The absorbance of a mixture of europium-xylenol orange (XO), or other members of lanthanides-XO, and the bivalent metal ions M2+-XO (M2+ = Mn, Co, Ni, Cu, Zn, Cd, and Pb) complexes in slightly acidic media (pH ~ 6) is equal to the sum of the individual absorbances measured alone, within ?, only Cu-XO shows greater deviation (within ?.5). With both purified and com. XO reagent, good additivity can be obtained. This may be due to similar visible change in the spectrum or sharp color change when the metal ions combine with phenolic oxygen of XO at 4

Key words: ULTRAVIOLET SPECTROPHOTOMETRY, MOLECULAR STRUCTURE, ACIDITY, XYLENOL ORANGE, RARE EARTH METAL COMPLEX, EUROPIUM COMPLEX, ABSORBANCE SUBTRACTION