Standard preparation— Dissolve an accurately weighed quantity of USP Benoxinate Hydrochloride RSin 0.1Nhydrochloric acid to obtain a solution having a known concentration of about 400µg per mL.

Assay preparation— Transfer a volume of Ophthalmic Solution,equivalent to about 20mg of benoxinate hydrochloride,to a separator containing 15mLof water,add 1mLof ammonium hydroxide,and extract with five 20-mLportions of ether.Wash the combined ether extracts with 10mLof water,extract the water washing with 10mLof ether,and add this ether extract to the main extract.Extract the ether solution with three 5-mLportions of 0.1Nhydrochloric acid,collect the acid extracts in a 50-mLvolumetric flask,dilute with 0.1Nhydrochloric acid to volume,and mix.

Procedure— Transfer 5.0mLeach of the Standard preparation,the Assay preparation,and 0.1Nhydrochloric acid to provide a blank,to separate 200-mLvolumetric flasks.Dilute the contents of each flask with water to volume,and mix.Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance at about 308nm,with a suitable spectrophotometer,using the blank to set the instrument.Calculate the quantity,in mg,of C17H28N2O3·HCl in each mLof the Ophthalmic Solution taken by the formula: in which Cis the concentration,in µg per mL,of USP Benoxinate Hydrochloride RSin the Standard preparation;Vis the volume,in mL,of Ophthalmic Solution taken;and AUand ASare the absorbances of the solutions from the Assay preparationand the Standard preparation,respectively.