目的　建立一种测定水中痕量三价铁的新方法。方法　三价铁、硫氰酸铵和吐温 - 80形成红色配合物 ,然后用分光光度法测定 ,用单因素改变法选择出显色反应的最佳条件。结果　配合物最大吸收波长 (λmax) 480nm ,摩尔吸光系数 (ε) 1.54× 10 5mol·L·cm- 1,Fe3+ 浓度在 0～ 0 .8mg/L内服从比尔定律 ,相关系数 (r) 0 .9997,检出限 (L) 1.4 6× 10 - 4μg/L。回收率 98.4 %～ 10 4 .3%。结论　新建方法具有选择性高、准确度好、简便实用等优点 ,是一种测定水中痕量三价铁的可靠方法。 Objective To establish a new method for the determination of trace ferric iron in water. Methods Trivalent iron, ammonium thiocyanate and Tween - 80 were used to form the red complex. The complex was determined by spectrophotometry. The optimum conditions for the chromogenic reaction were selected by single factor method. Results The maximum absorption wavelength (λmax) of the complex was 480 nm, the molar absorptivity (ε) was 1.54 × 10 5 mol·L · cm-1, and the Beer’s Law was obeyed with the concentration of Fe3 + ranging from 0 to 0.8 mg / L. 9997, the detection limit (L) 1.4 6 × 10 - 4μg / L. The recovery rate of 98.4% ~ 104.3%. Conclusion The new method has the advantages of high selectivity, good accuracy, simple and practical, and is a reliable method for the determination of trace ferric iron in water.