Aluminum speciation using morin: I. Morin and its complexes with aluminum

In this and a companion study, we developed a procedure that uses the fluorescing chelating agent morin (2,3,4,5,7-pentahydroxy-flavone) to speciate aqueous Al in the acidic pH range. Morin is a pentaprotic acid with five weakly acidic functional groups. However, only the first proton dissociation reaction was found to be significant in the acidic pH range, and a simple monoprotic model (pK(al) = 5.04) accurately described proton binding. Aluminum and morin formed complexes of 1:1 and 1:2 and/or 1:3 (Al/morin) stoichiometry, but at low total morin concentrations (near 1 μM) only 1:1 species were prevalent. The equilibrium chemistry of 1:1 Al/morin complexes in the acidic pH range was closely described by a parabolic model consistent with the formation of three complexes: AIH₄MOR²⁺, Al(OH)H₄MOR⁺, and Al(OH)₂H₄MOR⁰. For Al and six potentially interfering cations, the order of affinity for morin relative to Al in the acidic pH range was approximately as follows: Al > Fe(III) > Cu > Fe(II) > Ca > Mg > Mn = Zn. Formation of competing cation-morin complexes was enhanced with increasing pH. None of the tested cations formed complexes with morin with fluorescence spectra overlapping the spectra of the Al-morin complexes.