pH4Phosphate buffer— Place 38mLof 0.2Mdibasic sodium phosphate in a 100-mLvolumetric flask.Dilute with 0.1Mcitric acid to volume,and mix.Adjust the pH,if necessary,with either the dibasic sodium phosphate solution or the citric acid solution.

pH5.6Phosphate buffer— Place 58mLof 0.2Mdibasic sodium phosphate in a 100-mLvolumetric flask.Dilute with 0.1Mcitric acid to volume,and mix.Adjust the pH,if necessary,with either the dibasic sodium phosphate solution or the citric acid solution.

Bromocresol green solution— Transfer 125mg of bromocresol green to a 25-mLvolumetric flask,dissolve in 3.5mLof 0.05Nsodium hydroxide,dilute with water to volume,and mix.

Standard preparation— Dissolve an accurately weighed quantity of USP Oxybutynin Chloride RSin 0.05Nsulfuric acid to obtain a solution having a known concentration of about 100µg per mL.

Assay preparation— Transfer an accurately measured volume of Oral Solution,equivalent to about 10mg of oxybutynin chloride,to a 100-mLvolumetric flask,dilute with water to volume,and mix.

Procedure— Separately transfer 10.0mLof the Standard preparationand the Assay preparationto separate 125-mLseparators.Add 20mLof pH4Phosphate bufferto each separator,and extract each solution with a 25-mLportion of chloroform.[NOTE—Allow at least 10minutes for the layers to separate.]Collect the chloroform extracts in respective 125-mLseparators,each containing a mixture of 2mLof pH5.6Phosphate bufferand 1mLof Bromocresol green solution.Shake the separators,and filter the chloroform extracts through rayon pledgets,collecting the extracts in respective 100-mLvolumetric flasks.Repeat the double extractions with 25-mLportions of chloroform.Wash the rayon pledgets with chloroform,collecting the washings in the respective 100-mLvolumetric flasks.Dilute both solutions with chloroform to volume,and mix.Concomitantly determine the absorbances of both solutions at the wavelength of maximum absorbance at about 415nm,with a suitable spectrophotometer,against a blank prepared using 10mLof 0.05Nsulfuric acid treated in the same manner as the Standard preparationand the Assay preparation.Calculate the quantity,in mg,of C22H31NO3·HCl in each mLof Oral Solution taken by the formula: in which Cis the concentration,in µg per mL,of USP Oxybutynin Chloride RSin the Standard preparation;Vis the volume,in mL,of Oral Solution taken;and AUand ASare the absorbances of the solutions from the Assay preparationand the Standard preparation,respectively.