Phosphoric Acid
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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)
H3PO4 98.00
Phosphoric acid CAS RN®: 7664-38-2.
1 DEFINITION
Phosphoric Acid contains NLT 85.0% and NMT 88.0%, by weight, of H3PO4 .
[Caution—Avoid contact, because Phosphoric Acid rapidly destroys tissues.]
2 IDENTIFICATION
A. Identification Tests—General, Phosphate〈191〉: When carefully neutralized with 1 N sodium hydroxide, phenolphthalein TS being used as the indicator, it meets the requirements.
3 ASSAY
Procedure
Sample: 1 g
Blank: 120 mL of water
Titrimetric system
(See Titrimetry 〈541〉.)
Mode: Direct titration
Titrant: 1 N sodium hydroxide VS
Endpoint detection: Visual
Analysis: Place the Sample in a tared, glass-stoppered flask, and dilute it with water to 120 mL. Add 0.5 mL of thymolphthalein TS. Titrate with 1 N sodium hydroxide VS to the first appearance of a blue color. Perform a blank determination.
Calculate the percentage of phosphoric acid (H3PO4) in the Sample taken:
Result = {[(VS − VB ) × N × F]/W} × 100
VS = volume of Titrant consumed by the Sample (mL)
VB = volume of Titrant consumed by the Blank (mL)
N = actual normality of the Titrant (mEq/mL)
F = equivalency factor, 49.00 mg/mEq
W = weight of the Sample (mg)
Acceptance criteria: 85.0%–88.0% by weight
4 IMPURITIES
Limit of Nitrate
Sample solution: Dilute 6 mL of Phosphoric Acid with 14 mL of water.
Analysis: Mix 5 mL of the Sample solution with about 0.1 mL of indigo carmine TS, then add 5 mL of sulfuric acid.
Acceptance criteria: The blue color is not discharged within 1 min.
5 SPECIFIC TESTS
Chloride and Sulfate, Sulfate〈221〉
Sample solution: Dilute 6 mL of Phosphoric Acid with 90 mL of water.
Analysis: To the Sample solution add 1 mL of barium chloride TS.
Acceptance criteria: No precipitate is formed immediately.
Alkali Phosphates
Sample: 1 mL
Analysis: Transfer the Sample to a graduated cylinder, and add 6 mL of ether and 2 mL of alcohol.
Acceptance criteria: No turbidity is produced.
Phosphorous or Hypophosphorous Acid
Sample solution: Dilute 6 mL of Phosphoric Acid with 14 mL of water.
Analysis: Gently warm 5 mL of the Sample solution, and add 2 mL of silver nitrate TS.
Acceptance criteria: The mixture does not become brownish.
6 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in tight containers.
