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

This work provides the pKa at the biorelevant temperature of 37 oC 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 oC can be avoided too. In order to anchor the pKa values obtained through the IS-CE method in the pH scale, the acidity constant at 37 oC of some of the standards has been determined also by the reference potentiometric method. In general, a decrease in the pKa value is observed when changing from 25 to 37 oC, and the magnitude of the change depends on the nature of the compounds. Once the pKa values at 37 oC 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 pKa values. Due to the lack of reference data at 37 oC only the pKas of labetalol can be compared to values from literature, and a very good agreement is observed.