Fe 3+ and Al 3+ hydrolysis equilibria. Cooperativity in Al 3+ hydrolysis reactions

Abstract

The hydrolysis equilibria of Fe 3+ and Al 3+ play important roles in geology and biology. While the four successive deprotonations from the aquo Fe 3+ are spaced at about two pH unit intervals, the four Al 3+ 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 Fe 3+, but undergoes a regular reduction for Al 3+ 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 Fe 2(OH) 2 4+is too weak to be detected in aqueous solutions, in agreement with recent results.