Modified ferron assay for speciation characterization of hydrolyzed Al(III): a precise k value based judgment

The speciation of Al-OH complexes in terms of Al_a, Al_b and Al_c could be achieved by traditional ferron assay and Al_b is generally considered as Al₁₃, however, the inherent correlation between them remains an enigma. This paper presents a modified ferron assay to get precise determination of Al₁₃ using nonlinear least squares analysis, and to clarify the correlation between Al_b and Al₁₃. Two parallel reactions conforming to pseudo-first-order kinetics can simulate the complicate reactions between polynuclear complexes and ferron successfully. Four types of experimental kinetic constant (k value) of Al-OH complexes can be observed by this method when investigating three typical aluminium solutions. Comparing with the results of ²⁷Al NMR, the species with moderate kinetics around 0.001 s⁻¹ can be confirmed to resemble to Al₁₃ polycation. The other types of kinetics are also well-regulated in partially neutralized aluminium solutions with various OH/Al ratios (b values) in the range 0 ∼ 2.5. It would provide potential means to trace the in-situ formation of Al₁₃ in dilute solutions such as coagulation with Al-based coagulants