Potentiometric titration curves of aluminium salt solutions and its species conversion in the hydrolysis-polymerization course

Abstract

A new concept of critical point is expounded by analysing the potentiometric titration curves of aluminium salt solutions under the moderate slow rate of base injection. The critical point is defined as the characteristic spot of the Al³⁺ salt solutions potentiometric titration curve, which is related to the experiment conditions. In addition, the changes of critical points reflect the influence of experiment conditions on the course of the hydrolysis-polymerization and the conversion of hydroxyl polynuclear aluminum species. According to the OH/Al mole ratio, the Al species can be divided into four regions quantitatively by three characteristic points on the titration curves: Part I, Al³⁺/Al_a region, consist chiefly of Al³⁺ and mononuclear Al; Part II, the small/middle polynuclear Al region, including Al₂-Al₁₂; Part III, the large-size polynuclear aluminum region, consistent with predominantly Al₁₃-Al₅₄ and a little sol/gel Al(OH)₃; Part IV, the dissolving region of sol/gel Al_c, only Al(OH)₃ (aq or am) and Al(OH)₄^- species, which set up a base to study on the hydrolysis-polymerization of Al³⁺. At the same time, significant effects of total aluminum concentration, temperature, halide ion, silicate radical, and organic acid radical on the titration curves and its critical points were observed. Given the three critical points which demarcating the aluminum forms, we carry out a through investigation into the fundamental regulations of these factors’ influence, and offer a fresh train of thought to study the hydrolysis-polymerization of Al³⁺ in soil solutions.

KEY WORDS: Potentiometric titration, Hydroxyl polynuclear aluminum species, Hydrolysis-polymerization, Critical point, Factors affecting titration curves

Bull. Chem. Soc. Ethiop. 2008, 22(2), 155-164.