在0.30 mol·L-1盐酸中, 铁(Ⅲ)显著催化过硫酸钾氧化甲基红的褪色反应, 据此建立了测定痕量铁(Ⅲ)的“Fe(Ⅲ)-盐酸-过硫酸钾-甲基红”新体系, 研究了该体系测定痕量铁(Ⅲ)的最佳条件, 考察了共存离子对测定的影响, 分析了测定反应的动力学参数及方程, 并利用动力学性质探讨了测定痕量铁(Ⅲ)的反应机理, 从而构建了一种测定痕量铁(Ⅲ)的新催化动力学光度法。 研究表明, 在最大吸收波长518 nm以及最佳测定条件下, 该体系中甲基红的浓度变化与铁(Ⅲ)含量呈以下良好线性关系: ln(A0/A)=1.334 1+0.001 0, 相关系数为0.999 1; 痕量铁(Ⅲ)发生一级反应, 该测定体系符合准一级反应特征且反应的表观活化能为69.88 kJ·mol-1; 反应机理的研究进一步证明了该体系中痕量铁(Ⅲ)起催化作用; 该痕量铁(Ⅲ)的测定方法操作简便, 所用试剂廉价易得, 目前还未见报道, 其灵敏度优于普通光度法, 检出限达0.005 mg·L-1。 该方法应用于实测水样和食品中痕量铁(Ⅲ), 相对标准偏差为1.18%～2.11%, 平均回收率为98.0%～104.0%。 该测定体系稳定, 结果满意。

A catalytic kinetic spectrophotometric method, which is based on the catalytic effect of Fe(Ⅲ) on the fading reaction between potassium persulfate(K2S2O8) and methyl red(MR) in the solution of 0.30 mol·L-1 hydrochloric acid, for the determination of trace amounts of Fe(Ⅲ) has been investigated. A novel detection system, Fe(Ⅲ)-HCl-K2S2O8-MR, has been developed. The optimum experimental conditions for the determination of trace amounts of Fe(Ⅲ) were found on the basis of orthogonal test. The kinetics parameters and equation of this fading reaction of MR were studied. Its reaction mechanism was discussed. The results show that there is a good linear relationship between the variation of MR absorbance at the maximum absorption wavelength of 518 nm and the concentration of Fe(Ⅲ) under the optimum experimental conditions: ln(A0/A)=1.334 1+0.001 0, the correlation coefficient is 0.999 1. The kinetic research shows that the reaction order with respect to Fe(Ⅲ) is 1 and the overall fading reaction is a pseudo-first order reaction. The apparent activation energy of the fading reaction of MR is 69.88 kJ·mol-1. Furthermore, the catalytic effects of Fe(Ⅲ) on this fading reaction is confirmed by its reaction mechanism. This novel method for the determination of trace Fe(Ⅲ) has never previously been published so far. Trace amounts of Fe(Ⅲ) can be selectively determined by this catalytic kinetic spectrophotometric method with high precision and accuracy. This method is simple and its reagents used are cheap and available. Its sensitivity is higher than that of conventional spectrophtometry with detection limit of 0.005 mg·L-1. This detection system is stable. This proposed method has been applied to the determination of trace amounts of Fe(Ⅲ) in food and water samples with satisfactory results. Relative standard deviation of the detection results is 1.18%～2.11%. Average recovery rate of the detection results is 98.0%～104.0%.