Determination of the Formation Constant of the Copper Chelate of β-Cyclodextrin by Spectrophotometric Titration with Ethylenediaminetetraacetic Acid Using Window Factor Analysis

Abstract

Copper(II) is known to form a 2:1 complex with β-cyclodextrin in alkaline solution (pH ≥ 12), but its formation constant has not been determined. The present investigation involves the determination of the formation constant of the complex by spectrophotometric titration with ethylenediaminetetraacetic acid (EDTA) using window factor analysis (WFA). WFA is a self-modeling chemometric technique that extracts the concentration profiles of individual species from evolutionary processes such as complexometric titration. The visible absorption spectra of eleven aqueous solutions, at pH 12, containing fixed amounts of copper and cyclodextrin and different amounts of EDTA, were recorded. Principal factor analysis (PFA) of the absorbances gave evidence that three components were responsible for the spectral data. The concentration profiles of the components formed were extracted by WFA. Applying stoichiometric balance to the concentration profiles, the titration complex was determined to be (Cu₂HO₂CD·EDTA)⁻⁷, and the logarithm of its formation constant was calculated to be 8.51 ± 0.21. Using this information and the solubility product constant of Cu(OH)₂ as well as the formation constant of (CuOH·EDTA)⁻³, the logarithm of formation constant of (Cu₂HO₂CD)⁻³ was detemined to be 35.78 ± 0.38.