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 25ºC, 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) (logK_11-2, -3.95 ± 0.05), Al(TH_-3) (logK_11-3, -4.5 ± 0.1) and Al(TH_-4) (logK_l1-4, -5.7 ± 0.2). A small chelate effect arising from coordination of remote pairs of 1,2-diphenolato groups was observed.