Speciation of Hydroxy‐Aluminum Solutions by Wet Chemical and Aluminum‐27 NMR Methods

Abstract

Abstract Partially neutralized Al solutions with OH/Al molar (ñ) ratios ranging from 0.25 to 2.5 were studied employing 27 Al nuclear magnetic resonance spectroscopy and the ferron‐timed colorimetric assay. The monomeric Al concentrations (Al a ) estimated by the ferron assay were consistently higher than the actual concentration determined by NMR spectroscopy. The differences between the methods indicate the presence of a small polymer which reacts rapidly with ferron and is erroneously assigned to the monomeric Al fraction. The polymeric Al component (Al b ) estimated by the ferron method could be partitioned into a more rapidly reacting fraction (≤1 h) and a slowly reacting fraction (>1 h). The amount of Al associated with the rapidly reacting polymeric fraction corresponded to the amount of [AlO 4 Al 12 (OH) 24 (H 2 O) 12 ] 7+ polymer directly determined by NMR spectroscopy. The slower reacting component of the Al b fraction is believed to be a polymer of the hexameric ring morphology. Direct NMR measurements provided evidence for specific adsorption of Al 13 to a cation exchange resin and differential precipitation kinetics of this polymer with added sulfate. These data indicate that characterizing hydroxy Al solutions without direct measurements can lead to erroneous interpretations concerning the distribution between monomeric and polymeric Al and the nature and types of polymeric Al present.