Procedure for content uniformity— Dissolve the contents of 1 container in water to provide a solution containing about 20 µg of phentolamine mesylate per mL. Concomitantly determine the absorbances of this solution and of a solution of USP Phentolamine Mesylate RS, in the same medium, at a concentration of about 20 µg per mL, in 1-cm cells at the wavelength of maximum absorbance at about 278 nm, with a suitable spectrophotometer, using water as the blank. Calculate the quantity, in mg, of C17H19N3O·CH4O3S in the Phentolamine Mesylate for Injection taken by the formula: in which T is the labeled quantity, in mg, of phentolamine mesylate in the Phentolamine Mesylate for Injection, D is the concentration, in µg per mL, of phentolamine mesylate in the solution from the Phentolamine Mesylate for Injection, based on the labeled quantity per container and the extent of dilution, C is the concentration, in µg per mL, of USP Phentolamine Mesylate RS in the Standard solution, and AU and AS are the absorbances of the solution from the Phentolamine Mesylate for Injection and the Standard solution, respectively.

(Official January 1, 2007)

Standard preparation— Transfer about 25 mg of USP Phentolamine Mesylate RS, accurately weighed, to a 50-mL volumetric flask, add water to volume, and mix.

Assay preparation— Dissolve the contents of 10 containers of Phentolamine Mesylate for Injection in a volume of water corresponding to the volume of solvent specified in the labeling. Transfer an aliquot, equivalent to about 25 mg of phentolamine mesylate, to a 50-mL volumetric flask, add water to volume, and mix.

Procedure— Pipet 5-mL portions, respectively, of the Standard preparation, Assay preparation, and water to provide a blank, into separate 125-mL separators. Into each separator pipet 5-mL portions of 0.1 N hydrochloric acid and saturated picric acid solution. Extract with three 25-mL portions of chloroform, filtering each portion through chloroform-washed cotton into a 100-mL volumetric flask. Dilute with chloroform to volume, and mix. Concomitantly determine the absorbances of the solutions from the Assay preparation and the Standard preparation in 1-cm cells at the wavelength of maximum absorbance at about 410 nm, with a suitable spectrophotometer, against the blank. Calculate the quantity, in mg, of C17H19N3O·CH4O3S in the aliquot of Phentolamine Mesylate for Injection taken by the formula: in which C is the concentration, in mg per mL, of USP Phentolamine Mesylate RS in the Standard preparation, and AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively.