Shen Shigang, Cui Yuping, Sun Hanwen, Shan
Jinhuan, Liu Ying Received Feb. 18, 2002.Abstract This work was primarily an
experimental investigation of the bromate oscillator using glycerol and acetone as the
mixed organic substrates. The empirical equation of the induction period and oscillating
period with the concentrations of the reactants and temperature were obtained. The
oscillating characteristic and possible oscillation mechanism were analyzed. Chemical oscillating reactions represent a typical far-from-equilibrium phenomenon in a system that the concentrations are not uniform in the course of time. It is also called as the chemical clock, which is similar to the biological clock. The concentration of some substances in the reaction system exhibits a regular change with time, a kind of self-regulating function similar to what happened in an organism. The study of life assimilation and activation intermediate involved in the oscillating system during the metabolic process provides important information and theoretical basis in the sugar fermentation in organism in imitating organism. During the last twenty years, Vitamin C, glucose, pyruvic acid, fructose, amino acid, lactose, and malic acid have been reported[1-8] to be very important reaction substrates in oscillating reaction, they play an important part in the synthesis of cell substances in life[9]. As a dehydrator and lubricator, glycerol has been used clinically in curing many diseases. In the present work, a new experimental investigation of the bromate oscillator using glycerol and acetone as the mixed organic substrates was reported. The temperature effects on the oscillations of this system and the initial concentration range of the reactants in the oscillating system were examined. The relations of the oscillation periods with the substrate concentrations have been studied in detail. The oscillating characteristic and a possible oscillation mechanism were analyzed. 2 EXPERIMENTAL Experiments were performed in a self-made thermostat (The temperature was controlled at T¡À 0.1 K). The overall volume of the solution is 50 ml. Sulfuric acid, glycerol, manganous sulfate and acetone were added successively under stirring by a magnetic agitator. When the temperature of the reaction mixture reached constant (¡À 0.1 K), the solution of potassium bromate thermostated at the same temperature was instantly transferred into the mixture. The oscillating curves of the potential with time were recorded using a table x-t recorder. Smooth bright platinum was employed as indicator electrode and a 217-Type SCE was used as the reference electrode together with (1 mol dm-3 sulfuric acid) liquid junction having a sintered silica disc at the end dipping in the reaction mixture. Since only the characteristic of the oscillating curve with time was considered in the present work, the typical trace of potential oscillations was shown in Figure 1. 3 RESULTS AND DISCUSSION Table 1 gives the initial concentration range of the reactants and temperature range in the chemical oscillating system of Glycerol-Acetone-Bromate-MnSO4-H2SO4. Fig.1 The oscillating curve of the potential with time [System: [Act]0 = 1.35 mol dm-3, [GL]0 = 3.30¡Á 10 -2 mol dm-3, [BrO3-]0 = 0.090 mol dm-3, [Mn2+]0 = 6.0¡Á10-4 mol dm-3, [H2SO4]0 = 2.40 mol dm-3, Temp. = 298¡À 0.1 K] Table 1. The initial concentration range of the reactants and temperature range in the oscillating system
In the concentration range
listed in Table 1, the color of the solution changed from pink (the color of Mn2+)
to brown, and at the same time the potential of the system went up rapidly after adding
KBrO3 solution as required by the experiment. After a period of induction, the
potential decreased rapidly, and the system exhibited a periodic oscillation between brown
