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

Procedure— Transfer 5.0mLof Stock test solutionto each of three 25-mLvolumetric flasks.To these flasks,respectively,add 0.0,5.0,and 10.0mLof Stock standard solution,dilute with water to volume,and mix.These flasks contain 0.0,0.20,and 0.40µg 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 589nm with an atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering á851ñ)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 0.0mLof the Stock standard solution.Calculate the percentage of sodium in the portion of Potassium Nitrate taken by multiplying Cby 0.25:the limit is 0.1%.

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

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

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

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

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

Procedure— Dissolve about 400mg of Potassium Nitrate,accurately weighed,in 100mLof water.Pass this solution through the Cation-exchange columnat a rate of about 5mLper minute,and collect the eluate in a 500-mLconical flask.Wash the resin in the column with water at a rate of about 10mLper minute,collecting the eluate in the conical flask.Add 0.15mLof phenolphthalein TSto the flask,and after 5minutes titrate with 0.1Nsodium hydroxide VSto a pink endpoint.Continue collecting the wash from the column,and continue titrating,if necessary,until a 50-mLincrement of eluate requires no further addition of sodium hydroxide.Each mLof 0.1Nsodium hydroxide is equivalent to 10.11mg of KNO3.