The ε-Keggin [AlO4Al12(OH)24(H2O)12]7+ ion (AlAl127+) is a metastable precursor in the formation of aluminum oxyhydroxide solids. It also serves as a useful model for the chemistry of aluminous mineral surfaces. Herein we calculate the enthalpies of formation for this aqueous ion and its heterometal-substituted forms, GaAl127+ and GeAl128+, using solution calorimetry. Rather than measuring the enthalpies of the MAl127/8+ ions directly from solution hydrolysis, we measured the metathesis reaction of the crystallized forms with barium chloride creating an aqueous aluminum solution monospecific in MAl127/8+. Then, the contributions to the heat of formation from the crystallized forms were subtracted using referenced states. When comparing the aqueous AlAl127+ ion to solid aluminum (oxy)-hydroxide phases, we found that this ion lies closer in energy to solid phases than to aqueous aluminum monomers, thus explaining its role as a precursor to amorphous aluminum hydroxide phases. Downhill: Calorimetric measurements on the ε-AlAl127+ ion and its heterometal-substituted forms provide the first enthalpy of formation (ΔHf,el) values on these species in aqueous form independent of formation pathway. Data reveal the AlAl127+ ion lies closer in energy to solid aluminum (oxy)hydroxide phases than aqueous monomers, thus explaining its role as a precursor to amorphous aluminum hydroxides (flocs).
|Original language||English (US)|
|Number of pages||4|
|Journal||Angewandte Chemie - International Edition|
|State||Published - Aug 1 2015|
- Keggin ions
- aluminum hydroxide
ASJC Scopus subject areas