分光光度法测定单宁酸-Fe^(3+)反应结合常数及其化学计量比

Spectrophotometric determination of formation constants and stoichiometry of tannic acid complexes with Fe^(3+)

选择一种典型的单宁酸化合物1,2,3,4,6-O-五没食子酰葡萄糖(PGG)为研究对象,利用紫外-可见分光光度法分析其与Fe^(3+)离子的络合反应过程及特性。结果表明:利用等摩尔连续变化法(又称Job's法)分析显示在pH 6.0条件下1个PGG分子可结合2个Fe^(3+)离子,即形成PGG-2Fe络合物。利用数学模拟计算法对得到的不同反应吸光度进行非线性模拟,单宁酸-Fe^(3+)络合反应的结合常数(K)为1.607×1012(mol/L)^(-2),同时计算出PGG与其Fe^(3+)络合物的摩尔吸光系数差值为0.046μ/(mol·L^(-1))。此外,利用pH滴定技术分析了单宁酸中的酚羟基去质子化反应过程。结果表明,随着溶液pH的升高,PGG分子中有6个酚羟基首先发生去质子化形成氧负离子状态。随着溶液pH的进一步升高,PGG分子中最终约有9个酚羟基团发生去质子化。

The active interactions between tannins and metals have been long observed in the natural environment while the tanninmetal compounds and the related processes have been widely applied in industry.However,little is known about quantitative study on the interactions between the metals and the tannins,especially the pure and chemically defined tannins.In this paper,coordination reactions between 1,2,3,4,6penta-O-galloylglucose(PGG),a typical tannic acid and Fe3+at pH 6.0 were studied by using ultravioletvisible(UVVis)spectrophotometric method.The results of a method of continuous variation(Jobs method)showed that the PGG formed 1〖DK〗∶2(molar ratio of PGG to metal)complexes with Fe3+at pH 6.0.The formation constants were obtained by nonlinear fitting the UV titration data and gave the result of 1.607×1012(mol/L)–2,and 0.046μ/(mol〖DK〗·L-1)for extinction coefficient difference,orΔε=0.046μ/(mol〖DK〗·L-1).Furthermore,the deprotonation reaction of phenolic hydroxyl groups in the structure of the PGG was studied by pH titration experiment.The results of pH titration suggested that six phenolic hydroxyls in the PGG can be deprotonated firstly with the increasing of pH value.When continued addition of more NaOH into the PGG solution,the spectra kept redshift proportionally with the NaOH addition until a ratio of NaOH to the PGG reached 9,suggesting nine phenolic hydroxyls in the PGG can be deprotonated.In other words,a 9fold excess of NaOH is necessary to obtain a full deprotonation PGG.A new analytical method for the quantitative analysis of the complexation reaction between polyphenols and metal ions is developed which will improve the understanding of the structureactivity relationship between the tannins and the metals and its complexation reaction mechanism.