Na2HPO4·xH2O (anhydrous) 141.96

Phosphoric acid, disodium salt, dodecahydrate.
Disodium hydrogen phosphate, dodecahydrate 358.14 [10039-32-4].

Phosphoric acid, disodium salt, heptahydrate.
Disodium hydrogen phosphate heptahydrate 268.07 [7782-85-6].

Phosphoric acid, disodium salt, dihydrate.
Disodium hydrogen phosphate, dihydrate 177.99 [10028-24-7].

Phosphoric acid, sodium salt, monohydrate.
Disodium hydrogen phosphate, monohydrate 159.94 [118830-14-1].

Phosphoric acid, disodium salt, hydrate.
Disodium hydrogen phosphate hydrate [10140-65-5].

Anhydrous [7558-79-4].

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.

Identification— A solution (the equivalent of 1 part of Na2HPO4 in 30) responds to the tests for Sodium 191 and for Phosphate 191.

Loss on drying 731— Dry it at 130 to constant weight: the dried form loses not more than 5.0% of its weight, the monohydrate loses between 10.3% and 12.0% of its weight, the dihydrate loses between 18.5% and 21.5% of its weight, the heptahydrate loses between 43.0% and 50.0% of its weight, and the dodecahydrate loses between 55.0% and 64.0% of its weight.

Insoluble substances— Dissolve the equivalent of 5.0 g of Na2HPO4 in 100 mL of hot water, filter through a tared filtering crucible, 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 (0.4%).

Chloride 221— A portion equivalent to 0.5 g of Na2HPO4 shows no more chloride than corresponds to 0.42 mL of 0.020 N hydrochloric acid (0.06%).

Sulfate 221— A portion equivalent to 0.1 g of Na2HPO4 shows no more sulfate than corresponds to 0.2 mL of 0.020 N sulfuric acid (0.2%).

Arsenic, Method I 211— Prepare a Test Preparation by dissolving a portion equivalent to 187.5 mg of Na2HPO4 in 35 mL of water: the limit is 16 ppm.

Heavy metals 231— Dissolve a portion equivalent to 2.1 g of Na2HPO4 in enough water to make 50 mL of stock solution. Transfer 12 mL of this stock solution to a 50-mL color-comparison tube (Test Preparation). Transfer 11 mL of the stock solution to a second color-comparison tube containing 1.0 mL of Standard Lead Solution (Monitor Preparation). Transfer 1.0 mL of Standard Lead Solution and 11 mL of water to a third color-comparison tube (Standard Preparation). Proceed as directed for Procedure, omitting the dilution to 50 mL: the limit is 0.002%.

Assay— Transfer 40.0 mL of 1 N hydrochloric acid to a 250-mL beaker, add 50 mL of water, and titrate potentiometrically with 1 N sodium hydroxide VS to the endpoint. Record the volume of 1 N sodium hydroxide VS consumed as the blank. Transfer a portion of Dibasic Sodium Phosphate, accurately weighed, equivalent to about 2.5 g of Na2HPO4, to a 250-mL beaker, add 40.0 mL of the 1 N hydrochloric acid and 50 mL of water, and stir until dissolved. 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 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 to pH 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.

Topic/Question	Contact	Expert Committee
Monograph	Elena Gonikberg, Ph.D. Senior Scientist 1-301-816-8251	(MDGRE05) Monograph Development-Gastrointestinal Renal and Endocrine

USP32–NF27 Page 3581