Aluminum-27 nuclear magnetic resonance studies of ferron—hydroxo—polynuclear Al interactions

The [AIO4AI12(OH)24+n(H2O)12−n] 7−n+(AI13) polynuclear species was found to interact with the commonly employed complexation agent ferron (8-hydroxy-7-iodo-5-quinoline-sulfonic acid) according to multiple reaction pathways. After the addition of ferron to the partially neutralized Al solution, a large portion of the Al13 polymer was no longer observed by 27Al NMR spectroscopy. The data indicated that the observed initial loss in Al13 concentration resulted from precipitation with acetate (OAc) in the buffer, since the loss was not related to the ferron: Al molar ratio, because no corresponding increase in the Al-ferron (δ = 3.4 ppm) resonance was observed and also because similar losses were noted in solutions where only NaOAc was added. The Al13 remaining in solution was characterized by a resonance with an identical chemical shift (δ = 62.5 ppm), yet having wider line widths at half-maximum (v1/2) than normally associated with the Al13 polynuclear species. This suggested that the ferron is complexing with octahedrally coordinated Al comprising the cage-like structure, and thus distorting the symmetry of the centrally located tetrahedral Al. The Al13—OAc solid phase was also being degraded by ferron more slowly, as indicated by the quantitatively larger increase in the ferron-Al resonance than could be accounted for by the subsequent disappearance of the Al13 peak. That a single polynuclear, the Al13, reacts with ferron via multiple reaction pathways helps to explain disparate interpretations in the literature based on indirect timed spectrophotometric assays regarding the existence of a large number of Al polynuclear structures.