Standard preparation— Dissolve with agitation an accurately weighed quantity of USP Hydroxypropyl Cellulose RS in water to obtain a solution having a known concentration of about 0.05 mg per mL.

Assay preparation— Transfer a sufficient number of Ocular Systems, to provide about 25 mg of hydroxypropyl cellulose, to a 500-mL volumetric flask, add about 250 mL of water, and dissolve with agitation on a mechanical shaker. Dilute with water to volume, and mix.

Procedure— Separately pipet 2 mL of the Standard preparation, the Assay preparation, and water, to provide a blank, into individual 50-mL centrifuge tubes. Add to each tube 6.0 mL of a 1 in 2000 solution of anthrone in sulfuric acid, and mix on a vortex mixer. After 40 minutes, again mix, and concomitantly determine the absorbances of the solutions obtained from the Standard preparation and the Assay preparation at 620 nm, with a suitable spectrophotometer, against the solution from the blank. Calculate the quantity, in mg, of hydroxypropyl cellulose in each Ocular System taken by the formula: in which C is the concentration, in mg per mL, of USP Hydroxypropyl Cellulose RS in the Standard preparation; N is the number of Ocular Systems taken for the Assay; and AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively.