Nature of amorphous aluminum hydroxycarbonate

The titration of sodium carbonate with aluminum nitrate is shown to produce amorphous aluminum hydroxycarbonate. This compound is not stoichiometric, although the maximum carbonate to aluminum ratio appears to be 0.5. The pH conditions for achieving the maximum carbonate content are concentration dependent. A model for the particle surface at the solution interface is proposed. This model accounts for the presence of carbonate directly coordinated to the aluminum and carbonate adsorbed by electrostatic forces. Sodium is present in the diffuse layer and is, therefore, not an integral part of the structure.