Synthesis of 5-(5-Iodine-2-pyridylazo)-2,4-diaminotoluene and Its Color Reaction with Rhodium(III)

A new reagent,5-(5-iodine-2-pyridylazo)-2,4-diaminotoluene(5-I-PADAT)has been synthesized,and characterized by infrared spectroscopy,elemental analysis and nuclear magnetic resonance spectroscopy. The color reaction of 5-I-PADAT with rhodium(Ⅲ) was studied. The experimental results show that rhodium(Ⅲ)reacts with 5-I-PADAT at the pH range of 4.0~5.2 to form a violet-red complex and its maximum absorbance is at 580 nm. The molar ratio of Rh(Ⅲ)to 5-I-PADAT is 1:2. The complex changes into a blue one in 1.2 mol\5L-1 HClO4 acid solution with an apparent molar absorptivity of 1.81×105 L\5moL-1\5cm-1 and Beer's law is obeyed in the range of 0~0.8 μg/mL for rhodium. The method is highly sensitive and selective and has been successfully applied to the determination of trace rhodium in catalysts with satisfactory results.