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在过量溴化物存在下的稀磷酸介质中， I－被 Cr（Ⅵ）氧化成 I2后与 Br－结合形成 [I2Br]－配阴离子，该配阴离子能进一步与罗丹明 6G、罗丹明 B、吖啶红等碱性吨染料阳离子形成离子缔合配合物。在聚乙烯醇存在下，缔合物体系稳定且溶液颜色有明显的变化，可用于 I－离子的光度测定。方法具有高灵敏度，不同体系的摩尔吸光系数在 4． 96× 104～ 1． 1× 105 L· mol－ 1· cm－ 1之间，以罗丹明 6G和罗丹明 B体系灵敏度较高。碘离子质量浓度分别在 0～ 0.8 mg/L(罗丹明 B和罗丹明 6G体系 )、 0～ 1.0 mg/L（吖啶红体系）之间遵守比尔定律。方法具有良好的选择性，用于海带、黄豆和含碘药片的测定结果令人满意。“,”In dilute phosphoric acid medium, chromium(Ⅵ ) oxidizes I-to I2, which then binds Br-to form [I2Br]-anion in the presence of excess bromide, and [I2Br]-anion further reacts with basic xanthene dyes such as rhodamine 6G, rhodamine B and acridine red to form ion -association complexes. In the presence of polyvinyl alcohol(PVA) the solution is stable and a distinct color change occurred. The method can be applied to the spectrophotometric determination of iodide. The method have a high sensitivity and selectivity. Its molar absorptivity is 1.1× 105 L· mol-1· cm-1 for rhodamine 6G and rhodamine B systems， and 4.96× 104 L· mol-1· cm-1 for acridine red system. The Beer’ s law is obeyed in the range over 0～ 0.8 mg/L and 0～ 1.0 mg/L of I-for rhodamine 6G, B systems, and acridine red system, respectively. It was used to determine I-in kelp, soybean and cydiodine tablets with satisfactory results.