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

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

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

Mercury Stock Solution and Standard Mercury Solution— Proceed as directed for Method I under Mercury 261.

Mercury Detection Instrument, Aeration Apparatus, and Stannous Chloride Solution— Proceed as directed for Method IIa and Method IIb under Mercury 261.

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

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

Procedure— Add 5 mL of stannous chloride solution (1 in 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 solutions versus 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 10 µg per g is found.

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

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

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

Procedure— Using a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-Scattering 851) 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 solution and 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 C is the concentration, in µg per mL, of lead in the Standard solution; W is the weight, in g, of Ferric Ammonium Citrate taken; and AU and AS are the absorbances of the Test solution and the Standard solution, respectively: not more than 10 µg per g is found.