Potassium Nitrate

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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KNO₃ 101.10

Potassium nitrate CAS RN^®: 7757-79-1; UNII: RU45X2JN0Z.

1 DEFINITION

Potassium Nitrate contains NLT 99.0% and NMT 100.5% of KNO₃.

2 IDENTIFICATION

A. IDENTIFICATION TESTS-GENERAL, Potassium (191): Meets the requirements

B. IDENTIFICATION TESTS-GENERAL, Nitrate (191): Meets the requirements

3 ASSAY

PROCEDURE

[NOTE-Use water that is free of carbon dioxide and ammonia.]

Cation-exchange column: Transfer strongly acidic styrene-divinylbenzene cation-exchange resin (16-to 50-mesh) to a 2-cm diameter chromatographic column to a depth of about 20 cm.

Sample solution: 4 mg/mL of Potassium Nitrate in water

Analysis: Pass 100 mL of Sample solution through the Cation-exchange column at a rate of 5 mL/min, and collect the eluate in a 500-mL conical flask. Wash the resin in the column with water at a rate of 10 mL/min, collecting the eluate in the conical flask. Add 0.15 mL of phenolphthalein TS to the flask, and after 5 min titrate with 0.1 N sodium hydroxide VS to a pink endpoint. Continue collecting the wash from the column, and continue titrating, if necessary, until a 50-mL increment of eluate requires no further addition of sodium hydroxide. Each mL of 0.1 N sodium hydroxide is equivalent to 10.11 mg of KNO₃.

Acceptance criteria: 99.0%-100.5%

4 IMPURITIES

4.1 CHLORIDE AND SULFATE, Chloride (221)

A 500-mg portion shows no more chloride than corresponds to 0.21 mL of 0.020 N hydrochloric acid (0.03%).

4.2 CHLORIDE AND SULFATE, Sulfate (221)

Sample solution: 100 mg of Potassium Nitrate in 10 mL of water. Add 15 mL of 6 N hydrochloric acid, and evaporate to dryness on a steam bath. To the residue add 7 mL of 6 N hydrochloric acid, and evaporate to dryness on a steam bath. Dissolve the residue in 35 mL of water and, if necessary, neutralize with hydrochloric acid using a litmus paper indicator. Filter, if necessary, to obtain a clear Sample solution.

Acceptance criteria: The Sample solution shows no more sulfate than corresponds to 0.10 mL of 0.020 N sulfuric acid (0.1%).

Change to read:

4.3 ARSENIC (211), Procedures, Procedure 1 (CN 1-JUN-2023)

NMT 3 ppm

Change to read:

4.4 LEAD (251), Procedures, Procedure 1 (CN 1-JUN-2023)

Sample solution: 500 mg in 20 mL of water

Acceptance criteria: NMT 10 ppm

Change to read:

4.5 IRON (241), Procedures, Procedure 1 (CN 1-JUN-2023)

NMT 10 ppm

4.6 LIMIT OF SODIUM

Standard stock solution: [NOTE-Sodium chloride is previously dried at 105° for 2 h.] 2.542 µg/mL of sodium chloride in water. This solution contains 1.0 µg/mL of sodium.

Sample stock solution: 2 mg/mL of Potassium Nitrate. [NOTE-The concentration of potassium nitrate in this solution may be modified by using a different quantity or by further dilution to bring the absorption response within the working range of the atomic absorption spectrometer.]

Instrumental conditions

(See Atomic Absorption Spectroscopy (852).)

Mode: Atomic absorption spectrophotometry

Analytical wavelength: Sodium emission line of 589 nm

Lamp: Sodium hollow-cathode

Flame: Oxidizing

Blank: Water

Analysis: Transfer 5.0 mL of the Sample stock solution to each of three 25-mL volumetric flasks. To these flasks, respectively, add 0.0, 5.0, and 10.0 mL of the Standard stock solution, dilute with water to volume, and mix. These flasks contain 0.0, 0.20, and 0.40 µg of added sodium/mL, respectively. [NOTE-Concentrations of sodium in these solutions may be modified to fit the linear or working range of the atomic absorption spectrophotometer.]

Determine the absorbances of these solutions. Plot the absorbances of the three solutions versus concentration, in µg/mL of added sodium, draw the straight line best fitting the plotted points, and extrapolate the line until it intercepts the concentration axis. From the graph determine the concentration, C, in µg/mL of sodium, of the solution containing 0.0 mL of the Standard stock solution.

Calculate the percentage of sodium in the portion of Potassium Nitrate taken by multiplying C by 0.25.

Acceptance criteria: NMT 0.1%

4.7 LIMIT OF NITRITE

Solution A: 1 mg/mL of sulfanilic acid

Solution B: 1 mg/mL of N-(1-naphthyl)ethylenediamine dihydrochloride. [NOTE-When stored in a low-actinic glass bottle, this solution may be used for 1 week.]

Standard stock solution: 15 µg/mL of sodium nitrite (10 µg/mL of nitrite)

Standard solutions: Transfer 1.0 and 2.0 mL of Standard stock solution to separate 50-mL beakers, and add 19 and 18 mL of water to the respective beakers.

Sample solution: Transfer 4.0 g of Potassium Nitrate to a 50-mL beaker, add 20 mL of water, and swirl to dissolve.

Analysis: To the beakers containing the Standard solutions and the Sample solution add 5.0 mL of Solution A and 5.0 mL of diluted hydrochloric acid, and allow to stand for 3 min. Add 5.0 mL of Solution B to each beaker, mix, and allow to stand for 15 min. Determine the absorbances of the solutions at 550 nm.

Acceptance criteria: The absorbance of the solution from the Sample solution does not exceed that of the solution from the Standard solution containing 20 µg of nitrite (5 µg/g).

5 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in tight containers.