Test solution— Prepare as directed for the Assay preparation in the Assay.

Standard solution— Prepare as directed for the Standard preparation in the Assay.

Cell size: 0.5 mm.

Standard preparation— Transfer about 60 mg of USP Polydimethylsiloxane RS, accurately weighed, to a 25-mL volumetric flask, add 15 mL of hexanes, and sonicate for 3 minutes. Allow to cool, dilute with hexanes to volume, and mix. Transfer 10 mL of this solution to a capped test tube, add about 1 g of anhydrous sodium sulfate, mix for about 1 minute, and centrifuge. Use the clear supernatant as the Standard preparation. The Standard preparation has a concentration of about 2.4 mg of USP Polydimethylsiloxane RS per mL.

Assay preparation— Transfer an accurately measured quantity of Oral Suspension, equivalent to about 240 mg of simethicone, to a glass-stoppered centrifuge tube. Add 5 mL of methanol, and mix for about 15 seconds. Add 30.0 mL of hexanes, and mix for about 10 seconds. Loosen the stopper, and heat the tube for about 10 minutes in a water bath at 65 ± 1. Mix for 1 minute, and centrifuge. Using a glass syringe, transfer the upper hexanes layer to a 100-mL volumetric flask. Repeat the extraction with two 30.0-mL portions of hexanes, combining the hexanes extracts in the 100-mL volumetric flask. [note—If an emulsion forms during any of the extractions, as much as 2 mL of methanol may be added to disperse the emulsion.] Allow the combined extracts to cool, dilute with hexanes to volume, and mix. Transfer 10 mL of this solution to a capped test tube, add about 1 g of anhydrous sodium sulfate, mix for about 1 minute, and centrifuge. Use the clear supernatant as the Assay preparation.

Dry hexanes— Mix 100 mL of hexanes and 10 g of anhydrous sodium sulfate, allow to settle, and centrifuge. Use the clear supernatant.

Procedure— Concomitantly determine the absorbances of the Standard preparation and the Assay preparation at the wavelength of maximum absorption at about 7.9 µm, using Dry hexanes as the blank to set the instrument. Calculate the quantity, in mg, of [–(CH3)2SiO–]n in each mL of the Oral Suspension taken by the formula: in which C is the concentration, in mg per mL, of USP Polydimethylsiloxane RS in the Standard preparation; V is the volume, in mL, of Oral Suspension taken; and AU and AS are the absorbances of the Assay preparation and the Standard preparation, respectively.