ALUMINUM(III) SPECIATION WITH ACETATE AND OXALATE - A POTENTIOMETRIC AND AL-27 NMR-STUDY

Aluminum(III) hydrolysis and precipitation in the presence of acetic acid and oxalic acid have been studied by combining potentiometric titration and liquid-state Al-27 NMR. The main aluminum species have thus been identified and quantified: unreacted, hydrolyzed, complexed monomers, and the Al13 tridecamer. A "solid" species appeared when precipitation occurred and was quantified by difference with the other species. The quantitative evolution of these species was followed for pH values up to 5. Acetate forms weak complexes with aluminum and practically does not affect the hydrolysis of aluminum. The precipitated phase was hypothesized to be aggregated Al13. Oxalate forms strong multiligand complexes at the lowest pH values. The hydrolysis of these complexes to form Al13 requires higher hydroxyl content. High oxalate contents (L/M > 1) inhibit tridecamer formation and precipitation occurs only at high pH values. With oxalate the precipitated phase seems to be devoid of Al13 and of a more condensed nature than it is with acetate.