Abstract: Dual-wavelength spectrophotometry on the color reaction of 4,6-dibromoarsenazo with rare earths has been studied and applied to assay steel and agricultural products.The color reaction of 4,6-dibromoarsenazo with rare earths has high sensitivity and forms stable purple red complexes(1:2)in chloroacetic ucid-sodium hydroxide buffer medium at pH 2.0.The maximum absorption wavelengthes of reagent and complexes are at 516 run and 638 nm,respectively,and the apparent molar absorptivity of the complexes for the dual-wavelength spectrophotometry isε=1.56×10~5.The dual-wavelength spectrophotometry is more sensitive than single-wavelength spectrophotometry.Beer’s law is obeyed in the concentration range of 0～2.0μg/ml for total rare earths.The method shows simplicity,high sensitivity,better selectivity and accuracy and can be used direcOy to determine trace rare earths in actual samples without separation and concentration.The recovery of the method is in the range of 92% 101%.The result is satisfactory.

Key words: 4,6-dibromoarsenazo, rare earths, dual-wavelength spectrophotometry