Potassium Nitrate
(poe tas' ee um nye' trate).

KNO3 101.10
Potassium nitrate     [7757-79-1].
DEFINITION
Potassium Nitrate contains NLT 99.0% and NMT 100.5% of KNO3.
IDENTIFICATION
•  A. Identification Tests—General, Potassium 191: Meets the requirements
•  B. Identification Tests—General, Nitrate 191: Meets the requirements
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%
IMPURITIES
•  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%).
•  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%).
•  Arsenic, Method I 211: NMT 3 ppm
•  Lead 251
Sample solution:  500 mg in 20 mL of water
Acceptance criteria:  NMT 10 ppm
•  Iron 241: NMT 10 ppm
•  Heavy Metals, Method I 231: NMT 20 ppm
•  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 
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%
•  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).
ADDITIONAL REQUIREMENTS
•  Packaging and Storage: Preserve in tight containers.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Sujatha Ramakrishna, Ph.D.
Senior Scientific Liaison
1-301-816-8349
(SM22010) Monographs - Small Molecules 2
USP35–NF30 Page 4374
Pharmacopeial Forum: Volume No. 28(3) Page 717