THE DETERMINATION OF BENZALKONIUM CHLORIDE IN EYE-DROPS BY DIFFERENCESPECTROPHOTOMETRY

A direct, extraction-free spectrophotometric method was developed for the determination of benzalkonium chloride (BAC) in various eye-drops. The procedure is based on ion-pair formation between BAC and 2',4',5' ,7'-tetrabromofluorescein (eosin-Y) which decreases the absorbance and induces a bathochromic shift of the maximum in the eosin-Y spectrum. The effects of pH, excess of reagent and ionic strength on the ion-pai r formation have been studied in detail. At pH 4.40 and 9.62, the work ing curve is linear in the 1.98 x 10(-6) to 2.40 x 10(-5) M (0.7-8.5 m u g cm(-3)) concentration range; however, the sensitivity drops to abo ut one third in the basic solution. At pH 4.40, the analytical signal is stable for more than 60 min, while at pH 9.62 the signal changes in time and reaches the maximum value 3 min after mixing the reagent and the sample. When the active substance is beta-5-isopropyl-2'deoxyurid ine and the sample contains typical additives, the reproducibility of the analytical signal at pH 4.40 is R.S.D. = 2.36% (n = 81). In the ca se of such samples, the linearity of the method is somewhat dependent on the composition, but generally acceptable at the 50-150% concentrat ion levels. Eye-drops containing tobramycin, an aminoglycoside-type an tibiotic, as the active substance were analyzed at pH 9.62. This was n ecessary to avoid strong interference from the analyte in acidic solut ion. In this case the linearity of the method is limited to a narrower concentration range however, the recovery is still acceptable at the 100% level. (C) 1998 Elsevier Science B.V.