FE3+ AND AL3+ HYDROLYSIS EQUILIBRIA - COOPERATIVITY IN AL3+ HYDROLYSIS REACTIONS

The hydrolysis equilibria of Fe3+ and Al3+ play important roles in geology and biology. While the four successive deprotonations from the aquo Fe3+ are spaced at about two pH unit intervals, the four Al3+ deprotonations occur cooperatively and squeeze into an interval of less than one pH unit. This dramatic contrast is accounted for by proposing that throughout the deprotonations, the hexacoordinate aquo ion remains six-coordinate for Fe3+, but undergoes a regular reduction for Al3+ from six through five to four-coordinate in the aluminate ion, Al(OH)4-. An acidity constant comparison predicts that the aluminum dimer analogous to Fe2(OH)24+ is too weak to be detected in aqueous solutions, in agreement with recent results.