Spectrophotometric Determination of Trace Amount of Iodidewith Chromium(VI)-Bromide-Basic Xanthene Dye System

In dilute phosphoric acid medium, chromium(Ⅵ) oxidizes I- to I2, which then binds Br- to form - anion in the presence of excess bromide, and - 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.