A: Dissolve 5g of Sodium Sulfite in 100mLof water:the pHof the solution is between 8.0and 10.0.[NOTE—Reserve portions of the solution so obtained for use inIdentification testBand in the test forColor and clarity of solution.]

B: To 5mLof the solution fromIdentificationtestA,add 0.5mLof iodine TS:the solution is colorless and it meets the requirements of the barium chloride test for Sulfate á191ñ.

C: It meets the requirements of the pyroantimonate precipitate test for Sodium á191ñ.

Standard solution— Immediately before use,dilute 1volume of Standard Iron Solution,prepared as directed under Iron á241ñ,to 10mLwith water.[NOTE—This solution contains the equivalent of 1µg of iron per mL.]

Test solution— To 10.0g of Sodium Sulfite add 25mLof water.Shake until mostly dissolved,and carefully and slowly add 15mLof hydrochloric acid.Heat to boiling.Cool,and dilute with water to 100.0mL.Use a 10-mLportion.

Procedure— To theStandard solutionand theTest solutionseparately add 2mLof a citric acid solution having a concentration of 200g per L,and then add 0.1mLof thioglycolic acid.Mix,make alkaline with stronger ammonia water,and dilute with water to 20mL.Allow to stand for 5minutes.Any pink color in theTest solutionis not more intense than that in theStandard solution:not more than 10µg per g is found.

Selenium standard solution— [Caution—Selenium is toxic;handle it with care. ]Dissolve about 0.1g of metallic selenium,accurately weighed,in 2mLof nitric acid.Evaporate to dryness,add 2mLof water,and evaporate to dryness.Repeat the addition of water and the evaporation to dryness three more times.Dissolve the residue so obtained in 50mLof diluted hydrochloric acid,transfer to a 1000-mLvolumetric flask,dilute with diluted hydrochloric acid to volume,and mix to obtain a solution having a known concentration of about 100µg of selenium per mL.

Procedure— Prepare a test solution as follows.To 3.0g of Sodium Sulfite add 10mLof formaldehyde TS,and slowly and carefully add 2mLof hydrochloric acid.Heat in a water bath for 20minutes.In parallel prepare a Standard solution as follows.To 1.0g of Sodium Sulfite add 0.2mLofSelenium standard solutionand 10mLof formaldehyde TS,and slowly and carefully add 2mLof hydrochloric acid.Heat in a water bath for 20minutes.Any pink color in the test solution is not more intense than that in the Standard solution:not more than 10µg per g is found.

Zinc standard stock solution— Prepare a solution containing 1mLof acetic acid and the amount of zinc sulfate equivalent to 0.440g of ZnSO4·7H2Oin 100.0mLof water.[NOTE—This solution contains the equivalent of 1000µg of Zn per mL.]

Zinc standard solutions— Dilute a volume of the Zinc standard stock solutionquantitatively,and stepwise if necessary,with water to obtain a solution having a known concentration of about 25µg of zinc per mL.Transfer 1.0-mL,2.0-mL,and 4.0-mLportions of this solution to separate 100-mLvolumetric flasks.Dilute the contents of each flask with water to volume,and mix to obtain solutions having known concentrations of about 0.25µg of zinc per mL,0.5µg of zinc per mL,and 1.0µg of zinc per mL,respectively.

Test solution— To 10.0g of Sodium Sulfite add 25mLof water.Shake until mostly dissolved,and carefully and slowly add 15mLof hydrochloric acid.Heat to boiling.Cool,and dilute with water to 100.0mL.Transfer 2.0mLof this solution to a 10-mLvolumetric flask,dilute with water to volume,and mix.

Procedure— Concomitantly determine the absorbances of theZinc standard solutions and theTest solutionat the zinc emission line at 213.9nm with an atomic absorption spectrophotometer (seeSpectrophotometry and Light-Scattering á851ñ),equipped with a zinc hollow-cathode lamp and an air–acetylene flame.Plot the absorbances of theZinc standard solutionsversus concentration of zinc,in µg per mL,and draw the straight line best fitting the plotted points.From the graph so obtained,determine the concentration of zinc,in µg per mL,in theTest solution:not more than 25µg per g is found.