A: A solution (1 in 10) responds to the tests for Zinc 191.

B: It responds to Identification test B under Calcium Gluconate.

Standard preparation— Transfer 137.2 mg of cadmium nitrate to a 1000-mL volumetric flask, dissolve in water, dilute with water to volume, and mix. Pipet 25 mL of the resulting solution into a 100-mL volumetric flask, add 1 mL of hydrochloric acid, dilute with water to volume, and mix. Each mL of this Standard preparation contains 12.5 µg of Cd.

Test preparation— Transfer 10.0 g of Zinc Gluconate to a 50-mL volumetric flask, dissolve in and dilute with water to volume, and mix.

Procedure— To three separate 25-mL volumetric flasks add, respectively, 0, 2.0, and 4.0 mL of the Standard preparation. To each flask add 5.0 mL of the Test preparation, dilute with water to volume, and mix. These test solutions contain, respectively, 0, 1.0, and 2.0 µg per mL of cadmium from the Standard preparation. Concomitantly determine the absorbances of the test solutions at the cadmium emission line at 228.8 nm, with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering 851) equipped with a cadmium hollow-cathode lamp and an air–acetylene flame, using water as the blank. Plot the absorbances of the test solutions versus their contents of cadmium, in µg per mL, as furnished by the Standard preparation, draw the straight line best fitting the three points, and extrapolate the line until it intercepts the concentration axis. From the intercept determine the amount, in µg, of cadmium in each mL of the test solution containing 0 mL of the Standard preparation. Calculate the quantity, in ppm, of Cd in the specimen by multiplying this value by 25: the limit is 5 ppm.

Ascorbic acid–sodium iodide solution— Dissolve 20 g of ascorbic acid and 38.5 g of sodium iodide in water in a 200-mL volumetric flask, dilute with water to volume, and mix.

Trioctylphosphine oxide solution— [Caution—This solution causes irritation. Avoid contact with eyes, skin, and clothing. Take special precautions in disposing of unused portions of solutions to which this reagent is added. ] Dissolve 5.0 g of trioctylphosphine oxide in 4-methyl-2-pentanone in a 100-mL volumetric flask, dilute with the same solvent to volume, and mix.

Standard solution and Blank— Transfer 5.0 mL of Lead Nitrate Stock Solution, prepared as directed in the test for Heavy Metals 231, to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 2.0 mL of the resulting solution to a 50-mL volumetric flask. To this volumetric flask and to a second, empty 50-mL volumetric flask (Blank) add 10 mL of 9 N hydrochloric acid and about 10 mL of water. To each flask add 20 mL of Ascorbic acid–sodium iodide solution and 5.0 mL of Trioctylphosphine oxide solution, shake for 30 seconds, and allow to separate. Add water to bring the organic solvent layer into the neck of each flask, shake again, and allow to separate. The organic solvent layers are the Blank and the Standard solution, and they contain 0.0 µg and 2.0 µg of lead per mL, respectively.

Test solution— Add 1.0 g of Zinc Gluconate, 10 mL of 9 N hydrochloric acid, about 10 mL of water, 20 mL of Ascorbic acid–sodium iodide solution, and 5.0 mL of Trioctylphosphine oxide solution to a 50-mL volumetric flask, shake for 30 seconds, and allow to separate. Add water to bring the organic solvent layer into the neck of the flask, shake again, and allow to separate. The organic solvent layer is the Test solution.

Procedure— Concomitantly determine the absorbances of the Blank, Standard solution, and Test solution at the lead emission line at 283.3 nm, with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-Scattering 851) equipped with a lead hollow-cathode lamp and an air–acetylene flame, using the Blank to set the instrument to zero. In a suitable analysis, the absorbance of the Standard solution and the absorbance of the Blank are significantly different: the absorbance of the Test solution does not exceed that of the Standard solution (0.001%).