Dibasic 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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Na₂HPO₄ · 12H₂O 358.14

Na₂HPO₄ · 7H₂O 268.07

Na₂HPO₄ · 2H₂O 177.99

Na₂HPO₄ · H₂O 159.94

Na₂HPO₄ 141.96

Phosphoric acid, disodium salt, dodecahydrate;

Disodium hydrogen phosphate, dodecahydrate CAS RN®: 10039-32-4.

Phosphoric acid, disodium salt, heptahydrate;

Disodium hydrogen phosphate heptahydrate CAS RN®: 7782-85-6.

Phosphoric acid, disodium salt, dihydrate;

Disodium hydrogen phosphate, dihydrate CAS RN®: 10028-24-7.

Phosphoric acid, sodium salt, monohydrate;

Disodium hydrogen phosphate, monohydrate CAS RN®: 118830-14-1.

Phosphoric acid, disodium salt, hydrate;

Disodium hydrogen phosphate hydrate CAS RN®: 10140-65-5.

Anhydrous CAS RN®: 7558-79-4.

1 DEFINITION

Dibasic Sodium Phosphate is dried or contains one, two, seven, or twelve molecules of water of hydration. It contains NLT 98.0% and NMT 100.5% of Na₂HPO₄, calculated on the dried basis.

2 IDENTIFICATION

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

Sample solution: Equivalent to 1 part of Na₂HPO₄ in 30

Acceptance criteria: The solution meets the requirements.

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

Sample solution: Equivalent to 1 part of Na₂HPO₄ in 30

Acceptance criteria: The solution meets the requirements.

3 ASSAY

3.1 Procedure

Sample: Equivalent to 2.5 g of Na₂HPO₄

Sample solution: Transfer the Sample to a 250-mL beaker. Add 50 mL of water and 40.0 mL of 1 N hydrochloric acid, and stir until dissolved.

Blank: Transfer 40.0 mL of 1 N hydrochloric acid to a 250-mL beaker. Add 50 mL of water.

Analysis: Titrate the Blank with 1 N sodium hydroxide VS, and record the volume of 1 N sodium hydroxide VS consumed. Titrate the excess acid in the Sample solution potentiometrically with 1 N sodium hydroxide VS to the inflection point at about pH 4, and record the buret reading. Subtract this buret reading from that of the Blank, and designate the volume of 1 N sodium hydroxide VS resulting from this subtraction as A. Continue the titration with 1 N sodium hydroxide VS to the inflection point at about pH 8.8, record the buret reading, and calculate the volume (B) of 1 N sodium hydroxide required in the titration between the two inflection points (pH 4–8.8). Where A is equal to or less than B, each mL of the volume A of 1 N sodium hydroxide is equivalent to 142.0 mg of Na₂HPO₄. Where A is greater than B, each mL of the volume (2B − A) of 1 N sodium hydroxide is equivalent to 142.0 mg of Na₂HPO₄.

Acceptance criteria: 98.0%–100.5% on the dried basis

4 IMPURITIES

4.1 Insoluble Substances

Sample solution: Equivalent to 5.0 g of Na₂HPO₄ in 100 mL of hot water

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

Acceptance criteria: NMT 20 mg (NMT 0.4%)

4.2 Chloride and Sulfate, Chloride 〈221〉

Sample: Equivalent to 0.5 g of Na₂HPO₄

Acceptance criteria: Shows no more chloride than corresponds to 0.42 mL of 0.020 N hydrochloric acid (NMT 0.06%)

4.3 Chloride and Sulfate, Sulfate 〈221〉

Sample: Equivalent to 0.1 g of Na₂HPO₄

Acceptance criteria: Shows no more sulfate than corresponds to 0.2 mL of 0.020 N sulfuric acid (NMT 0.2%)

Change to read:

4.4 Arsenic 〈211〉, Procedures, Procedure 1

Test preparation: Equivalent to 187.5 mg of Na₂HPO₄ in 35 mL of water

Acceptance criteria: NMT 16 ppm

5 SPECIFIC TESTS

Loss on Drying 〈731〉: Dry a sample at 130° to constant weight.

Acceptance criteria: See Table 1.

Table 1

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.

Labeling: Label it to indicate whether it is dried or is the monohydrate, the dihydrate, the heptahydrate, or the dodecahydrate.