Phenylalanine
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
C9H11NO2 165.19
l-Phenylalanine CAS RN®: 63-91-2; UNII: 47E5O17Y3R.
1 DEFINITION
Phenylalanine contains NLT 98.5% and NMT 101.5% of l-phenylalanine (C9H11NO2), calculated on the dried basis.
2 IDENTIFICATION
A. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197K
3 ASSAY
Procedure
Sample: 160 mg of Phenylalanine
Blank: Mix 3 mL of formic acid and 50 mL of glacial acetic acid.
Titrimetric system
(See Titrimetry 〈541〉.)
Mode: Direct titration
Titrant: 0.1 N perchloric acid VS
Endpoint detection: Potentiometric
Analysis: Dissolve the Sample in 3 mL of formic acid and 50 mL of glacial acetic acid. Titrate with the Titrant. Perform the Blank determination.
Calculate the percentage of phenylalanine (C9H11NO2) in the Sample taken:
Result = {[(VS − VB) × N × F]/W} × 100
VS = Titrant volume consumed by the Sample (mL)
VB = Titrant volume consumed by the Blank (mL)
N = actual normality of the Titrant (mEq/mL)
F = equivalency factor, 165.2 mg/mEq
W = Sample weight (mg)
Acceptance criteria: 98.5%–101.5% on the dried basis
4 IMPURITIES
Residue on Ignition 〈281〉: NMT 0.4%
Chloride and Sulfate, Chloride〈221〉
Standard solution: 0.50 mL of 0.020 N hydrochloric acid
Sample: 0.73 g of Phenylalanine
Acceptance criteria: NMT 0.05%
Chloride and Sulfate, Sulfate〈221〉
Standard solution: 0.10 mL of 0.020 N sulfuric acid
Sample: 0.33 g of Phenylalanine
Acceptance criteria: NMT 0.03%
Change to read:
Iron 〈241〉, Procedures, Procedure 1 (CN 1-Jun-2023) : NMT 30 ppm
Related Compounds
Diluent: Glacial acetic acid and water (1:1)
Standard solution: 0.05 mg/mL of USP l-Phenylalanine RS in Diluent. [Note—This solution has a concentration equivalent to 0.5% of that of the Sample solution.]
System suitability solution: 0.4 mg/mL each of USP l-Phenylalanine RS and USP l-Tyrosine RS in Diluent
Sample solution: 10 mg/mL of Phenylalanine in Diluent
Chromatographic system
(See Chromatography 〈621〉, Thin-Layer Chromatography.)
Mode: TLC
Adsorbent: 0.25-mm layer of chromatographic silica gel mixture
Application volume: 5 μL
Developing solvent system: Butyl alcohol, glacial acetic acid, and water (3:1:1)
Spray reagent: 2 mg/mL of ninhydrin in a mixture of butyl alcohol and 2 N acetic acid (95:5)
System suitability
Suitability requirements: The chromatogram of the System suitability solution exhibits two clearly separated spots.
Analysis
Samples: Standard solution, System suitability solution, and Sample solution
After air-drying the plate, spray with Spray reagent, and heat between 100° and 105° for 15 min. Examine the plate under white light.
Acceptance criteria: Any secondary spot of the Sample solution is not larger or more intense than the principal spot of the Standard solution.
Individual impurities: NMT 0.5%
Total impurities: NMT 2.0%
5 SPECIFIC TESTS
Optical Rotation, Specific Rotation〈781S〉
Sample solution: 20 mg/mL in water
Acceptance criteria: −32.7° to −34.7°
pH 〈791〉
Sample solution: 10 mg/mL solution
Acceptance criteria: 5.4–6.0
Loss on Drying 〈731〉: Dry a sample at 105° for 3 h: it loses NMT 0.3% of its weight.
6 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in well-closed containers.
USP Reference Standards 〈11〉
USP l-Phenylalanine RS
USP l-Tyrosine RS
