Speciation determination of aluminium (Al3+) by flow injection amperometry via the 1,2-dihydroxyanthraquinone-3-sulfonic acid complex

Two flow injection methods for the determination of ‘free Al’ (Al3+ and Al(OH)2+) in natural samples have been developed and compared. Both employed amperometric detection by measuring the decrease (in the presence of Al) in the oxidation current for DASA (1,2-dihydroxyanthraquinone-3-sulfonic acid) at pH 5.0 at a glassy carbon electrode. One method sought to achieve selective determination of free Al by use of a short reaction time (10 s or 3 s) and a low concentration of DASA (3 μM at the detector). The other effected an initial fractionation of Al on a column (22 μl) of oxine(8-quinolinol)-derivatised Fractogel (< 2 s contact time) followed by elution of Al with NaOH solution. The latter protocol achieved better speciation determination and countered the interference of Fe(III) and Ca2+. The 3σ detection limits for the two methods were 75 and 120 nM, respectively. The procedures were applied to the determination of free Al in synthetic Al-ligand solutions and of the Al complexation capacity of a soil fulvic acid and humic water.