á291ñSELENIUM

Stock Solution— Dissolve 40.0mg of metallic selenium in 100mLof dilute nitric acid (1in 2)in a 1000-mLvolumetric flask,warming gently on a steam bath if necessary to effect solution,add water to volume,and mix.Pipet 5mLof this solution into a 200-mLvolumetric flask,add water to volume,and mix.Each mLof the resulting solution contains the equivalent of 1µg of selenium (Se).

Diaminonaphthalene Solution— Dissolve 100mg of 2,3-diaminonaphthalene and 500mg of hydroxylamine hydrochloride in 0.1Nhydrochloric acid to make 100mL.Prepare this solution fresh on the day of use.

Standard Solution— Pipet 6mLof Stock Solutioninto a 150-mLbeaker,and add 25mLof dilute nitric acid (1in 30)and 25mLof water.

Test Solution— Clean combustion of the test material is an important factor in conducting the test.For compounds that burn poorly and produce soot,the addition of magnesium oxide usually results in more thorough combustion and reduces soot formation.Where the need to add magnesium oxide has been identified,it is specified in the individual monograph.Using a 1000-mLcombustion flask and using 25mLof dilute nitric acid (1in 30)as the absorbing liquid,proceed as directed under Oxygen Flask Combustion á471ñ,using a test specimen weighing 100to 200mg,unless directed otherwise in the individual monograph.Upon completion of the combustion,place a few mLof water in the cup,loosen the stopper,and rinse the stopper,the specimen holder,and the sides of the flask with about 10mLof water.Transfer the solution with the aid of about 20mLof water to a 150-mLbeaker,and heat gently to the boiling temperature.Boil for 10minutes,and allow the solution to cool to room temperature.

Procedure— Treat the Standard Solution,the Test Solution,and the reagent blank consisting of 25mLof dilute nitric acid (1in 30)and 25mLof water,concomitantly and in parallel,as follows.Add ammonium hydroxide solution (1in 2)to adjust to a pHof 2.0±0.2.Dilute with water to 60mL,and transfer to a low-actinic separator with the aid of 10mLof water,adding the 10mLof rinsings to the separator.Add 200mg of hydroxylamine hydrochloride,swirl to dissolve,immediately add 5.0mLof Diaminonaphthalene Solution,insert the stopper,and swirl to mix.Allow the solution to stand at room temperature for 100minutes.Add 5.0mLof cyclohexane,shake vigorously for 2minutes,and allow the layers to separate.Discard the aqueous layer,and centrifuge the cyclohexane extract to remove any dispersed water.Determine the absorbances of the cyclohexane extracts of the Test Solutionand the Standard Solutionin a 1-cm cell at the wavelength of maximum absorbance at about 380nm,with a suitable spectrophotometer,using the cyclohexane extract of the reagent blank as the blank,and compare the absorbances:the absorbance of the Test Solutionis not greater than that of the Standard Solutionwhere a 200-mg test specimen has been taken,or is not greater than one-half that of the Standard Solutionwhere a 100-mg test specimen has been taken.