A: Transfer a volume of Injection, equivalent to about 500 mg of phenylbutazone, to a 250-mL conical flask, add 100 mL of solvent hexane, and heat the mixture under reflux for 15 minutes. Filter the hot mixture, and allow the filtrate to cool. Separate the crystals thus formed by filtration, and dry in vacuum at 80 for 30 minutes: the phenylbutazone so obtained responds to Identification test A under Phenylbutazone.

B: The retention time of the phenylbutazone peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation as obtained in the Assay.

Acetate buffer, Mobile phase, Internal standard solution, Standard preparation, and Chromatographic system— Proceed as directed in the Assay under Phenylbutazone.

Assay preparation— Transfer an accurately measured volume of Injection, equivalent to about 200 mg of phenylbutazone, to a 100-mL volumetric flask. Dilute with acetonitrile to volume, and mix. Transfer 7.0 mL of this solution to a 50-mL volumetric flask, add 10.0 mL of Internal standard solution, dilute with acetonitrile to volume, and mix. [note—Use this solution within 8 hours of its preparation.]

Procedure— Proceed as directed for Procedure in the Assay under Phenylbutazone. Calculate the quantity, in mg, of C19H20N2O2 in each mL of the Injection taken by the formula: in which C is the concentration, in mg per mL, of USP Phenylbutazone RS in the Standard preparation, V is the volume, in mL, of Injection taken to prepare the Assay preparation, and RU and RS are the ratios of the peak responses of phenylbutazone to that of the internal standard obtained from the Assay preparation and the Standard preparation, respectively.