FORMATION OF AMORPHOUS PRECIPITATES FROM ALUMINUM ORGANIC-LIGANDS SOLUTIONS - MACROSCOPIC AND MOLECULAR STUDY

The precipitation mechanisms in mixtures of 0.02M aluminum chloride and 0.01M organic acid (acetate, oxalate, lactate, salicylate) were studied in the pH range from 3.5 to 6.5. The speciation of soluble Al was analyzed by Al-27 nuclear magnetic resonance (NMR); the formation of a colloidal phase was recorded by turbidimetry; the dried precipitates were analyzed by magic angle spinning (MAS)-Al-27 NMR. The hydrolysis of aluminum is oriented by the organic acids towards less polymerization and early precipitation. The organic acids hinder the formation of Al13, depending on their affinity for oligomeric rather than for monomeric aluminum. This hindrance is strong in the cases of acetate, salicylate and oxalate, and moderate in the case of lactate. Precipitation is enhanced by organic acids. The precipitates contain mainly poorly polymerized octahedral aluminum, hypothetically complexed oligomers.