Isoelectric points of some sulfonamides: determination by microelectrophoresis and by calculations involving acid-base strength.

The isoelectric points of four very slightly soluble sulfonamides were measured by microelectrophoresis of dilute suspensions as a function of pH. Ionic strength and pH were adjusted with KCl, KOH, and HCl only. The isoelectric points were also calculated from published values of acid and basic ionization constants which had been determined by potentiometric titration, and from changes in ultraviolet absorption spectra and in solubility as a function of pH. Including one sulfonamide whose isoelectric point as measured by microelectrophoresis was published, rather good agreement between the two methods was observed for all but one compound. All values were between 3.5 and 4.6, indicating that the sulfonamides function as weak acids rather than as amphoteric compounds at physiological pH.