NaH2PO4·xH2O
(anhydrous) 119.98

Phosphoric acid, monosodium salt, monohydrate.
Monosodium phosphate monohydrate 137.99 [10049-21-5].

Phosphoric acid, monosodium salt, dihydrate.
Monosodium phosphate dihydrate 156.01 [13472-35-0].

Anhydrous [7558-80-7].

Packaging and storage— Preserve in well-closed containers.

Labeling— Label it to indicate whether it is anhydrous or is the monohydrate or the dihydrate.

Identification— A solution (1 in 20) responds to the tests for Sodium 191 and for Phosphate 191.

pH 791: between 4.1 and 4.5, in a solution containing the equivalent of 1.0 g of NaH2PO4·H2O in 20 mL of water.

Water, Method I 921: less than 2.0% (anhydrous form); between 10.0% and 15.0% (monohydrate); between 18.0% and 26.5% (dihydrate). For the monohydrate, the sample may be ground to a fine powder in an atmosphere of temperature and relative humidity known not to influence the results, prior to performing the test.

Insoluble substances— Dissolve a portion equivalent to 10.0 g of NaH2PO4·H2O 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 (0.2%).

Chloride 221— A portion equivalent to 1.0 g of NaH2PO4·H2O shows no more chloride than corresponds to 0.20 mL of 0.020 N hydrochloric acid (0.014%).

Sulfate 221— A portion equivalent to 0.20 g of NaH2PO4·H2O shows no more sulfate than corresponds to 0.30 mL of 0.020 N sulfuric acid (0.15%).

Aluminum, calcium, and related elements— A solution containing the equivalent of 1.0 g of NaH2PO4·H2O in 10 mL of water does not become turbid when rendered slightly alkaline to litmus paper with 6 N ammonium hydroxide.

Arsenic, Method I 211— Dissolve a portion equivalent to 0.375 g of NaH2PO4·H2O in 35 mL of water: the limit is 8 ppm.

Heavy metals 231— Dissolve a portion equivalent to 1.0 g of NaH2PO4·H2O in 20 mL of water, and add 1 mL of 3 N hydrochloric acid and water to make 25 mL: the limit is 0.002%.

Assay— Dissolve about 2.5 g of Monobasic Sodium Phosphate, accurately weighed, in 10 mL of cold water, add 20 mL of a cold, saturated solution of sodium chloride, then add phenolphthalein TS, and titrate with 1 N sodium hydroxide VS, 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 1 N sodium hydroxide is equivalent to 120.0 mg of NaH2PO4.

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 3582

MONOBASIC SODIUM PHOSPHATE

Chromatographic columns text is not derived from, and not part of, USP 32 or NF 27.