Sodium chloride solution— Dissolve 450 mg of sodium chloride in water, dilute with water to 500 mL, and mix.

Standard preparations— Transfer 3.11 g of zinc oxide, accurately weighed, to a 250-mL volumetric flask, add 80 mL of 1 N sulfuric acid, warm to dissolve, cool, dilute with water to volume, and mix. This stock solution contains 10.0 mg of zinc per mL. Dilute an accurately measured volume of this solution quantitatively with water to obtain a solution containing 125 µg of zinc per mL. Transfer 2.0, 3.0, and 4.0 mL, respectively, of this solution to three separate 500-mL volumetric flasks, each containing 5 mL of Sodium chloride solution, dilute the contents of each flask with water to volume, and mix. These Standard preparations contain, respectively, 0.50, 0.75, and 1.0 µg of zinc per mL.

Assay preparation— Transfer an accurately measured volume of Injection, equivalent to about 5 mg of zinc, to a 500-mL volumetric flask, dilute with water to volume, and mix. Transfer 10.0 mL of this solution to a 100-mL volumetric flask. From the labeled amount of sodium chloride, if any, in the Injection, calculate the amount, in mg, of sodium chloride in the 10.0-mL portion and add sufficient Sodium chloride solution to bring the total sodium chloride content of the 100-mL volumetric flask to 0.9 mg. Dilute with water to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparations and the Assay preparation at the zinc emission line at 213.8 nm, with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering 851) equipped with a zinc hollow-cathode lamp and an air–acetylene flame, using water as the blank. Plot the absorbances of the Standard preparations versus the concentration, in µg per mL, of zinc, and draw the straight line best fitting the three plotted points. From the graph so obtained, determine the concentration, in µg per mL, of zinc in the Assay preparation. Calculate the quantity, in mg, of zinc in each mL of the Injection taken by the formula: in which C is the concentration, in µg per mL, of zinc in the Assay preparation; and V is the volume, in mL, of Injection taken.