Determination of iron in solutions with the ferric-xylenol orange complex

Formation of a colored complex between ferric iron and xylenol orange has b een used in a variety of applications, but there is disagreement about the number of complexes that exist, their stoichiometry, pH dependence, solvent effects, and other parameters. In this study we investigated the use of th e complex to measure micromolar concentrations of iron in aqueous and alcoh ol solutions. The results show that the optimum wavelength for the measurem ent is 560 nm, that only a 1:1 ferric:xylenol orange complex forms, and tha t its molar absorption coefficient is affected by the solvent, the source o f the xylenol orange, and the pH. Under the recommended conditions, formati on of the complex is completed in less than 5 min and it is stable in sever al solvents. Its molar absorption coefficient in 25 mM H2SO4 is 20,100 or 1 4,500 M-1 cm(-1), depending on the source of the dye. A recommended protoco l for the use of the assay is given. (C) 1999 Academic Press.