Spectrophotometric determination of conditional acidity constant as a function of β-cyclodextrin concentration for some organic acids using rank annihilation factor analysis

Rank annihilation factor analysis (RAFA) was used to the spectrophotometric studies of acid–base equilibria in the presence of β-cyclodextrin (β-CD). When the conditional acidity constants (β-CD concentration dependent acidity constant) acts as an optimizing object, and simply combined with the pure spectrum of acidic and basic forms, the rank of original data matrix can be reduced. The residual standard deviation (R.S.D.) of the residual matrix after bilinearization of the background matrix is regarded as the valuation function. The performance of the method has been evaluated by using synthetic data. Spectrophotometric studies of neutral red, 4-nitrophenol and 4-nitrocatechol in the presence of varying amounts of β-CD are used as experimental model systems and the amounts of conditional acidity constants and formation constants of the inclusion complexes of β-CD with acidic and basic forms are calculated. The conditional acidity constants increased by increasing β-CD concentration for all three investigated acids. The calculated stability constants show that 4-nitrophenolate, 4-nitrocatecholate anions and basic form of neutral red (neutral form) form more stable inclusion complexes with β-CD than their acidic forms.