Monobasic 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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KH₂PO₄ 136.09

Phosphoric acid, monopotassium salt;

Monopotassium phosphate CAS RN®: 7778-77-0.

1 DEFINITION

Monobasic Potassium Phosphate, dried at 105° for 4 h, contains NLT 98.0% and NMT 100.5% of monobasic potassium phosphate (KH₂PO₄).

2 IDENTIFICATION

A. Identification Tests - General, Potassium 〈191〉

Sample solution: 50 mg/mL

Acceptance criteria: Meets the requirements

B. Identification Tests - General, Phosphate 〈191〉

Sample solution: 50 mg/mL

Acceptance criteria: Meets the requirements

3 ASSAY

Procedure

Sample solution: Transfer 5 g of Monobasic Potassium Phosphate, previously dried, to a 250-mL beaker. Add 100 mL of water and 5.0 mL of 1 N hydrochloric acid VS, and stir until the assay specimen is completely dissolved.

Titrimetric system

(See Titrimetry 〈541〉.)

Mode: Direct titration

Titrant: 1 N sodium hydroxide VS

Endpoint detection: Potentiometrically

Analysis: Slowly titrate the excess acid in the Sample solution, stirring constantly, with Titrant to the inflection point occurring at about pH 4 (V_S1). Continue the titration with Titrant until the inflection point occurring at about pH 8.8 is reached (V_S2).

Calculate the percentage of monobasic potassium phosphate (KH₂PO₄) in the sample taken:

Result = {[(V_S2 − V_S1) × N × F]/W} × 100

V_S2 = Titrant volume consumed by the Sample solution to the second inflection point (mL)

V_S1 = Titrant volume consumed by the Sample solution to the first inflection point (mL)

N = actual normality of the Titrant (mEq/mL)

F = equivalency factor, 0.1361 g/mEq

W = weight of monobasic potassium phosphate taken to prepare the Sample solution (g)

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

4 IMPURITIES

Change to read:

Arsenic 〈211〉, Procedures, Procedure 1 (CN 1-Jun-2023) : NMT 3 μg/g

Change to read:

Lead 〈251〉, Procedures, Procedure 1 (CN 1-Jun-2023)

Test preparation: 1 g in 20 mL of water

Acceptance criteria: NMT 5 μg/g

Limit of Fluoride

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

Buffer solution: 294 mg/mL of sodium citrate dihydrate

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

Standard solution: Dilute 20.0 mL of Standard stock solution and 50.0 mL of Buffer solution with water to 100 mL. Each mL of this solution contains 100 μg of fluoride ion.

Sample solution: Transfer 2.0 g of Monobasic Potassium Phosphate 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 solution and sufficient water to make 100 mL.

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〉).

Analysis

Standard response line: Transfer 50.0 mL of Buffer solution 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.

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 content of fluoride in the portion of Monobasic Potassium Phosphate taken:

Result = (V × C)/W

V = volume of the Sample solution (mL)

C = concentration of fluoride ion, determined from the Standard response line, in the Sample solution (μg/mL)

W = weight of Monobasic Potassium Phosphate taken to prepare the Sample solution (g)

Acceptance criteria: NMT 10 μg/g

5 SPECIFIC TESTS

Insoluble Substances

Sample solution: 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: NMT 20 mg (0.2%)

Loss on Drying 〈731〉

Analysis: Dry a sample at 105° for 4 h.

Acceptance criteria: NMT 1.0%

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.

USP Reference Standards 〈11〉

USP Sodium Fluoride RS