Study on the color reaction of rhodium with 4-(5-nitro-2-pyridylazo)-1,3-diaminobenzene and its application

A new reagent 4-(5-nitro-2-pyridylazo)-1,3-diaminobenzene(5-NO2-PADAB) is recommended for the determination of rhodium(Ⅲ).It is shown that 5-NO2-PADAB reacts with rhodium at pH 3.3～4.7 to form a violet-red complex showing a maximum absorption at 524 nm.The complex can be changed into another deeply colored stable species by addition of mineral acid.The maximum absorption of the complex shifts to 580 nm in a medium of 1.2 mol/L HClO4.The apparent molar absorptivity is 1.52×105 L·mol-1·cm-1.Beer′s law is obeyed in the concentration range of 0～0.8 μg/mL for rhodium.The method is highly sensitive and selective.Most common ions do not interfere with it.It has been successfully applied to the determination of trace rhodium in catalysts with satisfactory results.