Dibasic Sodium Phosphate

Na2HPO4·12H2O 358.14
Na2HPO4·7H2O 268.07
Na2HPO4·2H2O 177.99
Na2HPO4·H2O 159.94
Na2HPO4 141.96

Phosphoric acid, disodium salt, dodecahydrate;    
Disodium hydrogen phosphate, dodecahydrate     [10039-32-4].
Phosphoric acid, disodium salt, heptahydrate;    
Disodium hydrogen phosphate heptahydrate     [7782-85-6].
Phosphoric acid, disodium salt, dihydrate;    
Disodium hydrogen phosphate, dihydrate     [10028-24-7].
Phosphoric acid, sodium salt, monohydrate;    
Disodium hydrogen phosphate, monohydrate     [118830-14-1].
Phosphoric acid, disodium salt, hydrate;    
Disodium hydrogen phosphate hydrate     [10140-65-5].
Anhydrous     [7558-79-4].
DEFINITION
Dibasic Sodium Phosphate is dried or contains one, two, seven, or twelve molecules of water of hydration. It contains NLT 98.0% and NMT 100.5% of Na2HPO4, calculated on the dried basis.
IDENTIFICATION
•  A. Identification Tests—General, Sodium 191
Sample solution:  Equivalent to 1 part of Na2HPO4 in 30
Acceptance criteria:  The solution meets the requirements.
•  B. Identification Tests—General, Phosphate 191
Sample solution:  Equivalent to 1 part of Na2HPO4 in 30
Acceptance criteria:  The solution meets the requirements.
ASSAY
•  Procedure
Sample:  Equivalent to 2.5 g of Na2HPO4
Sample solution:  Transfer the Sample to a 250-mL beaker. Add 50 mL of water and 40.0 mL of 1 N hydrochloric acid, and stir until dissolved.
Blank:  Transfer 40.0 mL of 1 N hydrochloric acid to a 250-mL beaker. Add 50 mL of water.
Analysis:  Titrate the Blank with 1 N sodium hydroxide VS,and record the volume of 1 N sodium hydroxide VS consumed. Titrate the excess acid in the Sample solution 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 4–8.8). 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.
Acceptance criteria:  98.0%–100.5% on the dried basis
IMPURITIES
•  Insoluble Substances
Sample solution:  Equivalent to 5.0 g of Na2HPO4 in 100 mL of hot water
Analysis:  Filter through a tared filtering crucible, wash the insoluble residue with hot water, and dry at 105 for 2 h.
Acceptance criteria:  NMT 20 mg (NMT 0.4%)
•  Chloride and Sulfate, Chloride 221
Sample:  Equivalent to 0.5 g of Na2HPO4
Acceptance criteria:  Shows no more chloride than corresponds to 0.42 mL of 0.020 N hydrochloric acid (NMT 0.06%)
•  Chloride and Sulfate, Sulfate 221
Sample:  Equivalent to 0.1 g of Na2HPO4
Acceptance criteria:  Shows no more sulfate than corresponds to 0.2 mL of 0.020 N sulfuric acid (NMT 0.2%)
•  Arsenic, Method I 211
Test preparation:  Equivalent to 187.5 mg of Na2HPO4 in 35 mL of water
Acceptance criteria:  NMT 16 ppm
•  Heavy Metals 231
Sample stock solution:  Dissolve the equivalent to 2.1 g of Na2HPO4 in enough water to make 50 mL.
Analysis:  Transfer 12 mL of the Sample stock solution to a 50-mL color-comparison tube (Test Preparation). Transfer 11 mL of the Sample 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.
Acceptance criteria:  NMT 20 ppm
SPECIFIC TESTS
•  Loss on Drying 731: Dry a sample at 130 to constant weight.
Acceptance criteria:  See Table 1.
Table 1
Hydrate Form Acceptance Criteria
Dried NMT 5.0%
Monohydrate 10.3%–12.0%
Dihydrate 18.5%–21.5%
Heptahydrate 43.0%–50.0%
Dodecahydrate 55.0%–64.0%
ADDITIONAL REQUIREMENTS
•  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.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Elena Gonikberg, Ph.D.
Principal Scientific Liaison
1-301-816-8251		 (SM32010) Monographs - Small Molecules 3
USP35–NF30 Page 4667
Pharmacopeial Forum: Volume No. 29(5) Page 1578