Monobasic Sodium Phosphate

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].

» Monobasic Sodium Phosphate contains one or two molecules of water of hydration, or is anhydrous. It contains not less than 98.0 percent and not more than 103.0 percent of NaH2PO4, calculated on the anhydrous basis.
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 221A 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 221A 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 211Dissolve a portion equivalent to 0.375 g of NaH2PO4·H2O in 35 mL of water: the limit is 8 ppm.
Heavy metals 231Dissolve 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.
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Topic/QuestionContactExpert Committee
MonographElena Gonikberg, Ph.D.
Principal Scientific Liaison
(SM32010) Monographs - Small Molecules 3
USP35–NF30 Page 4668
Pharmacopeial Forum: Volume No. 29(5) Page 1579
Chromatographic Column—
Chromatographic columns text is not derived from, and not part of, USP 35 or NF 30.