On the Interpretation of Labile Aluminum as Determined by Reaction with 8-Hydroxyquinoline
- Jörg Luster,
- Andrew Yang and
- Garrison Sposito *
The interpretation of labile Al, as defined operationally by a 15-s reaction with 8-hydroxyquinoline, remains problematic. In solutions free of complexing ligands other than OH−, one question is whether the chemical species (Al(OH)+2 should be included. To resolve this issue, labile Al was measured in Al(ClO4)3 solutions at total Al (AlT) concentrations in the range 4 to 60 µmol kg−1 and pH in the range 4 to 6. In solutions equilibrated for 4 h without shaking, no effect of pH in the range 4 ≤ pH ≤ 5 on the values of labile Al was observed. Thus, Al3+ and all significant mononuclear hydrolytic species of Al that formed under the experimental conditions were reactive with 8-hydroxyquinoline within 15 s. Shaking the sample solutions at pH 5 throughout equilibration, however, produced a gradual decline in labile Al with concomitant changes in light scattering that indicated the formation of colloidal or macromolecular Al species that were unreactive with 8-hydroxyquinoline during a 15-s period. Even in unshaken solutions, a decline in labile Al (and the inferred formation of colloidal Al species) was observed at pH 5 if solutions with AlT > 1 mmol kg−1 were used to prepare standard solutions for calibration. These results are important not only for the application of the 15-s 8-hydroxyquinoline method, but also for any experimental study of aqueous Al at pH values where Al hydrolysis is significant.
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