Tribasic Sodium Phosphate
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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)
Na3PO4 (anhydrous) 163.94
Trisodium phosphate, monohydrate 181.96
Phosphoric acid, trisodium salt, dodecahydrate;
Trisodium phosphate, dodecahydrate 380.13 CAS RN®: 10101-89-0.
Anhydrous CAS RN®: 7601-54-9.
1 DEFINITION
Tribasic Sodium Phosphate is anhydrous or contains one to twelve molecules of water of hydration. Na3PO4 (anhydrous) and Na3PO4.H2O (monohydrate) contain NLT 97.0% of Na3PO4, calculated on the ignited basis. Na3PO4.12H2O(dodecahydrate) contains NLT 92.0% of Na3PO4, calculated on the ignited basis
2 IDENTIFICATION
A. Identification Tests—General, Sodium 〈191〉 and Phosphate 〈191〉: A solution (1 in 20) meets the requirements.
3 ASSAY
Procedure
Sample: 5.5 g of Tribasic Sodium Phosphate, on the anhydrous basis
Blank: 100.0 mL of 1 N hydrochloric acid, accurately measured
Titrimetric system
(See Titrimetry 〈541〉.)
Mode: Residual titration
Titrant: 1 N sodium hydroxide VS
Endpoint detection: Potentiometric
Analysis: Transfer the Blank to a 400-mL beaker, and titrate with the Titrant to the endpoint at a pH of 7.0. Record as the volume consumed, and designate as A. Transfer the Sample to a 400-mL beaker, add 100.0 mL of 1 N hydrochloric acid, and stir until dissolved. Pass a stream of carbon dioxide-free air, in fine bubbles, through the solution for 30 min to expel carbon dioxide, covering the beaker loosely to prevent any loss by spraying. Wash the cover and sides of the beaker with a few mL of water.
Titrate the excess acid potentiometrically with the Titrant to the inflection point at a pH of 4. Record the buret reading, and designate as B. Protect the solution from carbon dioxide absorbed from the air, and continue the titration with 1 N sodium hydroxide VS to the inflection point at a pH of 8.8. Record the buret reading, and designate as C.
Calculate the amount of Titrant consumed by the Sample to the first inflection point, correcting for the Blank (V1 = A − B) and the amount of Titrant consumed by the Sample between the two inflection points (V2 = C − B). If V1 is equal to or greater than 2V2, calculate the amount of Na3PO4 in the portion of Sample taken:
D = V2 × N × F
V2 = volume of Titrant consumed between the two inflection points (mL)
N = actual normality of the Titrant (mEq/mL)
F = equivalency factor, 163.9 mg/mEq
If V is less than 2V , calculate the amount of Na3PO4 in the portion of Sample taken:
D = (V1 − V2) × N × F
V1 = volume of the Titrant consumed to the first inflection point, correcting for the Blank (mL)
N = actual normality of the Titrant (mEq/mL)
F = equivalency factor, 163.9 mg/mEq
Calculate the percentage of Na3PO4 on the ignited basis in the portion of Tribasic Sodium Phosphate taken:
Result = [10/(100 – L)] × (D/W)
L = percentage calculated in the test for Loss on Ignition 〈733〉
D = amount of Na3PO4 found (mg)
W = weight of the Sample (g)
Acceptance criteria: NLT 97.0% of Na3PO4 on the ignited basis. Na3PO4.12H2O(dodecahydrate) contains NLT 92.0% of Na3PO4 on the ignited basis.
4 IMPURITIES
Loss on Ignition 〈733〉
Sample: 2 g
Analysis: Dry the Sample at 110° for 5 h, and then ignite at 800° for 30 min.
Acceptance criteria: The anhydrous form loses NMT 2.0% of its weight, the monohydrate loses 8.0%–11.0% of its weight, and the dodecahydrate loses 45.0%–57.0% of its weight.
Change to read:
Arsenic 〈211〉, Procedures, Procedure 1
Test preparation: Dissolve a portion equivalent to 1.0 g of anhydrous tribasic sodium phosphate in 35 mL of water. Analysis: Proceed as directed in the chapter.
Acceptance criteria: NMT 3 ppm
5 SPECIFIC TESTS
Insoluble Substances
Sample solution: Dissolve a portion equivalent to 10.0 g of anhydrous tribasic sodium phosphate in 100 mL of hot water. Analysis: Filter the Sample solution through a tared filtering crucible. [Note—Do not use glass.] Wash the insoluble residue with hot water, and dry at 105° for 2 h.
Acceptance criteria: The weight of the residue so obtained does not exceed 20 mg (0.2%).
6 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in tight containers. No storage requirements specified.
Labeling: Label it to indicate whether it is anhydrous, the monohydrate, or the dodecahydrate.
