Determination of the Formation Constants of Aluminum(III) with Gallic Acid Studied by Spectrophotometry

Abstract

Complex formation (1 : 1) between Al(III) and gallic acid (H₄L) was studied by spectrophotometry at 25 °C and at ionic strength I = 0.1. At pH 4–5 buffered with acetate ions, the complex species can be analyzed by measuring the absorbance of the free ligand, which decreases along with the increase in Al(III) concentration. The method is based on the competition between gallic acid and buffer ligands for Al(III) ions, or between hydrogen ions and Al(III) ions for gallic acid. The concentration of the Al(III)-gallate complex formed at constant pH or at constant acetate (buffer) concentration decreased along with the increase in acetate ion or hydrogen ion concentration, respectively. The results showed that the complex species Al(H₂A) was formed but Al(H₃A) was not in the pH 4–5. By using the acid dissociation constants of gallic acid and the formation constant of Al(III)-acetate complex the intrinsic formation constant of Al(H₂A) was determined to be log K_Al(H2A) = 8.61 ± 0.02 which was close to the value of pK₂ (K₂ = the proton dissociation constant of H₃L), the protonation constant of the phenolate group of gallic acid.