Flow injection determination of Al3+ and Al13O4(OH)24(H2O)127+ species using a 1.3-s reaction with 8-quinolinol-derivatised Fractogel

Speciation of Al is determined by a 1.3-s reaction with 8-quinolinol (oxine)-derivatised Fractogel in a 22 μl column reactor in a flow injection (FI) manifold. Al (pre)concentrated on the column from a 650 μl sample is selectively eluted with 250 μl of 0.02 M NaOH and detected spectrophotometrically as the Al-chrome azurol S (CAS) complex at pH 5.0. This Al (referred to as ‘free Al’) comprises Al3+ + A1(OH)2+ + Al in very labile complexes. Tests with synthetic solutions established that Al is not significantly sequestered from the citrate, oxalate and malonate complexes. Al-hydroxo polymers [Al13(OH)327+] are quantitatively retained by the column but are not desorbed by 0.02 M NaOH in the time frame of the FI method; therefore they do not contribute to the analytical signal. However, they can be quantified after stopped-flow elution with 0.2 M NaOH. The AlF2+ and AlF2+ complexes are retained and eluted quantitatively and therefore contribute to the measurement of ‘free Al’. The method has been applied to humic waters and soil solutions and the results for ‘free’ Al3+ compared with those obtained by the 7-s CAS method. The method has a 2σ detection limit of 70 nM, a linear working range of 0.3–16 μM and relative standard deviations of 7% and 1% at 0.5 and 16 μM, respectively.