Monobasic Sodium Phosphate

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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NaH₂PO₄ 119.98

NaH₂PO₄ · H₂O 137.99

NaH₂PO₄ · 2H₂O 156.01

Phosphoric acid, monosodium salt, monohydrate;

Monosodium phosphate monohydrate CAS RN®: 10049-21-5.

Phosphoric acid, monosodium salt, dihydrate;

Monosodium phosphate dihydrate CAS RN®: 13472-35-0.

Anhydrous CAS RN®: 7558-80-7.

1 DEFINITION

Monobasic Sodium Phosphate contains one or two molecules of water of hydration, or is anhydrous. It contains NLT 98.0% and NMT 103.0% of monobasic sodium phosphate (NaH₂PO₄), calculated on the anhydrous basis.

2 IDENTIFICATION

2.1 A. Identification Tests-General, Sodium 〈191〉

Sample solution: 50 mg/mL

Acceptance criteria: Meets the requirements

2.2 B. Identification Tests-General, Phosphate 〈191〉

Sample solution: 50 mg/mL

Acceptance criteria: Meets the requirements

3 ASSAY

3.1 Procedure

Sample: 2.5 g

Titrimetric system

• (See Titrimetry 〈541〉.)
• Mode: Direct titration
• Titrant: 1 N sodium hydroxide VS
• Endpoint detection: Visual

Analysis: Dissolve the Sample in 10 mL of cold water. Add 20 mL of a cold, saturated solution of sodium chloride, add phenolphthalein TS, and then titrate with Titrant, keeping the temperature of the solution between 10° and 15° during the entire titration. Perform a blank determination, and make any necessary correction. Each mL of Titrant is equivalent to 120.0 mg of monobasic sodium phosphate (NaH₂PO₄).

Acceptance criteria: 98.0%–103.0% on the anhydrous basis

4 IMPURITIES

4.1 Insoluble Substances

Sample solution: Equivalent to 10.0 g of NaH₂PO₄ · H₂O in 100 mL of hot water

Analysis: Pass through a tared filtering crucible, wash the insoluble residue with hot water, and dry at 105° for 2 h.

Acceptance criteria: NMT 20 mg (0.2%)

4.2 Chloride and Sulfate, Chloride 〈221〉

Sample: Equivalent to 1.0 g of NaH₂PO₄ · H₂O

Control: 0.20 mL of 0.020 N hydrochloric acid

Acceptance criteria: NMT 0.014%; the Sample shows no more chloride than the Control.

4.3 Chloride and Sulfate, Sulfate 〈221〉

Sample: Equivalent to 0.20 g of NaH₂PO₄ · H₂O

Control: 0.30 mL of 0.020 N sulfuric acid

Acceptance criteria: NMT 0.15%; the Sample shows no more sulfate than the Control.

4.4 Aluminum, Calcium, and Related Elements

Sample solution: Equivalent of 1.0 g of NaH₂PO₄ · H₂O in 10 mL of water

Acceptance criteria: The Sample solution does not become turbid when rendered slightly alkaline to litmus paper with 6 N ammonium hydroxide.

Change to read:

4.5 Arsenic 〈211〉, Procedures, Procedure 1

Test preparation: Equivalent of 0.375 g of NaH₂PO₄ · H₂O in 35 mL of water

Acceptance criteria: NMT 8 ppm

5 SPECIFIC TESTS

5.1 pH 〈791〉

Sample solution: Equivalent to 1.0 g of NaH₂PO₄ · H₂O in 20 mL of water

Acceptance criteria: 4.1–4.5

5.2 Water Determination, Method I 〈921〉

Test preparation: For the monohydrate, before performing the test, the sample may be ground to a fine powder in an atmosphere of temperature and relative humidity known not to influence the results.

Acceptance criteria: See Table 1.

Table 1

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in well-closed containers.

Labeling: Label it to indicate whether it is anhydrous, monohydrate, or dihydrate.