食品中常见有机物对Cr(Ⅵ)还原反应动力学
首发时间:2012-12-26
摘要:采用二苯碳酰二肼分光光度法,测定了食品中15种常见有机物在不同pH值、不同温度下还原Cr(Ⅵ)的数据,结果表明:L-抗坏血酸、L-半胱氨酸盐酸盐、L-甲硫氨酸表现出较强的还原Cr(Ⅵ)能力,其余12种有机物在5h内基本未还原Cr(Ⅵ)。L-抗坏血酸在短时可将Cr(Ⅵ)全部还原,L-半胱氨酸盐酸盐在60min内可将Cr(Ⅵ)全部还原,L-甲硫氨酸在5h内能将Cr(Ⅵ)还原23%,其还原作用能力次序为:L-抗坏血酸> L-半胱氨酸盐酸盐> L-甲硫氨酸。pH值对还原反应动力学影响较大,L-半胱氨酸盐酸盐对Cr(Ⅵ)的还原作用随pH的升高而增强,而L-甲硫氨酸对Cr(Ⅵ)的还原作用则随pH的升高而减弱直至不反应。以零级反应动力学方程拟合了pH=3.0时L-甲硫氨酸还原Cr(Ⅵ)的动力学数据,得到L-甲硫氨酸还原Cr(Ⅵ)的表观活化能为35.5 kJ•mol-1;以一级反应动力学方程拟合了半胱氨酸盐酸盐还原Cr(Ⅵ)动力学数据,得到pH=3.0、5.0和7.0时,半胱氨酸盐酸盐还原Cr(Ⅵ)的表观活化能分别为34.4, 45.3, 34.9 kJ•mol-1。
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Reduction Kinetics of Cr(Ⅵ) with Common Organic Compounds in Food
Abstract:In this paper, the data of Cr(Ⅵ) reduction with 15 kinds of common organic compounds in food were determined by the diphenylcarbazide spectrophotometry at different pH values and temperatures. The results indicated that L-ascorbic acid, L-cysteine hydrochloride salt and L-methionine had relatively strong reduction activities for Cr(Ⅵ), while Cr(Ⅵ) hardly underwent the reduction with other 12 kinds of organics within 5 hours. Cr(Ⅵ) was completely reduced by L-ascorbic acid in a short period of time, and completely reduced by L-cysteine hydrochloride salt within 60 minutes, but reduced for 23% by L-methionine after 5 hours. Therefore, the sequence of the reduction activity of the three organic compounds was L-ascorbic acid >L-cysteine hydrochloride salt>L-methionine. The values of pH had a great effect on the reduction kinetics. As pH value increased, the reduction with L-cysteine hydrochloride salt was enhanced, while the reduction with L-methionine was decreased, and even decreased to none. The kinetics data of Cr(Ⅵ) reduction with L-methionine were fitted by a zero-order kinetics equation at a pH of 3.0, and the apparent activation energy was obtained as 35.5 kJ•mol-1. In addition, the kinetics data of Cr(Ⅵ) reduction with L-methionine at the pH of 3.0, 5.0 and 7.0 were fitted by a first-order kinetics equation, and the apparent activation energies of the reduction were obtained as 34.4, 45.3, 34.9 kJ•mol-1, respectively.
Keywords: chemical engineering organic compound Cr(Ⅵ) reduction kinetics
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