206 ALUMINUM

Nitric Acid Diluent— Transfer 40 mL of nitric acid to a 1000-mL volumetric flask, and dilute with water to volume.

Standard Preparations— Treat some aluminum wire with 6 N hydrochloric acid at 80 for a few minutes. Dissolve about 100 mg of the treated wire, accurately weighed, in a mixture of 10 mL of hydrochloric acid and 2 mL of nitric acid by heating at about 80 for approximately 30 minutes. Continue heating until the volume is reduced to about 4 mL. Cool to room temperature, and add 4 mL of water. Evaporate to about 2 mL by heating. Cool, and transfer this solution, with the aid of water, to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 10.0 mL of this solution to a second 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 1.0 mL of this solution to a third 100-mL volumetric flask, dilute with water to volume, and mix. The concentration of aluminum in this Standard Preparation is about 1.0 µg per mL. If a more diluted Standard Preparation is required, transfer 1.0-, 2.0-, and 4.0-mL portions of this solution to separate 100-mL volumetric flasks, dilute with Nitric Acid Diluent to volume, and mix. These solutions contain 0.01, 0.02, and 0.04 µg of Al per mL, respectively.

Test Preparation— Unless otherwise directed in the monograph, transfer an accurately weighed amount (in g) of the test substance, as specified in the monograph, to a 100-mL plastic volumetric flask, add 50 mL of water, and sonicate for 30 minutes. Add 4 mL of nitric acid, dilute with water to volume, and mix.

Procedure— Determine the absorbances of the Standard Preparations and the Test Preparation at the aluminum emission line at 309.3 nm with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-Scattering 851) equipped with an aluminum hollow-cathode lamp and a flameless electrically heated furnace, using the Nitric Acid Diluent as the blank. Plot the absorbances of the Standard Preparations versus the content of Al, in µg per mL, drawing a straight line best fitting the three points. From the graph so obtained, determine the quantity, in µg, of Al in each mL of the Test Preparation. Calculate the amount of Al in the specimen taken, in µg per g, by multiplying this value by 100/W, where W is the weight, in g, of the substance taken to prepare the Test Preparation.

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