Standard preparation— Dissolve an accurately weighed quantity of USP Guaifenesin RS in chloroform to obtain a solution having a known concentration of about 40 µg per mL.

Assay preparation— Constitute a container of Guaifenesin for Injection with a volume of warm (30 to 35) water, accurately measured, corresponding to the volume of solvent specified in the labeling. Transfer an accurately measured volume of the constituted solution, equivalent to about 100 mg of guaifenesin, to a 250-mL separator containing 10 mL of a saturated solution of sodium bicarbonate. Extract with four 25-mL portions of chloroform, combining the chloroform extracts in a second 250-mL separator. Wash the combined chloroform extracts with 5 mL of 1 N hydrochloric acid. Filter the washed chloroform extracts through chloroform-moistened filter paper, collecting the filtrate in a 100-mL volumetric flask. Dilute with chloroform to volume, and mix. Transfer 4.0 mL of this solution to a second 100-mL volumetric flask, dilute with chloroform to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparation and the Assay preparation at the wavelength of maximum absorbance at about 276 nm, using chloroform to zero the instrument. Calculate the quantity, in mg, of guaifenesin (C10H14O4) in each mL of the constituted solution of Guaifenesin for Injection taken by the formula: in which C is the concentration, in µg per mL, of USP Guaifenesin RS in the Standard preparation, V is the volume, in mL, of constituted solution taken to prepare the Assay preparation, and AU and AS are the absorbances of the Assay preparation and the Standard preparation, respectively.