Aluminum Complexation by Protocatechuic and Caffeic Acids as Determined by Ultraviolet Spectrophotometry

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Abstract
Ultraviolet (UV) spectrophotometry was used to study the complexation of Al by protocatechuic and caffeic acids at pH 3.8 to 4.5. The stoichiometry of the Al‐organic acid complex was found to be 1:1 through Job's method of continuous variation in the pH range studied. Increased pH and Al concentrations resulted in bathochromic shifts in the UV spectra of the organic acids due to the formation of an Al‐organic complex absorbing UV radiation differently than the free organic. Aluminum was concluded to be chelated by the ortho‐hydroxy group of the organic acids, as opposed to carboxyl‐group binding. The deprotonation of the carboxyl group in the free organic and Al‐organic complex had to be considered in the calculation of association constants. Association constants of 1015.33 and 1015.06 for protocatechuic and caffeic acids, respectively, were determined by Al titrations. Calculated absorbance as a function of pH agreed well with experimental titrations. Application of the determined association constants to a simplified model of a forest soil solution is presented.
Funding Information
  • National Science Foundation (EAR‐8512226)