Lead—
[NOTE—For the preparation of all aqueous solutions and for the rinsing of glassware before use, employ water that has been passed through a strong-acid, strong-base, mixed-bed ion-exchange resin before use. Select all reagents to have as low a content of lead as practicable, and store all reagent solutions in containers of borosilicate glass. Cleanse glassware before use by soaking in warm 8
N nitric acid for 30 minutes and by rinsing with deionized water.
]
Standard preparation—
Transfer 10.0 mL of
Lead Nitrate Stock Solution, prepared as directed in the test for
Heavy Metals 
231
, to a 100-mL volumetric flask, add 40 mL of water and 5 mL of nitric acid, dilute with water to volume, and mix. Transfer 0.40 mL of this solution to a second 100-mL volumetric flask, add 50 mL of water and 1 mL of nitric acid, dilute with water to volume, and mix. This solution contains 0.04 µg of lead per mL.
Test preparation—
Transfer about 4 g of Copper Gluconate, accurately weighed, to a 100-mL volumetric flask. Add 50 mL of water and 5 mL of nitric acid, and sonicate to dissolve the specimen. Dilute with water to volume, and mix. Transfer 4.0 mL of this solution to a second 100-mL volumetric flask, add 50 mL of water and 1 mL of nitric acid, dilute with water to volume, and mix.
Blank—
Transfer 1.2 mL of nitric acid to a 100-mL volumetric flask, dilute with water to volume, and mix.
Test solutions—
Prepare mixtures of the Test preparation, the Standard preparation, and the Blank with the following proportional compositions, by volume: 10.0:0:10.0, 10.0:4.0:6.0, 10.0:7.0:3.0, and 10.0:10.0:0. These Test solutions contain, respectively, 0, 0.008, 0.014, and 0.020 µg per mL, of lead from the Standard preparation.
Procedure—
Separately inject equal volumes (about 20 µL) of the
Test solutions and the
Blank into the graphite tube of a suitable graphite furnace atomic absorption spectrophotometer, temperature-programmed as follows to reach 2000
in about 2 minutes, using an argon gas flow of about 3 L per minute, except where indicated: 70
for 10 seconds, 90
for 60 seconds, 120
for 15 seconds, 250
for 5 seconds (no gas flow), 250
for 10 seconds, 250
for 2 seconds (no gas flow), and 2000
for 3.2 seconds. When the temperature reaches 2000
, determine the absorbance at the lead emission line at 283.3 nm, corrected for background absorption. Correct the absorbance values obtained from the
Test solutions by subtracting from each the absorbance value obtained from the
Blank. Plot the corrected absorbances of the
Test solutions versus their contents of lead, in µg per mL, as furnished by the
Standard preparation, draw the straight line best fitting the four points, and extrapolate the line until it intercepts the concentration axis. From the intercept determine the concentration,
C, in µg per mL, of lead in each mL of the test solution containing 0 µg of lead from the
Test preparation. Calculate the percentage of lead in the specimen taken by the formula:
0.5C / W,
in which
W is the weight, in g, of Copper Gluconate taken to prepare the
Test preparation: the limit is 0.0025%.
Reducing substances—
Transfer 1.0 g to a 250-mL conical flask, dissolve in 10 mL of water, and add 25 mL of
alkaline cupric citrate TS. Cover the flask, boil gently for 5 minutes, accurately timed, and cool rapidly to room temperature. Add 25 mL of 0.6 N acetic acid, 10.0 mL of 0.1 N iodine VS, and 10 mL of 3 N hydrochloric acid, and titrate with 0.1 N sodium thiosulfate VS, adding 3 mL of
starch TS as the endpoint is approached. Perform a blank determination, omitting the specimen, and note the difference in volumes required. Each mL of the difference in volume of 0.1 N sodium thiosulfate consumed is equivalent to 2.7 mg of reducing substances (as dextrose): the limit is 1.0%.
;)