and pale yellow. In the latter stage, the solution color became light, and the oscillating
period did not change much during the whole oscillating process. With the duration of time, the oscillating amplitude began to show a trend of
slow increase, and after a period of stable amplitude, the amplitude began to decrease
gradually until the oscillation stopped. ln(ti-1s) = -Ei/RT + Ai ln(tp-1s) = -Ep/RT + Ap in which Ai and Ap are the intercept of the lines, Ei/R and Ep/R are the slopes of the fitting lines, respectively. Compared with the Arrhenius Equation ln(k) = -EA/RT + A, ti-1 and tp-1 are very similar to the reaction rate constants, but Ei and Ep should be corresponding to the activation energies, which are called the apparent activation energies in this paper, and their values are Ei = 69.7 kJ mol-1 and Ep = 72.0 kJ mol-1 respectively. The results are in good agreement with the data in the literature[10]. Fig.2 The figure of ln(t-1s) with T-1K A: ln(ti-1s) with T-1K; B: ln(tp-1s) with T-1K. [System: [Act]0 £½ 1.35 mol dm-3, [GL]0 £½ 3.30¡Á10 -2 mol dm-3, [BrO3-]0£½ 0.090 mol dm-3, [Mn2+]0 £½ 6.0¡Á10-4 mol dm-3, [H2SO4]0 = 2.40 mol dm-3.] 3.3 The effects of the reactant concentration
ln(t i/s) = ai + bi ln([Act]0 mol-1 dm3) + ci ln([GL]0 mol-1 dm3) + di ln[BrO3- ]0 mol-1 dm3) +ei ln([Mn2+]0 mol-1 dm3) + fi ln[H2SO4]0 mol-1 dm3) ln(tp/s) = ap + bp ln([Act]0 mol-1 dm3) + cp ln([GL]0 mol-1 dm3) + dp ln[BrO3- ]0 mol-1 dm3) + ep ln([Mn2+]0 mol-1 dm3) + fp ln[H2SO4]0 mol-1 dm3) Based on the experimental data (which are composed of 45
experimental data, each represents the average of three parallel experimental results),
the correlation coefficients in the above two formulae can be determined by plural linear
fitting: ai = 5.80, bi = -0.48, ci =-1.01, di
= 0.71, ei =- 0.21, fi = -2.24; ap = 6.64, bp
= -0.63, cp = -0.99, dp = 0.99, ep =0.57, fp
=-0.86. The plural linear correlation coefficients are 0.999 and 0.998, respectively.
Therefore, the relationship of the induction period t i(s) and oscillating
period tp(s) with the initial concentration of the reactants can be expressed
as: Tp(s) = 768[Act]0-0.63[GL]0-0.99[BrO3- ]00.99[Mn2+]00.57[H2SO4]0-0.86(mol dm-3)0.92s It can be seen from the above relationship, that increasing [Act]0, [GL]0, and [H2SO4]0 can increase the rate and shorten induction period ti(s) and oscillation period tp(s); Increasing [Mn2+]0 can shorten the induction period ti(s) and elongate the oscillating period tp(s); Increasing [BrO3-]0 can elongate the induction period ti(s) and oscillating period tp(s). 3.4 The function of acetone
Keeping the other conditions constant, adding small amount of bromoacetone shortened the induction period. It indicates that the accumulation of bromoacetone is very important during the induction period. Bromoacetone was partly oxidized by Mn3+ to produce Br-, which was favorable for the accumulation of HBrO2 and the induction period was shortened. The reaction process is as follows: Mn3++CH3COCH2Br Mn2++fBr-+Oxidation Product I (2) 3.5 The function of GL GL played the role of the reductant and participated in the formation of Br2 at the same time. So with the increase of the initial concentration of GL, the rate of regeneration of Mn2+ through the reduction of Mn3+ by GL increased, and the induction period ti(s) and oscillating period tp(s) became shorter. 