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.

• A: Identification tests—general, lactate (191): Meets the requirements.

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)

• 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.
• 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.

• 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.

• Packaging and storage: Preserve in tight containers.
• Labeling: Label it to indicate whether it is levorotatory or racemic.

• United Stated Pharmacopeia - Lactic Acid