A: Add 10 mL of 6 N acetic acid to a quantity of powdered Tablets, equivalent to about 100 mg of nitrofurantoin, boil for a few minutes, and filter while hot. Cool to room temperature, collect the precipitate of nitrofurantoin, and dry at 105 for 1 hour: the IR absorption spectrum of a mineral oil dispersion of the precipitate so obtained exhibits maxima only at the same wavelengths as that of a similar preparation of USP Nitrofurantoin RS.

B: It responds to Identification test B under Nitrofurantoin.

Medium: pH 7.2 phosphate buffer (see Buffer Solutions in the section Reagents, Indicators, and Solutions); 900 mL.

Apparatus 1: 100 rpm.

Times: 60 minutes, 120 minutes.

Standard preparation— Dissolve about 50 mg, accurately weighed, of USP Nitrofurantoin RS in 25 mL of dimethylformamide, dilute with Dissolution Medium to 500 mL, mix, and dilute a suitable aliquot of the resulting solution with Dissolution Medium to obtain a solution having a known concentration of about 10 µg per mL.

Procedure— Determine the amount of C8H6N4O5 dissolved from absorbances at the wavelength of maximum absorbance at about 375 nm of filtered portions of the solution under test, suitably diluted with Dissolution Medium, if necessary, using Dissolution Medium as the blank, in comparison with the Standard preparation.

Tolerances— Not less than 25% (Q) of the labeled amount of C8H6N4O5 is dissolved in 60 minutes, and not less than 85% (Q) of the labeled amount of C8H6N4O5 is dissolved in 120 minutes.

pH 7.0 phosphate buffer, Mobile phase, Internal standard solution, Standard preparation, and Chromatographic system— Proceed as directed in the Assay under Nitrofurantoin.

Assay preparation— Weigh and finely powder not less than 20 Tablets. Weigh accurately a portion of the powder, equivalent to about 50 mg of nitrofurantoin, into a glass-stoppered flask. Add 40.0 mL of dimethylformamide, and shake by mechanical means for 15 minutes. Add 50.0 mL of Internal standard solution, mix, and cool to room temperature. Pass a portion of the solution through a 0.45-µm pore size nylon filter, discarding the first few mL of the filtrate.

Procedure— Proceed as directed for Procedure in the Assay under Nitrofurantoin. Calculate the quantity, in mg, of C8H6N4O5 in the portion of the powder taken by the formula: in which W is the weight, in mg, of USP Nitrofurantoin RS in the Standard preparation and RU and RS are the ratios of the peak responses of the nitrofurantoin to the internal standard obtained from the Assay preparation and the Standard preparation, respectively.