Complex formation between rare earth ions and polyhalogenated bisazo chromotropic acid was studied. The dissociation constants of tribromoarsenazo determine by spectrophotometric and potentiometric methods are pK1 = 0.8, pK2 = 1.7, pK3 = 2.5, pK4 = 6.5, pK5 = 9.6, and pK6 = 10.3, resp. These dissociation constants are larger than those of Arsenazo III and the stability constants of the complexes formed are higher than those for other reagents of the same type (log KS = 52 ?0.4). The molar ratio Ce/DBC-Arsenazo in the complex is estimated to be 1:3 by the Jobs continuous variation and the equilibrium shift methods. When halogen atom are introduced into the ligand, red- or blue-shifts of the absorption peaks occur. The shape of the absorption spectrum changes from asym. to sym. with decreasing electronegativity of the halogen atom.

Key words: REACTION MECHANISM, HALIDE, RARE EARTH SERIES, COLOUR REAGENTS, STABILITY CONSTANT, RARE EARTH METAL COMPLEX, ARSENAZO, AZO COMPOUNDS, NAPHTHALENEDISULFONIC ACID P, NAPHTHALENEDIOL P