(meth'' oh kar' ba mol).
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C11H15NO5 241.24

1,2-Propanediol, 3-(2-methoxyphenoxy)-, 1-carbamate, (±)-.
(±)-3-(o-Methoxyphenoxy)-1,2-propanediol 1-carbamate [532-03-6].
» Methocarbamol contains not less than 98.5 percent and not more than 101.5 percent of C11H15NO5, calculated on the dried basis.
Packaging and storage— Preserve in tight containers.
USP Reference standards 11
USP Guaifenesin RS Click to View Structure
USP Methocarbamol RS Click to View Structure
Solution: 40 µg per mL.
Medium: alcohol.
Loss on drying 731 Dry it at 60 for 2 hours: it loses not more than 0.5% of its weight.
Residue on ignition 281: not more than 0.1%.
Heavy metals, Method I 231 Dissolve 1.0 g in a mixture of 7 mL of methanol and 3 mL of 1 N acetic acid, and dilute with water to 25 mL. The limit is 0.002%.
Chromatographic purity—
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 ± .05.
Mobile phase— Prepare a suitably filtered and degassed solution of pH 4.5 Buffer solution and methanol (about 75:25) (see System suitability).
Guaifenesin solution— Transfer 20.0 mg of USP Guaifenesin RS to a 50-mL volumetric flask. Dissolve in and dilute with methanol to volume, and mix.
Standard solution— Transfer 20.0 mg of USP Methocarbamol RS, accurately weighed, to a 10-mL volumetric flask. Add 1.0 mL of Guaifenesin solution and 2.0 mL of methanol to dissolve the methocarbamol. Dilute with pH 4.5 Buffer solution to volume, and mix. Use this solution within 24 hours.
Test solution— Transfer about 100 mg of Methocarbamol, accurately weighed, to a 50-mL volumetric flask, add 13 mL of methanol to dissolve, dilute with pH 4.5 Buffer solution to volume, and mix. Use this solution within 24 hours.
Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 274-nm detector and a 4-mm × 25-cm column that contains packing L1. Adjust the operating conditions so that System suitability requirements are met.
System suitability— Chromatograph three replicate 20-µL portions of the Standard solution as directed for the Test solution under Procedure. The analytical system is suitable for use if the peak area percentage of guaifenesin is 2.4 ± 1.0, the relative standard deviation for the peak area percentage is not greater than 4.0%, and the resolution, R, between guaifenesin and methocarbamol is not less than 2.0.
Procedure— By means of a suitable microsyringe or sampling valve, inject about 20 µL of the Test solution into the chromatograph. Determine the peak areas of the methocarbamol peak and all extraneous peaks having a retention time greater than 0.5 of the retention time of methocarbamol. The relative retention times are about 0.8 for guaifenesin and 1.0 for methocarbamol. Calculate the percentage of related impurities taken by the formula:
100(2.4 / G)(PE / PT)
in which G is the area percentage of the guaifenesin peak in the chromatogram of the Standard solution determined under System suitability; PE is the peak area of all extraneous peaks; and PT is the total of the peak areas of all extraneous peaks and the methocarbamol. The limit is 2.0%.
Assay— Transfer about 100 mg of Methocarbamol, accurately weighed, to a 100-mL volumetric flask, add methanol to volume, and mix. Transfer 4.0 mL of this solution to a second 100-mL volumetric flask, dilute with methanol to volume, and mix. Concomitantly determine the absorbances of this solution and a Standard solution of USP Methocarbamol RS in methanol having a known concentration of about 40 µg per mL at the wavelength of maximum absorbance at about 274 nm in 1-cm cells, using methanol as the blank. Calculate the quantity, in mg, of C11H15NO5 in the portion of Methocarbamol taken by the formula:
2.5C(AU / AS)
in which C is the concentration, in µg per mL, of USP Methocarbamol RS in the Standard solution; and AU and AS are the absorbances of the Assay solution and the Standard solution, respectively.
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Monograph Hariram Ramanathan, M.S.
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(SM42010) Monographs - Small Molecules 4
Reference Standards RS Technical Services
USP35–NF30 Page 3851
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