Equilibria of iron(III) complexes with xylenol orange and methylthymol blue

A potentiometric and spectrophotometric study of Fe(III) complexes with Xylenol Orange (XO) and Methylthymol Blue (MTB) has been made. The formation constants for the Fe(III) complexes with XO and MTB were determined. Evidence was found for the formations of 1:1 and 2:1 complexes (metal:ligand) and it was assumed that protonated and hydroxo-complexes exist in addition to the simple complex in each case. The hydroxo-complexes are stable over the pH range of 7–12. Suggestions are made concerning the probable structures of these complexes.