Diluted 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)
1 DEFINITION
Diluted Phosphoric Acid contains, in each 100 mL, NLT 9.5 g and NMT 10.5 g of phosphoric acid (H3PO4).
Diluted Phosphoric Acid may be prepared as follows.
| Phosphoric Acid | 69 mL |
| Purified Water, a sufficient quantity to make | 1000 mL |
Mix the ingredients.
2 IDENTIFICATION
A. IDENTIFICATION TESTS-GENERAL, Phosphate (191)
Sample: 100 mL
Analysis: Carefully neutralize the Sample with 1 N sodium hydroxide. Use phenolphthalein TS as the indicator.
Acceptance criteria: Meets the requirements
3 ASSAY
3.1 PROCEDURE
Sample: 10 mL
Blank: 50 mL of water
Titrimetric system
(See Titrimetry (541).)
Mode: Direct titration
Titrant: 1 N sodium hydroxide VS
Endpoint detection: Visual
FICIAL
Analysis: Dilute the Sample with water to 50 mL. Add 0.5 mL of thymolphthalein TS. Titrate with Titrant to the first appearance of a blue color.
Perform a blank determination.
Calculate the amount of phosphoric acid (H3PO4) in the portion of the sample taken:
Result = (VS - VB) × N × F
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, 4.9 x 10-2 g/mEq
Acceptance criteria: 9.5-10.5 g per 100 mL
4 IMPURITIES
4.1 LIMIT OF NITRATE
Sample: 100 mL
Analysis: To the Sample add 0.1 mL of indigo carmine TS, then 5 mL of sulfuric acid.
Acceptance criteria: The blue color is not discharged within 1 min.
5 SPECIFIC TESTS
5.1 ALKALI PHOSPHATES
Sample: 20 mL
Analysis: Evaporate the Sample on a steam bath to a weight of 5 g. Cool, transfer 2 mL to a graduated cylinder, and add 6 mL of ether and 2 mL of alcohol.
Acceptance criteria: No turbidity is produced
5.2 PHOSPHOROUS OR HYPOPHOSPHOROUS ACID
Sample: 100 mL
Analysis: Gently warm the Sample, and add 2 mL of silver nitrate TS.
Acceptance criteria: The mixture does not become brownish.
5.3 CHLORIDE AND SULFATE, Sulfate (221)
Sample: 100 mL
Analysis: To the Sample add 1 mL of barium chloride TS.
Acceptance criteria: No precipitate is formed immediately.
6 ADDITIONAL REQUIREMENTS
PACKAGING AND STORAGE: Preserve in tight containers.