3.6 The function of Mn2+ If replacing Mn2+ by water, though no oscillation occurred, the solution color changed to light-brown gradually. This indicates that though there is no Mn2+ as catalyst, BrO3- can partly oxidize GL. This is because alcohol is more easily oxidized than acid[6,7]. This reaction is shown as processes E which is the side reaction of the oscillating reaction, it consumes partial GL. In the presence of Mn2+, BrO3- and GL reacted rapidly to produce Br2, so that the color of the solution changed rapidly to brown. Here Mn2+ plays the catalytic role, and the catalytic process is shown as follows: BrO3-+Mn2++5H+ Mn3++HOBr+2H2O (3) Mn3++GL Mn2++Oxidation Product II (4) HOBr+GL l/2Br2+Oxidation Product III (5) ________________________________________________________________ BrO3-+2GL+5H+ l/2Br2+Oxidation Product II+Oxidation Product III+2H2O (I) Br2 produced was removed in Reaction (l) to form Br- at the same time. The Br- thus formed can be oxidized by BrO3-which is favorable for the accumulation of HBrO2.The reaction process11 took place as Processes A as follows. At the beginning of the reaction (the induction period), the concentration of Br- was very low. The catalytic process took place predominantly. With the increase of the concentration of Mn2+, it might cause Reaction (3) and (5) to proceed very fast and more Br2 was produced. This further caused Reaction (1) to produce more Br- and then Reaction (6) accelerated the accumulation of HBrO2. The time to reach the critical value of the oscillating was very short, so the induction period ti(s) became shorter with the increase of [Mn2+]0. At the beginning of the oscillation, Mn3+ returned to Mn2+ through Reaction (4) and (2). When [Mn2+]0 was in excess, though the rate of Reaction (4) became faster, every oscillation reaction consumed large amount of GL, causing [GL] to become lower; and during each period the time of Mn3+ returning to [Mn2+] needed became longer with the increase of [Mn2+]0; So the oscillating period tP(s) became longer with the increase of [Mn2+]. 3.7 The function of BrO3- If without the catalysis of Mn2+, BrO3- can still oxidize part of GL and consumes part of GL, which shown as Process E. So with increasing the concentration of BrO3-, more GL was consumed. Therefore with the increase of [BrO3- ], the induction period ti(s) and the oscillating period tP(s) became longer. 3.8 The effect of the radical inhibitor Keeping other conditions constant, when acrylonitrile was added into the reaction system which was oscillating, the oscillation stopped immediately, or when ethanol was added into the reaction system which was oscillating, the oscillation stopped immediately after a few oscillation. Since acrylonitrile and ethanol are both radical inhibitors, it indicates that radicals have been involved in the oscillation reaction. It is reported [11] that the possible radical reaction process may take place as Processes B in the following. 3.9 Discussion on the oscillating mechanism Based on the above discussions, it is proposed that the system may have undergone the following five processes[11,12,13]: Process A: Br-+BrO3-+2H+ HBrO2+HOBr (6) HBrO2 + Br-+ H+ HOBr (7) 5 Br-+ HOBr + H+ Br2 + H2O (8) ________________________________________________________________ 5Br-+BrO3-+H+ 3Br2+3H2O (II)
Process C: Mn3+ returned
to Mn2+ through Reaction (4) and (2) [1] Zhao X Z, Zhao H X, Xu Y T et al. Chemistry (Huaxue Tongbao), 1985, 30: 586. [2] Li H X, Liu M, Wang S X et al. Acta Physico-Chimica Sinica, 1990, (6): 609. [3] Lu C H, Xu Z Q, Xu J D et al. Chemistry (Huaxue Tongbao), 1992, (2): 30. [4] Shen S G, Zhao J G, San J H et al. J. Hebei University, 1995, 5: 21. [5] Yuan C L, Li Z X, Wang J C. Acta Physico-Chimica Sinica, 1994, 10: 87. [6] An C J, Zhuang L, Liu Y et al. Acta Chimica Sinica, 1997, 55: 259. [7] An C J, Gan N Q, Liu Y et al. Acta Chimica Sinica, 1998, 56: 973. [8] Rastogi R P, Verma M K, Yadav R D. Indian J. Chem., 1985, 24A: 721. [9] Trivett T L, Meyer E A. J.Bacteriol., 1971, 107: 770. [10] Pastapur S M, Kulkarni V R. J.Indian Chem. Soc., 1991, 88: 293. [11] Field R J, Burger M. Oscillations and travelling waves in chemical system. Wiley-interscience, New York, 1984. [12] Field R J, Koros E, Noyes R. J. Am. Chem. Soc., 1972, 94: 8649. [13] Noyes R M. J. Am. Chem. Soc., 1980, 102: 4644. ¡¡ ¡¡ |