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

Abstract

This work provides the pK_a at the biorelevant temperature of 37 °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 °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 °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 °C, and the magnitude of the change depends on the nature of the compounds. Once the pK_a values at 37 °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 °C only the pK_a values of labetalol can be compared to values from the literature, and a very good agreement is observed.