Stock test solution— Transfer 1.0 g of Potassium Nitrate to a 500-mL volumetric flask, add water to dissolve it, dilute with water to volume, and mix. [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 spectrophotometer.]

Stock standard solution— Transfer 127.1 mg of sodium chloride, previously dried at 105 for 2 hours and accurately weighed, to a 500-mL volumetric flask, add water to volume, and mix. Transfer 5.0 mL of this solution to a second 500-mL volumetric flask, dilute with water to volume, and mix. This solution contains 1.0 µg of sodium per mL.

Procedure— Transfer 5.0 mL of Stock test solution to each of three 25-mL volumetric flasks. To these flasks, respectively, add 0.0, 5.0, and 10.0 mL of Stock standard solution, dilute with water to volume, and mix. These flasks contain 0.0, 0.20, and 0.40 µg of added sodium per mL, respectively. [note—Concentrations of sodium in these solutions may be modified to fit the linear or working range of the atomic absorption spectrophotometer.] Concomitantly determine the absorbances of these solutions at the sodium emission line of 589 nm with an atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering 851) equipped with a sodium hollow-cathode lamp and an oxidizing flame, using water to zero the instrument. Plot the absorbances of the three solutions versus concentration, in µg of added sodium per mL, draw the straight line best fitting the plotted points, and extrapolate the line until it intercepts the concentration axis. From the graph so obtained determine the concentration, C, in µg of sodium per mL, of the solution containing 0.0 mL of the Stock standard solution. Calculate the percentage of sodium in the portion of Potassium Nitrate taken by multiplying C by 0.25: the limit is 0.1%.

Sulfanilic acid solution— Prepare a solution containing 1 mg of sulfanilic acid per mL.

N-(1-Naphthyl)ethylenediamine dihydrochloride solution— Prepare a solution containing 1 mg of N-(1-naphthyl)ethylenediamine dihydrochloride per mL. [note—When stored in a low-actinic glass bottle, this solution may be used for 1 week.]

Standard solutions— Transfer 150.0 mg of sodium nitrite to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 10.0 mL of this solution to a 1000-mL volumetric flask, dilute with water to volume, and mix. This solution contains 15 µg of sodium nitrite (10 µg of nitrite) per mL. Transfer 1.0 mL and 2.0 mL of this solution to separate 50-mL beakers, and add 19 and 18 mL of water to the respective beakers.

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

Procedure— To the beakers containing the Standard solutions and the Test solution add 5.0 mL of Sulfanilic acid solution and 5.0 mL of diluted hydrochloric acid, and allow to stand for 3 minutes. Add 5.0 mL of N-(1-Naphthyl)ethylenediamine dihydrochloride solution to each beaker, mix, and allow to stand for 15 minutes. Concomitantly determine the absorbances of the solutions at 550 nm. The absorbance of the solution from the Test solution does not exceed that of the solution from the Standard solution containing 20 µg of nitrite (5 µg per g).

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.

Procedure— Dissolve about 400 mg of Potassium Nitrate, accurately weighed, in 100 mL of water. Pass this solution through the Cation-exchange column at a rate of about 5 mL per minute, and collect the eluate in a 500-mL conical flask. Wash the resin in the column with water at a rate of about 10 mL per minute, collecting the eluate in the conical flask. Add 0.15 mL of phenolphthalein TS to the flask, and after 5 minutes 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.