Lactic Acid

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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Propanoic acid, 2-hydroxy-;

Lactic acid

CAS RN®: 50-21-5.

1 DEFINITION

Lactic Acid is a mixture of lactic acid (C₃H₆O₃) and lactic acid lactate (C₆H₁₀O₅), equivalent to a total of NLT 88.0% and NMT 92.0%, by weight, of lactic acid (C₃H₆O₃). It is obtained by the lactic fermentation of sugars or is prepared synthetically. Lactic Acid obtained by fermentation of sugars is levorotatory, whereas that prepared synthetically is racemic.

[Note—Lactic Acid prepared by fermentation becomes dextrorotatory on dilution, which hydrolyzes l-(−)-lactic acid lactate to l-(+)-lactic acid.]

2 IDENTIFICATION

A. Identification Tests—General, Lactate〈191〉: Meets the requirements

3 ASSAY

Procedure

Sample: 2.5 mL, accurately weighed

Titrimetric system

(See Titrimetry 〈541〉.)

Mode: Residual titration

Titrant: 1 N sodium hydroxide VS

Back-titrant: 1 N sulfuric acid VS

Endpoint detection: Visual

Analysis: Transfer the Sample to a tared 250-mL flask, add 50.0 mL of Titrant, and boil the mixture for 20 min. Add phenolphthalein TS, and titrate the excess alkali in the hot solution with Back-titrant. Perform a blank determination. Each mL of Titrant is equivalent to 90.08 mg of lactic acid (C₃H₆O₃).

Acceptance criteria: 88.0%–92.0% (w/w)

4 IMPURITIES

4.1 Chloride

Sample solution: 1 in 100

Analysis: To 10 mL of the Sample solution acidified with nitric acid add a few drops of silver nitrate TS.

Acceptance criteria: No opalescence is produced immediately.

4.2 Sulfate

Sample solution: 1 in 100

Analysis: To 10 mL of the Sample solution add 2 drops of hydrochloric acid and 1 mL of barium chloride TS.

Acceptance criteria: No turbidity is produced.

Residue on Ignition 〈281〉

Sample: 5 mL, accurately weighed

Acceptance criteria: NMT 3 mg (0.05%)

Limit of Citric, Oxalic, Phosphoric, or Tartaric Acid

Sample solution: 1 in 10

Analysis: To 10 mL of the Sample solution add 40 mL of calcium hydroxide TS, and boil for 2 min.

Acceptance criteria: No turbidity is produced.

5 SPECIFIC TESTS

Readily Carbonizable Substances

Sample: 5 mL

Analysis: Rinse a test tube with sulfuric acid, and allow to drain for 10 min. Add 5 mL of sulfuric acid to the test tube, carefully overlay it with the Sample, and maintain the tube at 15°.

Acceptance criteria: No dark color develops at the interface of the two acids within 15 min.

Optical Rotation, Angular Rotation〈781A〉: −0.05° to +0.05° for racemic Lactic Acid Sugars

Sample: 5 drops

Analysis: To 10 mL of hot alkaline cupric tartrate TS add the Sample.

Acceptance criteria: No red precipitate is formed.

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.

Labeling: Label it to indicate whether it is levorotatory or racemic.