Determination of acidity constants at 37 °C through the internal standard capillary electrophoresis (IS-CE) method: internal standards and application to polyprotic drugs

This work provides the pK(a)at the biorelevant temperature of 37 degrees C for a set of compounds proposed as internal standards for the internal standard capillary electrophoresis (IS-CE) method. This is a high throughput method that allows the determination of the acidity constants of compounds in a short time, avoiding the exact measurement of the pH of the buffers used. pH electrode calibration at 37 degrees C can be avoided too. In order to anchor the pK(a)values obtained through the IS-CE method in the pH scale, the acidity constant at 37 degrees C of some of the standards has been determined also by the reference potentiometric method. In general, a decrease in the pK(a)value is observed when changing the temperature from 25 to 37 degrees C, and the magnitude of the change depends on the nature of the compounds. Once the pK(a)values at 37 degrees C of the internal standards have been established, the method is applied to the determination of the acidity constants of seven polyprotic (5 diprotic and 2 triprotic) drugs. The obtained mobility-pH profiles show well-defined curves, and the fits provide precise pK(a)values. Due to the lack of reference data at 37 degrees C only the pK(a)values of labetalol can be compared to values from the literature, and a very good agreement is observed.