Dependence on ionic strength of stability constants, protonation and complexation of 2,X-dihydroxybenzoic acids(X=4, 5 or 6) with aluminium(III)

The complexation of aluminium(III) with 2,X(X = 4, 5, 6)-dihydrobenzoic acids (DHBA) has been investigated potentiometrically. The temperature was kept constant at 25 degrees C and the ionic strengths were kept (0.1, 0.3, 0.5, 0.7 and 0.9) mol/l in sodium chloride media. It was shown that aluminium(III) form a mononuclear Al(HL)(+) type 1:1 complex with 2,X(-DHBA), where L3- represents the fully dissociated 2,X(X = 4, 5, 6) anion at pH = 4 - 5. The stochiometries of the complexes were determined by continuous varitions method. The protonation constants of 2,X(-DHBA) (using the potentiometric technique) and the stability constant of the complex were determined at different ionic strengths. The dependence of ionic strength on the protonation and stability constants was evaluated using a simple Debye - Huckel type equation.