Aluminum-Tannin Equilibria: A Potentiometric Study

Abstract

The equilibrium reactions between aluminium(III) and an epicatechin polymer or tannin (T) have been studied by potentiometric titration at 25�C, I = 0.001 M. The tannin, which has on average ten 1,2-dihydroxybenzene units per molecule, encapsulates the aluminium ion to form a bis(l,2-diphenolato) complex at pH 5-7. Stability constants are reported for Al(TH-2) (logK11-2, -3.95 � 0.05), Al(TH-3) (logK11-3, -4.5 � 0.1) and Al(TH-4) (logKl1-4, -5.7 � 0.2). A small chelate effect arising from coordination of remote pairs of 1,2-diphenolato groups was observed.