Determination of thermodynamic pKa values of benzimidazole and benzimidazole derivatives by capillary electrophoresis

Thermodynamic pK_a values for benzimidazole and several substituted benzimidazoles were determined by CE. Electrophoretic mobilities of benzimidazoles were determined by CE at different pH levels and ionic strengths. The dependence of electrophoretic mobilities on pH was used to obtain pK_a values at different ionic strengths. Extrapolations to zero ionic strength were used to determine the thermodynamic pK_a values. Using this method the thermodynamic pK_a values of 15 benzimidazoles were determined and found to range from 4.48 to 7.38. Results from the CE measurements were compared with spectrophotometric measurements which were evaluated at wavelengths where the highest absorbance difference for varying pH was recorded. The two analytical techniques were in good agreement.