiley, New York) 1985. 2 Paul S C & Banerjee R S, J Indian Chem Soc, 75 (1998) 73. 3 Kulkarni V R, Indian J Chem, 32A (1993) 794. 4 Chen Y F, Lin H P, Sun S S & Jwo J J, Int J Chem Kinet, 28 (1996) 345. 5 Sun S S, Lin H P, Chen Y F & Jwo J J, J Chinese Chem Soc, 41 (1994) 651. 6 Rastogi R P & Misra G P, Indian J Chem Soc A, 29 (1990) 1205. 7 Pastapur S M & Kulkarni V R, Indian J Chem Soc A, 31 (1992) 182. 8 Lin H P & Jwo J J, J Phys Chem, 99 (1995) 6897. 9 Rastogi R P, Ishwar Das & Sharma A, J Chem Soc Faraday Trans I, 85 (1989) 2011. 10 Rastogi R P & Kumar A, J Phys Chem, 80 (1976) 2548. 11 Rastogi R P & Rastogi P, Indian J Chem, 16A (1978) 374. 12 Dutta A K & Banerjee R S, J Indian Chem Soc, 64 (1987) 185. 13 Koros E & Orban M, Nature, 273 (1978) 371. 14 Pastapur S M & Kulkarni B R, J Indian Chem Soc A, 31 (1974) 182.