Tribasic Sodium Phosphate

Na3PO4 (anhydrous)

163.94

Phosphoric acid, trisodium salt, dodecahydrate.
Trisodium phosphate, dodecahydrate 380.13

[10101-89-0].

Trisodium phosphate, monohydrate 181.96

Anhydrous

[7601-54-9].

» Tribasic Sodium Phosphate is anhydrous or contains one to twelve molecules of water of hydration. Na3PO4 (anhydrous) and Na3PO4·H2O (monohydrate) contain not less than 97.0 percent of Na3PO4, calculated on the ignited basis; Na3PO4·12H2O (dodecahydrate) contains not less than 92.0 percent of Na3PO4, calculated on the ignited basis.

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.

Identification— A solution (1 in 20) meets the requirements of the tests for Sodium

and for Phosphate

— Weigh accurately about 2 g, dry at 110

for 5 hours, then ignite at about 800

for 30 minutes: the anhydrous form loses not more than 2.0% of its weight, the monohydrate loses between 8.0% and 11.0% of its weight, and the dodecahydrate loses between 45.0% and 57.0% of its weight.

Insoluble substances— Dissolve a portion equivalent to 10.0 g of anhydrous tribasic sodium phosphate in 100 mL of hot water, and filter through a tared filtering crucible. [note—Do not use glass.] Wash the insoluble residue with hot water, and dry at 105

for 2 hours: the weight of the residue so obtained does not exceed 20 mg (0.2%).

Arsenic, Method I

211

— Dissolve a portion equivalent to 1.0 g of anhydrous tribasic sodium phosphate in 35 mL of water: 3 µg per g.

Heavy metals, Method I

231

— Dissolve a portion equivalent to 2.0 g of anhydrous tribasic sodium phosphate in 25 mL of water: 0.001%.

Assay— Transfer 100.0 mL of 1 N hydrochloric acid to a 400-mL beaker, and titrate potentiometrically with 1 N sodium hydroxide VS to the endpoint at a pH of 7.0. Record the volume of 1 N sodium hydroxide VS consumed as the blank. Transfer a portion of Tribasic Sodium Phosphate, accurately weighed, equivalent to about 5.5 g of anhydrous tribasic sodium phosphate, 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 minutes 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 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 the subtraction as A.

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 about 8.8, record the buret reading, and calculate the volume of 1 N sodium hydroxide required in the titration between the two inflection points (pH 4 to pH 8.8) as B. Where A is equal to or greater than 2B, each mL of the volume of B of 1 N sodium hydroxide is equivalent to 163.9 mg of Na3PO4. Where A is less than 2B, each mL of the volume A – B of 1 N sodium hydroxide is equivalent to 163.9 mg of Na3PO4. Calculate the percentage of Na3PO4 on the ignited basis in the portion of Tribasic Sodium Phosphate taken by the formula:

[10/(100 – L)](D/W)

in which L is the percentage calculated in the test for Loss on ignition; D is the amount, in mg, of Na3PO4 found; and W is the weight, in g, of the sample.

Auxiliary Information— Please check for your question in the FAQs before contacting USP.

Topic/Question	Contact	Expert Committee
Monograph	Robert H. Lafaver, B.A. Scientist 1-301-816-8335	(EM105) Excipient Monographs 1

USP32–NF27 Page 1345

Pharmacopeial Forum: Volume No. 32(2) Page 402