Comparison of Three Spectrophotometric Methods for Differentiating Mono‐ and Polynuclear Hydroxy‐Aluminum Complexes

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Abstract
Increasing interest in the relationships between aluminum (Al) speciation and biological toxicity has created a need for accurate methods for speciating aqueous Al. Polynuclear hydroxy‐Al complexes are generally viewed as metastable species, and are thus more amenable to empirical than to computational approaches to speciation. The objectives of this study were to evaluate and compare the ferron (8‐hydroxy‐7‐iodo‐5‐quinoline‐sulfonic acid), aluminon (aurintricarboxylic acid, triammonium salt), and 8‐hydroxyquinoline methods for their ability to differentiate mono‐ and polynuclear Al in solutions typical of those used in related phytotoxicity studies. Test solutions were 20 µM in AlCl3, 0.4 mM in CaCl2 or 40 mM in NaCl, and had basicities (molar OH/Al ratios) ranging from −1 to 2.25. Reactions between test solutions and ferron or aluminon were kinetically modeled as two parallel irreversible reactions to yield estimates of the mononuclear fraction, fm. The reaction of 8‐hydroxyquinoline with mononuclear Al was essentially instantaneous, and use of an arbitrary cutoff was unavoidable. Estimates of fm were in the order 8‐hydroxyquinoline > ferron > aluminon at low basicities, while at a basicity of 2.25 the general order was aluminon ≥ 8‐hydroxyquinoline > ferron. Increasing ionic strength of test solutions increased estimates of fm by all methods. Significant quantities of polynuclear Al were detected even in solutions without added base or with slight acidification. Aging solutions up to 32 d resulated in decreased estimates of fm using aluminon, but estimates using ferron and 8‐hydroxyquinoline were only minimally affected. All three methods yield results of adequate precision for most purposes, although the ferron procedure is somewhat less sensitive than the other two. Additional studies using ferron demonstrated its utility for characterizing the nonmononuclear Al fraction using kinetic analyses. Ferron may be the preferred method based on its simplicity, level of precision, and moderate reaction rate with Al.