Humic substances in solution constitute a generally overlooked source of interference in the spectrophotometric determination of aluminium. We investigated humic interference in the determination of total monomeric and acid-reactive aluminium using three methods: (1) 8-hydroxyquinoline/MIBK extraction, (2) flow injection pyrocatechol violet, and (3) flow injection eriochrome cyanine R/CTA. Interference was apparent in all three methods from both humic and fulvic acid concentrations above 10mg CL#x2212;1in the measurement of total monomeric aluminium. The 8-hydroxyquinoline/MIBK extraction method was not subject to interference in the determination of acid-reactive aluminium (200mg CL#x2212;1), while interference appeared in the other two methods at 20#x2013;50 mg CL#x2212;1. The observed interferences were pH-dependent and were minimized by performing the reaction at higher pH-values.
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