Thermodynamic Dissociation Constants of Butorphanol and Zolpidem by the Least-Squares Nonlinear Regression of Multiwavelength Spectrophotometric pH-Titration Data

The mixed dissociation constants of two drugs—butorphanol and zolpidem—at temperatures of (25 and 37) °C were determined with the use of multiwavelength and multivariate treatments of spectral data using SPECFIT/32 and SQUAD(84) nonlinear regression. The factor analysis in the INDICES program predicts the correct number of components, that is, the number of dissociated and nondissociated forms of the molecules studied. The thermodynamic dissociation constant pKaT was estimated by nonlinear regression of {pKa, I} data at (25 and 37) °C: for butorphanol pKa,1T = 9.46(1) and 8.99(3) and pKa,2T = 9.64(2) and 9.34(3); for zolpidem pKa,1T = 6.33(3) and 6.14(1), where the standard deviation in last significant digits is in parentheses. The proposed procedure involves chemical model building, calculating the concentration profiles, and fitting the protonation constants of the chemical model to multiwavelength and multivariate data measured. If the proposed protonation model represents the data adequately, the res...