Aluminum speciation using morin: II. Principles and procedures

In this study we present an analytical technique that uses a dilute aqueous solution of the fluorescing chelating agent morin (2,3,4,5,7-pentahydroxyflavone) (1 μM) to distinguish between Al species in acidic solutions. Sensitive and accurate speciation estimates were obtained for synthetic solutions containing low concentrations of model Al binding ligands (1 μM F or 0.001 M SO₄). Good agreement existed with thermochemical modeling predictions, and accurate equilibrium constants were estimated for AlF²⁺ and Al(SO₄)⁺. For the F solutions, speciation measurements made with morin were superior to measurements made with the F ion selective electrode. For natural water samples (n = 108) exhibiting a wide range of pH and concentrations of Al binding ligands, the addition of morin did not obscure the subtleties of the Al species distribution. The average redistribution of Al species in the presence of morin was less than 3%, and a correction factor was applied to calculate the species distribution prior to morin addition. The fraction of total Al bound with organic solutes estimated using morin (74%) was significantly higher than in previous studies using other techniques. The magnitude of uncertainties in species concentration estimates, estimated by Monte Carlo simulation, varied with pH, Al concentration, and total morin concentration. For 1 μM morin solutions, maximum uncertainties for major Al species were low (approx. < 12% in the pH range between 3 and 7.