维生素C氧化反应的研究——用氧气氧化的动力学

STUDY OF ASCORBIC ACID OXIDATION REACTION——KINETICS OF OXIDATION BY MOLECULAR OXYGEN

维生索C(抗坏血酸,H_2A)的氧化反应是与生理过程密切相关的反应。在Cu^(++)催化下用氧气氧化H_2A的动力学研究已有不少报导。早期的研究所得“反应速率常数”往往与浓度有关,不是真正的常数。从六十年代末又有一些作者对此反应进行了动力学研究,得到不同形式的反应速率方程,这些研究大多是在较低pH值下进行的(一般实验条件pH值低于6,大多数低于4)。Russell虽在pH值为4.2—11.9的范围内研究了这一反应,但只报导了在pH值为6—8区间内反应速率有极大值,而没有提供相应的反应速率方程。Young等人提出在pH值为6—8范围内此反应可能出现化学振荡现象,虽然指出这种振荡现象不容易完全重复,但提供了比较容易出现振荡的条件。为了进一步探讨在此pH值范围内此反应的动力学行为,我们对此反应进行了较深入的研究。我们在pH值为6.0—7.5范围内研究了该反应非振荡条件下的动力学行为,得到相应的反应速率方程其中[Cu^(++)]在反应过程中视为不变的常数,K_1、K_2与pH值和温度有关。同时我们也提了相应的反应机理。(1) 式与文献所报导的反应速率方程有很大不同。

In this paper, the kinetics and mechanism of the copper( Ⅱ )-catalysed oxidation of ascorbic acid(vitamine C, H2A) by molecular oxygen have been analyzed in detail, and the oscillatory phenomenon of this reaction was discussed .Young et al found chemical oscillatory phonomenon of this reaction . In this paper, the oscillatory phonomenon was studied at pH 6-8, The oscillatory phonomenon take place easily in the range of such pH value(as Fig. 1).However, the exact number andamplitude of the oscillations could not be exactly duplicated.The kinetics and mechanism of the reaction on nonoscillatory condition were studied in detail.Experimental condition:Phosphate buffer pH = 6.0-7.5 concentration[Cu++] = (4 to 20)×10-7 mol L-1 [H2A] = (4 to 20)×10-5 mol L-1 reaction temperature 20.0 to 40.0 ℃ Experimetal method:The concentration change of ascorbic acid was measured by Bekman DU- 8B spectrophotometer. Ascorbic acid shows a maximum at 265 nm with an extinction coefficient of 15500 M-1 cm-1 in this buffer. Data treatment and mechanism ascertainment:Data obtained from experiment are treated by PC-8000 microcomputer The reaction rate constants and standard deviations (SA) were computed by means of nonlinear optimal imitation method for seven various reaction rate equation and listed in Tab.1.It is obvious; that the following equation is the best of that in Table 1:r=-d[H2A]/dt=K1[Cu++]0.5[H2A]/(1+K2[H2A])0.5 (1) According to eq.(1),the relative results have been analyzed and shown in Figures 2 to 3 and Tables 2 to 3. The corresponding rate constants, activation energies, pre - exponential factors and so on may be obtained from these figures and tables.Considering these results, a chain mechansm of this catalysic reaction had been supposed by us. Applying the steady-state approximation method, we can get the eq.(1) in accordance with such a chain mechanism