Cephalexin Capsules
Cephalexin Capsules contain the equivalent of NLT 90.0% and NMT 120.0% of the labeled amount of cephalexin (C16H17N3O4S).
•  The retention time of the major peak of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.
•  Procedure
Mobile phase:  0.985 g/L of sodium 1-pentanesulfonate in a mixture of acetonitrile, methanol, triethylamine, and water (20:10:3:170), adjusted with phosphoric acid to a pH of 3.0 ± 0.1
Standard stock solution:  1 mg/mL of USP Cephalexin RS in water
Standard solution:  0.4 mg/mL of cephalexin in Mobile phase from Standard stock solution
Sample stock solution:  Equivalent to 1 mg/mL of cephalexin from combined contents of NLT 20 Capsules in water. Sonicate, if necessary, to dissolve the cephalexin. Filter, if necessary, to obtain a clear solution.
Sample solution:  0.4 mg/mL of cephalexin in Mobile phase from Sample stock solution
Chromatographic system 
Mode:  LC
Detector:  UV 254 nm
Column:  4.6-mm × 25-cm; packing L1 of low acidity
Flow rate:  1.5 mL/min
Injection size:  20 µL
System suitability 
Sample:  Standard solution
Suitability requirements 
Relative standard deviation:  NMT 2.0%
Samples:  Standard solution and Sample solution
Calculate the percentage of cephalexin (C16H17N3O4S) in the portion of Capsules taken:
Result = (rU/rS) × (CS/CU) × P × F × 100
rU== peak response from the Sample solution
rS== peak response from the Standard solution
CS== concentration of USP Cephalexin RS in the Standard solution (mg/mL)
CU== nominal concentration of cephalexin in the Sample solution (mg/mL)
P== potency of cephalexin in USP Cephalexin RS (µg/mg)
F== conversion factor, 0.001 mg/µg
Acceptance criteria:  90.0%–120.0%
•  Dissolution 711
Medium:  Water; 900 mL
Apparatus 1:  100 rpm
Time:  30 min
Standard solution:  20 µg/mL of USP Cephalexin RS in Medium
Sample solution:  Pass a portion of the solution under test through a suitable filter. Dilute with Medium, if necessary, to a concentration of about 20 µg/mL.
Spectrometric conditions 
Mode:  UV
Analytical wavelength:  262 nm
Samples:  Standard solution and Sample solution
Tolerances:  NLT 80% (Q) of the labeled amount of cephalexin (C16H17N3O4S) is dissolved.
•  Uniformity of Dosage Units 905: Meet the requirements
•  Packaging and Storage: Preserve in tight containers.
•  USP Reference Standards 11
USP Cephalexin RS Click to View Structure
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Ahalya Wise, M.S.
Senior Scientific Liaison
(SM12010) Monographs - Small Molecules 1
711 Margareth R.C. Marques, Ph.D.
Senior Scientific Liaison
(GCDF2010) General Chapters - Dosage Forms
Reference Standards RS Technical Services
USP35–NF30 Page 2586
Pharmacopeial Forum: Volume No. 36(1) Page 44