Multiwavelength spectrophotometric resolution of the micro-equilibria of cetirizine.

The acid-base properties of the antihistamine (H1 receptor antagonist) cetirizine have been studied by using a previously developed multiwavelength spectrophotometric titration method. A new computational procedure called "Two-Step-Divide-and-Conquer" (TSDC), which applied evolving factor analysis (EFA) and target factor analysis (TFA), has been derived to unravel the micro-equilibria of the triprotic zwitterionic compound from the spectral data. We have demonstrated that a single spectrophotometric titration experiment is sufficient to determine the 12 unknown microconstants and the distribution of microspecies, which are in good agreement with literature values, where available.