Determination of the Formation Constants of Aluminum (III) with Gallic Acid Studied by Spectrophotometry
Masahiko KURIHARA*, Yuko HAMABE, Mika IKEDA†, Hidenori SHINKAWA†† and Emiko OHYOSHI
Department of Bioengineering, Yatsushiro National College of Technology; 2627 Hirayama-shinmachi, Yatsushiro-shi 866-8501 Japan
† A & T Corporation; 2-24-27 Sekido, Tama-shi 206-0011 Japan
†† Iwatani Industrial Gases Corp.; 5-14-5 Nishinakajima, Yodogawa-ku, Osaka-shi 532-0011 Japan
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 incease 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 KAl (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.
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