Structure of aqueous sodium aluminate solutions: A solution X-ray diffraction study

A structural analysis of six alkaline sodium aluminate aqueous solutions by the X-ray diffraction method is reported. On average, each Al atom is surrounded by four oxygens, indicative of the predominance of Al(OH)(4)(-)(aq) in these solutions. Detailed least-squares fitting indicates that a significant contraction of the Al-O distances occurs with increasing aluminate concentration, from 1.80 Angstrom at 2 M to 1.74 Angstrom at 6 M Al-(OH)(3) in 8 M NaOH. The local structure has been described by models that have separate hydrated ions in the most dilute aluminate solution but contact sodium aluminate ion pairs in the most concentrated solution. The hydration number of the sodium ion decreases with increasing concentration, but the overall coordination number appears to be unchanged by the ion pair formation. An extensive rearrangement in the hydrogen-bonded network of bulk water also occurs as the aluminum concentration rises, with the appearance of new diffraction distances at 3.3 and 3.9 Angstrom. A gradual appearance and disappearance of shorter hydrogen bonds between first neighboring O atoms is observed. The data are consistent with the occurrence of oligomeric aluminate species but are not conclusive within the limits of the experimental error.