Packaging and storage—
Preserve in single-dose containers, preferably of Type I glass.
Identification—
Mix a volume of Injection, equivalent to about 500 mg of methocarbamol, with 40 mL of water in a small separator. Extract with 10 mL of ethyl acetate, and dry the ethyl acetate layer over anhydrous sodium sulfate. Evaporate the ethyl acetate with the use of a water bath maintained at 40
under a stream of nitrogen to dryness: the residue so obtained meets the requirements for
Identification test
A under
Methocarbamol.
Aldehydes—
Transfer to a 25-mL volumetric flask an accurately measured volume of Injection, equivalent to 400 mg of methocarbamol, add 2.0 mL of a filtered 1 in 100 solution of phenylhydrazine hydrochloride in dilute alcohol (1 in 5), and allow to stand for 10 minutes. Add 1 mL of potassium ferricyanide solution (1 in 100), and allow to stand for 5 minutes. Add 4 mL of hydrochloric acid, dilute with alcohol to volume, and mix. Determine the absorbance of this solution in a 1-cm cell at the wavelength of maximum absorbance at about 515 nm, with a suitable spectrophotometer, using a reagent blank to set the instrument: the absorbance is not greater than that produced by 4 mL of formaldehyde solution (1 in 100,000), treated in the same manner as the portion of Injection taken and similarly measured, corresponding to not more than 0.01%, as formaldehyde, based upon the content of C11H15NO5 as determined in the Assay.
Other requirements—
It meets the requirements under
Injections 
1
.
Assay—
pH 4.5 Buffer solution—
Dissolve 6.8 g of monobasic potassium phosphate in 1000 mL of water. Adjust with 18 N phosphoric acid or 10 N potassium hydroxide to a pH of 4.5 ± 0.05.
Mobile phase—
Prepare a suitable filtered and degassed solution of
pH 4.5 Buffer solution and methanol (about 70:30) (see
System Suitability under
Chromatography 621).
Standard preparation—
Dissolve an accurately weighed quantity of
USP Methocarbamol RS in
Mobile phase to obtain a solution having a known concentration of about 1 mg per mL.
Assay preparation—
Transfer an accurately measured volume of Injection, equivalent to 100 mg of methocarbamol, to a 100-mL volumetric flask. Dilute with Mobile phase to volume, and mix.
Chromatographic system
(see
Chromatography 621)—The liquid chromatograph is equipped with a 274-nm detector and a 4.6-mm × 10-cm column that contains 3- or 5-µm packing L1, operated at 30
. The flow rate is 1 mL per minute. Chromatograph the
Standard preparation, and record the peak responses as directed for
Procedure: the relative standard deviation for replicate injections is not more than 2.0%.
Procedure—
Separately inject equal volumes (about 20 µL) of the
Standard preparation and the
Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of methocarbamol (C
11H
15NO
5) in each mL of the Injection taken by the formula:
(100C/V)(rU / rS),
in which
C is the concentration, in mg per mL, of USP Methocarbamol RS in the
Standard preparation;
V is the volume, in mL, of Injection taken; and
rU and
rS are the peak responses of methocarbamol obtained from the
Assay preparation and the Standard preparation,
respectively.
