Chromatographic column— Mix 4g of chromatographic siliceous earth with 4mLof 0.1Nhydrochloric acid in a glass mortar until the mixture is fluffy.Transfer to a 19-×250-mm chromatographic tube (see Chromatography á621ñ)that contains a pledget of glass wool and is fitted with a stopcock at the bottom.Tamp gently to compress to a uniform mass.

Standard preparation— Transfer about 25mg of USP Idoxuridine RS,accurately weighed,to a 50-mLvolumetric flask,add methanol to volume,and mix.Dilute 5.0mLof this solution with a mixture of 1volume of butyl alcohol and 5volumes of chloroform to 100.0mL,and mix.

Assay preparation— Mix 4g of chromatographic siliceous earth with 2mLof 0.1Nhydrochloric acid in a glass mortar until the mixture is fluffy.Add a quantity of Ophthalmic Ointment,equivalent to about 5mg of idoxuridine and accurately weighed,to the mixture,and mix.

Procedure— Transfer the Assay preparationto the prepared Chromatographic column.Transfer 2g of chromatographic siliceous earth and 2mLof 0.1Nhydrochloric acid to the glass mortar,and mix until fluffy,using this material to rinse the mortar and pick up any remaining Ophthalmic Ointment.Transfer about half of this mixture to the tube,and tamp gently until the column appears uniform.Transfer the remaining portion to the Chromatographic column,and tamp as before.Wipe the walls of the mortar with a small pledget of glass wool,and insert the pledget in the top of the column.Pass 50mLof chloroform through the column at a flow rate of approximately 1mLper minute,and discard the chloroform.Elute with about 200mLof a mixture of 1volume of butyl alcohol and 5volumes of chloroform at the same flow rate,discarding the first 20mLof the eluate.Collect the remainder of the eluate in a 200-mLvolumetric flask,dilute with the eluting solvent to volume,and mix.Concomitantly determine the absorbances of this solution and the Standard preparationin 1-cm cells at 320nm and at the wavelength of maximum absorbance at about 283nm,with a suitable spectrophotometer,using a mixture of butyl alcohol and chloroform as the blank.Calculate the quantity,in mg,of C9H11IN2O5in the Ophthalmic Ointment taken by the formula: in which Cis the concentration,in µg per mL,of USP Idoxuridine RSin the Standard preparation;and the parenthetic expressions are the differences in the absorbances of the two solutions at the wavelengths indicated by the subscripts,for the solution from the Ophthalmic Ointment (U)and the Standard preparation(S),respectively.