Xylenol orange (XO) is one of the complexometric indicators, that can bind to metal cations at both their amino and acidic groups. In this study the protonation constants and distribution diagrams of XO were studied pH-metrically, and the corresponding six protonation constants were calculated. The complex formation between XO (L) and alkaline earth ions (M) was investigated and the formation constants of the resulting complexes ML, MHL, M2L and M2HL were determined. The stabilities of both ML and M2L complexes were found to vary in the order Mg2\ Ca2\ Sr2\ Ba2. Studying the complex formation between Al3 ion (M) and XO (L), it was observed that four complexed species with stoichiometries ML, ML2, MHL and MH2L could be formed in solution. It was also found that the Al L2 complex can act as a chelating agent for further complexation with two cations other than Al3 ion (i.e. Ba, L, Al, L, Ba, Mg, L, Al, L, Mg, and Mg, L, Al, L, Ba). The formation constants of the resulting mixed complexes were determined and their distribution diagrams were investigated. .
