A: Ignite about 0.5g:it chars,and leaves a residue of iron oxide.

B: To 10mLof a solution of Ferric Ammonium Citrate (1in 100)add 6Nammonium hydroxide dropwise:the solution darkens,but no precipitate forms.

C: To 5mLof a solution of Ferric Ammonium Citrate (1in 100)add 0.3mLof potassium permanganate TSand 4mLof mercuric sulfate TS,and heat the mixture to boiling:a white precipitate forms.

Mercury Stock Solution andStandard Mercury Solution— Proceed as directed for Method Iunder Mercury á261ñ.

Mercury Detection Instrument,Aeration Apparatus,and Stannous Chloride Solution— Proceed as directed for Method IIaand Method IIbunder Mercury á261ñ.

Standard solutions— Transfer 0.25,0.50,1.0,and 3.5mLof Mercury Stock Solutionto four separate glass-stoppered bottles,such as biological oxygen-demand bottles,of about 300-mLcapacity.Dilute the contents of each bottle with water to 100mL,and mix.These solutions contain the equivalent of 2.5,5.0,10.0,and 35.0µg of mercury per mL,respectively.

Test solution— Transfer about 1.000g of Ferric Ammonium Citrate,accurately weighed,to a 200-mLcentrifuge bottle with a polytef-lined screw cap,and add 5mLof nitric acid and 5mLof hydrochloric acid.Close the bottle tightly,digest on a steam bath for 1hour,and cool.Quantitatively transfer the solution to a suitable glass-stoppered bottle,dilute with water to 100mL,and bubble air through the solution for 2minutes.Prepare a reagent blank in the same manner.

Procedure— Add 5mLof stannous chloride solution (1in 10)to each solution,and immediately insert the bubbler of the Aeration Apparatus.Obtain the absorbances as directed by the instrument manufacturer's operating instructions.Perform a blank determination,and make any necessary correction.Plot the absorbances of the Standard solutionsversus concentrations,in µg per mL,of mercury,and draw the straight line best fitting the plotted points.From the graph so obtained,determine the concentration,in µg per g,of mercury in the Test solution:not more than 10µg per g is found.

Standard stock solution— Dissolve about 159.8mg of lead nitrate,accurately weighed,in 100mLof water containing 1mLof nitric acid.Dilute with water to 1000.0mL,and mix.

Standard solution— [NOTE—Prepare this solution on the day of use.]Transfer 10.0mLof Standard stock solutionto a 500-mLvolumetric flask,dilute with water to volume,and mix.Each mLcontains the equivalent of 2µg of lead (Pb).

Test solution— Transfer about 15g of Ferric Ammonium Citrate,accurately weighed,to a 100-mLvolumetric flask (previously rinsed with nitric acid and water),dissolve in a mixture of 50mLof water and 1mLof nitric acid,dilute with water to volume,and mix.

Procedure— Using a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-Scattering á851ñ)equipped with a deuterium arc background corrector,a digital readout device,and a burner head capable of handling 15%solids content,perform a blank determination with water,following the manufacturer's operating instructions.Separately aspirate portions of the Standard solutionand the Test solution,and record the absorbances.Calculate the lead content,in µg per g,in the portion of Ferric Ammonium Citrate taken by the formula: in which Cis the concentration,in µg per mL,of lead in the Standard solution;Wis the weight,in g,of Ferric Ammonium Citrate taken;and AUand ASare the absorbances of the Test solutionand the Standard solution,respectively:not more than 10µg per g is found.