Characteristics of simplified ferron colorimetric solution and its application in hydroxy-aluminum speciation

Despite the wide application of ferron assay in hydroxy-Al speciation, the complexation mechanism in function remains unclear and there is no generally accepted quantification stipulation. In this study, a simplified ferron colorimetric solution was suggested and characterized in its application to the speciation of different hydroxy-Al samples with the aid of 27Al NMR spectroscopy. Results show that with no addition of hydroxylamine and orthophenanthroline, the simplified ferron colorimetric solution can become more stable and is applicable in speciation for Al solutions with no or low content of iron ions (i.e., [Fe]/[Al] ≤ 0.05). Polymeric aluminum species, especially Al13 ([AlO4Al12 (OH) 24(H2O)12]7+) may not react with ferron directly but via the process of Al decomposition. Al species of the similar form may have the same level of reaction rate with ferron. The Ala-ferron interaction time adopted to quantify Ala can greatly affect the speciation of hydroxy-Al solutions. The longer the time adopted, the higher the content of oligomer Al species included in Ala (mononuclear and oligomer Al species). However, the adoption of the duration of ferron interaction with Ala + Alb has little effect on the speciation of Alb (soluble polymeric Al species) only if the time is long enough to ensure no further detectable absorbance increase. At least five kinds of Al species, such as mononuclear Al (Almon), Al2, Al6 polymers, Al13 and colloidal Al (Alc) exist in hydroxy-Al solutions. Generally, Ala includes Almon, Al2 and possibly part of Al6 polymers. In most cases, Alb is mainly composed of Al13 and other species like Al6 polymers can also contribute. Only under certain conditions (B value ranges), Alb can be equivalent to Al13.