FOR ORAL SOLUTION PACKAGED IN SINGLE-UNIT CONTAINERS: meets the requirements.

FOR ORAL SOLUTION PACKAGED IN MULTIPLE-UNIT CONTAINERS: meets the requirements.

(Official January 1, 2007)

Chromotropic acid solution— Mix 100 mg of chromotropic acid with 50 mL of water in a 100-mL volumetric flask. Cool in an ice bath and, while cooling, cautiously and slowly add 50 mL of sulfuric acid, and mix. Allow the solution to reach room temperature, and add dilute sulfuric acid (1 in 2) to volume. [NOTE—If excessive heat generated during mixing causes a violet color to appear in the solution, discard the solution, and prepare another, taking precautions to avoid excessive heat.]

Standard preparation— Transfer about 50 mg of USP Methenamine RS, accurately weighed, to a 1000-mL volumetric flask, dissolve in and dilute with water to volume, and mix. Proceed as directed for Assay preparation, beginning with “Transfer a 2.0-mL portion of this stock solution to a 100-mL volumetric flask.” The concentration of USP Methenamine RS in the Standard preparation is about 1 µg per mL.

Assay preparation— Transfer an accurately measured volume of Oral Solution, equivalent to about 1.5 g of methenamine, to a 500-mL volumetric flask, dissolve in and dilute with water to volume, and mix. Transfer 2.0 mL of this solution to a 100-mL volumetric flask, dilute with water to volume, and mix to obtain the stock solution. Transfer a 2.0-mL portion of this stock solution to a 100-mL volumetric flask, add 25 mL of Chromotropic acid solution and 50 mL of dilute sulfuric acid (1 in 2), and mix. Transfer another 2.0-mL portion of the stock solution to a second 100-mL volumetric flask to provide a blank, add 75 mL of dilute sulfuric acid (1 in 2), and mix. Place the two 100-mL flasks in a boiling water bath for 30 minutes, accurately timed, then remove them from the bath, cool immediately to room temperature, add dilute sulfuric acid (1 in 2) to volume, and mix.

Procedure— Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance at about 570 nm, with a suitable spectrophotometer, using dilute sulfuric acid (1 in 2) to set the instrument. Calculate the quantity, in mg, of methenamine (C6H12N4) in each mL of the Oral Solution taken by the formula: in which C is the concentration, in µg per mL, of USP Methenamine RS in the Standard preparation; V is the volume, in mL, of Oral Solution taken; AU and AS are the absorbances of the Assay preparation and the Standard preparation, respectively; and BU and BS are the absorbances of the blanks from the Assay preparation and the Standard preparation, respectively.