Dibasic Potassium Phosphate

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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K₂HPO₄ 174.18

Phosphoric acid, dipotassium salt;

Dipotassium hydrogen phosphate    CAS RN®: 7758-11-4.

1 DEFINITION

Dibasic Potassium Phosphate contains NLT 98.0% and NMT 100.5% of K₂HPO₄, calculated on the dried basis.

2 IDENTIFICATION

2.1 A. Identification Tests-General, Potassium 〈191〉

Sample solution: 1 in 20

Acceptance criteria: Meets the requirements

2.2 B. Identification Tests-General, Phosphate 〈191〉

Sample solution: 1 in 20

Acceptance criteria: Meets the requirements

3 ASSAY

3.1 Procedure

Sample: 5.2 g

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 potentiometrically 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 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 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 174.2 mg of K₂HPO₄. Where A is greater than B, each mL of the volume 2B − A of 1 N sodium hydroxide is equivalent to 174.2 mg of K₂HPO₄.

Acceptance criteria: 98.0%–100.5% on the dried basis

4 IMPURITIES

4.1 Insoluble Substances

Sample solution: Dissolve 10 g in 100 mL of hot water.

Analysis: Filter the Sample solution through a tared filtering crucible, wash the insoluble residue with hot water, and dry at 105° for 2 h.

Acceptance criteria: The weight of the residue is NMT 20 mg (NMT 0.2%).

4.2 Carbonate

Sample: 1 g

Analysis: Add 3 mL of water and 2 mL of 3 N hydrochloric acid to the Sample.

Acceptance criteria: NMT a few bubbles are evolved.

4.3 Chloride and Sulfate, Chloride 〈221〉

Sample: 1.0 g

Acceptance criteria: Shows no more chloride than corresponds to 0.40 mL of 0.020 N hydrochloric acid (NMT 0.03%)

4.4 Chloride and Sulfate, Sulfate 〈221〉

Sample: 0.20 g

Acceptance criteria: Shows no more sulfate than corresponds to 0.20 mL of 0.020 N sulfuric acid (NMT 0.1%)

Change to read:

4.5 Arsenic 〈211〉, Procedures, Procedure 1: NMT 3 ppm

Change to read:

4.6 Iron

Sample solution: Dissolve 0.33 g in 10 mL of water.

Analysis: To the Sample solution add 6 mL of hydroxylamine hydrochloride solution (1 in 10) and 4 mL of orthophenanthroline solution prepared by dissolving 1 g of orthophenanthroline in 1000 mL of water containing 1 mL of 3 N hydrochloric acid, and dilute with water to 25 mL.

Acceptance criteria: Any red color produced within 1 h is not darker than that of a control prepared from 1 mL of Standard Iron Solution (see Iron 〈241〉, Procedures, Procedure 1): NMT 30 ppm.

4.7 Sodium

A solution (1 in 10) tested on a platinum wire imparts no pronounced yellow color to a nonluminous flame.

4.8 Limit of Fluoride

[Note-Prepare and store all solutions in plastic containers.]

Buffer: 294 g/L of sodium citrate dihydrate in water

Standard stock solution: 1.1052 mg/mL of USP Sodium Fluoride RS in water

Standard solution: Transfer 20.0 mL of Standard stock solution to a 100-mL volumetric flask containing 50.0 mL of Buffer, and dilute with water to volume. Each mL of this solution contains 100 µg of fluoride ion.

Sample: 2.0 g

Electrode system: Use a fluoride-specific ion-indicating electrode and a silver–silver chloride reference electrode connected to a pH meter capable of measuring potentials with a minimum reproducibility of ±0.2 mV (see pH 〈791〉).

Standard response line: Transfer 50.0 mL of Buffer and 2.0 mL of hydrochloric acid to a beaker, and add water to make 100 mL. Add a plastic-coated stirring bar, insert the electrodes into the solution, stir for 15 min, and read the potential, in mV. Continue stirring, and at 5-min intervals add 100, 100, 300, and 500 µL of Standard solution, reading the potential 5 min after each addition. Plot the logarithms of the cumulative fluoride ion concentrations (0.1, 0.2, 0.5, and 1.0 µg/mL) versus potential, in mV.

Analysis: Transfer the Sample to a beaker containing a plastic-coated stirring bar. Add 20 mL of water and 2.0 mL of hydrochloric acid, and stir until dissolved. Add 50.0 mL of Buffer and sufficient water to make 100 mL (Sample solution). Rinse and dry the electrodes, insert them into the Sample solution, stir for 5 min, and read the potential, in mV. From the measured potential and the Standard response line determine the concentration, C, in µg/mL, of fluoride ion in the Sample solution.

Calculate the percentage of fluoride ion in the portion of Dibasic Potassium Phosphate taken:

Result = (C/C_u) × 100

C = concentration of fluoride ion in the Sample solution (µg/mL), as defined above

C_u = concentration of the Sample solution (µg/mL)

Acceptance criteria: NMT 0.001%

4.9 Limit of Monobasic or Tribasic Salt

Sample solution: Dissolve 3 g in 30 mL of water, and cool to 20°.

Analysis: Add 3 drops of thymol blue TS to the Sample solution.

Acceptance criteria: A blue color is produced, which is changed to yellow (with a greenish tinge) by the addition of NMT 0.4 mL of 1 N hydrochloric acid.

5 SPECIFIC TESTS

pH 〈791〉: 8.5–9.6, in a solution (1 in 20)

Loss on Drying 〈731〉: Dry a sample at 105° to constant weight: it loses NMT 1.0% of its weight.

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in well-closed containers.

USP Reference Standards 〈11〉

USP Sodium Fluoride RS