Distribution of benzalkonium cation into mixed cationic/nonionic micelles: Verification of ion-selective electrode measurements by UV spectrophotometry

Coated-wire electrodes were fabricated to be selective for the cation portion of benzalkonium chloride (BAC), a preservative commonly used in ophthalmic pharmaceuticals. Measurements were conducted on formulations containing a nonionic alkyl ethoxylate surfactant phase into which benzalkonium could distribute, potentially reducing its effectiveness as a preservative. Distribution coefficients for benzalkonium into the micellar phase were measured independently with the use of UV spectroscopy and corroborated with electrode measurements. The positions of three peaks in the benzalkonium spectrum in the region 250 to 275 mn shifted by less than 1 nm in the limit of complete distribution. It was possible to determine the peak positions to within an accuracy of similar to 0.005 nm by using a deconvolution algorithm in GRAMS 386. A brief mathematical treatment shows good agreement between the free cation concentration as deduced from electrode and UV measurements.