Limitations of the Phenolphthalein Competition Method for Estimating Cyclodextrin Binding Constants

The phenolphthalein competitive spectrophotometric technique was used to obtain binding constants for cyclodextrin complexes for a variety of both inclusates and cyclodextrins . The former included alkane-1-sulfonates (C5-C12), alkyl sulfates (C6, C8, C12) and several heterocyclic ligands, and the latter included β-cyclodextrin, dimethyl-β-cyclodextrin and γ- cyclodextrin . The technique was not successful for either α- cyclodextrin or trimethyl -β- cyclodextrin inclusion complexes, or with many primary amines and quaternary ammonium cationic surfactants. However, for many colourless inclusates this method is a simple, convenient, reliable route to cyclodextrin binding constants.