Test solution— Use the Assay preparation.

Procedure— The Test solution gives a positive test for chromium, determined at 357.9 nm using an atomic absorption spectrophotometer as directed for Procedure in the Assay.

Standard stock preparation 1— Transfer about 2.829 g of potassium dichromate, previously dried at 120 for 4 hours and accurately weighed, to a 1000-mL volumetric flask, dissolve in and dilute with water to volume, and mix to obtain a solution having a known concentration of about 1000 µg of chromium per mL. Store in a polyethylene bottle.

Standard stock preparation 2— Transfer 10.0 mL of Standard stock preparation 1 to a 1000-mL volumetric flask, add 50.0 mL of 6 N hydrochloric acid, dilute with water to volume, and mix to obtain a solution having a known concentration of about 10 µg of chromium per mL.

Standard preparations— Transfer 10.0 mL and 20.0 mL of Standard stock preparation 2 to separate 100-mL volumetric flasks, and transfer 15.0 mL and 20.0 mL of Standard stock preparation 2 to separate 50-mL volumetric flasks. Dilute the contents of each of the four flasks with 0.125 N hydrochloric acid to volume, and mix to obtain solutions having known concentrations of about 1.0, 2.0, 3.0, and 4.0 µg of chromium per mL.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 5 Tablets, to a porcelain crucible, heat the crucible in a muffle furnace maintained at about 550 for 6 to 12 hours, and cool. Add about 60 mL of hydrochloric acid, and boil gently on a hot plate or steam bath for 30 minutes, intermittently rinsing the inner surface of the crucible with 6 N hydrochloric acid. Cool, and quantitatively transfer the contents of the crucible to a 100-mL volumetric flask. Rinse the crucible with small portions of 6 N hydrochloric acid, and add the rinsings to the flask. Dilute with water to volume, mix, and filter, discarding the first 5 mL of the filtrate. Dilute this solution quantitatively, and stepwise if necessary, with 0.125 N hydrochloric acid to obtain a solution having a concentration of about 1 µg of chromium per mL.

Procedure— Concomitantly determine the absorbances of the Standard preparations and the Assay preparation at the chromium emission line at 357.9 nm with an atomic absorption spectrophotometer (see Spectrophotometry and Light-Scattering 851) equipped with a chromium hollow-cathode lamp and an air–acetylene flame, using 0.125 N hydrochloric acid as the blank. Plot the absorbances of the Standard preparations versus concentration, in µg per mL, of chromium, and draw the straight line best fitting the four plotted points. From the graph so obtained, determine the concentration, C, in µg per mL, of chromium in the Assay preparation. Calculate the quantity, in µg, of chromium (Cr) in the portion of Tablets taken by the formula: in which C is as defined above; and D is the dilution factor used to prepare the Assay preparation.