Malic Acid
(mal' ik as' id).
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C4H6O5 134.09
Hydroxybutanedioic acid, (±)-;    
(±)-Malic acid;    
(±)-Hydroxysuccinic acid     [617-48-1].
DEFINITION
Malic Acid contains NLT 99.0% and NMT 100.5% of C4H6O5.
IDENTIFICATION
•  A. Infrared Absorption 197K: On undried sample
ASSAY
•  Procedure
Sample:  2 g
Titrimetric system 
Mode:  Direct titration
Titrant:  1 N sodium hydroxide VS
Endpoint detection:  Visual
Analysis:  Dissolve the Sample in 40 mL of recently boiled and cooled water. Add phenolphthalein TS, and titrate to the first appearance of a faint pink color that persists for NLT 30 s. Perform a blank determination.
Calculate the percentage of malic acid (C4H6O5) in the Sample taken:
Result = [(V B) × N × F × 100]/W
V== volume of Titrant consumed by the Sample (mL)
B== volume of Titrant consumed by the Blank (mL)
N== actual normality of the Titrant (mEq/mL)
F== equivalency factor, 67.04 mg/mEq
W== weight of the Sample (mg)
Acceptance criteria:  99.0%–100.5%
IMPURITIES
•  Residue on Ignition 281: NMT 0.1%
•  Heavy Metals, Method II 231: NMT 20 ppm
•  Fumaric and Maleic Acids
Mobile phase:  0.01 N sulfuric acid
System suitability solution:  1 mg/mL of Malic Acid, 10 µg/mL of USP Fumaric Acid RS, and 4 µg/mL of USP Maleic Acid RS in Mobile phase
Standard solution:  5.0 µg/mL of USP Fumaric Acid RS and 2.0 µg/mL of USP Maleic Acid RS in Mobile phase
Sample solution:  1.0 mg/mL of Malic Acid in Mobile phase
Chromatographic system 
Mode:  LC
Detector:  UV 210 nm
Column:  6.5-mm × 30-cm; packing L17
Column temperature:  37 ± 1
Flow rate:  0.6 mL/min
Injection size:  20 µL
System suitability 
Sample:  System suitability solution
[Note—The relative retention times for maleic acid, malic acid, and fumaric acid are 0.6, 1.0, and 1.5, respectively. ]
Suitability requirements 
Resolution:  NLT 2.5 for the maleic acid and malic acid peaks; NLT 7.0 for the malic acid and fumaric acid peaks
Relative standard deviation:  NMT 2.0% for the maleic acid peak
Analysis 
Samples:  Standard solution and Sample solution
Calculate the percentage of maleic acid and of fumaric acid in the portion of Malic Acid taken:
Result = (rU/rS) × (CS/CU) × 100
rU== peak response of maleic acid or fumaric acid from the Sample solution
rS== peak response of maleic acid or fumaric acid from the Standard solution
CS== concentration of USP Maleic Acid RS or USP Fumaric Acid RS in the Standard solution (mg/mL)
CU== concentration of Malic Acid in the Sample solution (mg/mL)
Acceptance criteria 
Fumaric acid:  NMT 1.0%
Maleic acid:  NMT 0.05%
•  Water-Insoluble Substances
Sample:  25 g
Analysis:  Dissolve the Sample in 100 mL of water, filter the solution through a tared filtering crucible, wash the filter with hot water, and dry at 100 to constant weight.
Acceptance criteria:  The increase in weight is NMT 25 mg (0.1%).
ADDITIONAL REQUIREMENTS
•  Packaging and Storage: Preserve in well-closed containers.
•  USP Reference Standards 11
USP Fumaric Acid RS Click to View Structure
USP Maleic Acid RS Click to View Structure
USP Malic Acid RS Click to View Structure
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Robert H. Lafaver, M.S.
Scientific Liaison
1-301-816-8335
(EXC2010) Monographs - Excipients
Reference Standards RS Technical Services
1-301-816-8129
rstech@usp.org
USP35–NF30 Page 1851