A: Dissolve 5 g of Sodium Sulfite in 100 mL of water: the pH of the solution is between 8.0 and 10.0. [NOTE—Reserve portions of the solution so obtained for use in Identification test B and in the test for Color and clarity of solution.]

B: To 5 mL of the solution from Identification test A, add 0.5 mL of iodine TS: the solution is colorless and it meets the requirements of the barium chloride test for Sulfate 191.

C: It meets the requirements of the pyroantimonate precipitate test for Sodium 191.

Standard solution— Immediately before use, dilute 1 volume of Standard Iron Solution, prepared as directed under Iron 241, to 10 mL with water. [NOTE—This solution contains the equivalent of 1 µg of iron per mL.]

Test solution— To 10.0 g of Sodium Sulfite add 25 mL of water. Shake until mostly dissolved, and carefully and slowly add 15 mL of hydrochloric acid. Heat to boiling. Cool, and dilute with water to 100.0 mL. Use a 10-mL portion.

Procedure— To the Standard solution and the Test solution separately add 2 mL of a citric acid solution having a concentration of 200 g per L, and then add 0.1 mL of thioglycolic acid. Mix, make alkaline with stronger ammonia water, and dilute with water to 20 mL. Allow to stand for 5 minutes. Any pink color in the Test solution is not more intense than that in the Standard solution: not more than 10 µg per g is found.

Selenium standard solution— [Caution—Selenium is toxic; handle it with care. ] Dissolve about 0.1 g of metallic selenium, accurately weighed, in 2 mL of nitric acid. Evaporate to dryness, add 2 mL of water, and evaporate to dryness. Repeat the addition of water and the evaporation to dryness three more times. Dissolve the residue so obtained in 50 mL of diluted hydrochloric acid, transfer to a 1000-mL volumetric flask, dilute with diluted hydrochloric acid to volume, and mix to obtain a solution having a known concentration of about 100 µg of selenium per mL.

Procedure— Prepare a test solution as follows. To 3.0 g of Sodium Sulfite add 10 mL of formaldehyde TS, and slowly and carefully add 2 mL of hydrochloric acid. Heat in a water bath for 20 minutes. In parallel prepare a Standard solution as follows. To 1.0 g of Sodium Sulfite add 0.2 mL of Selenium standard solution and 10 mL of formaldehyde TS, and slowly and carefully add 2 mL of hydrochloric acid. Heat in a water bath for 20 minutes. Any pink color in the test solution is not more intense than that in the Standard solution: not more than 10 µg per g is found.

Zinc standard stock solution— Prepare a solution containing 1 mL of acetic acid and the amount of zinc sulfate equivalent to 0.440 g of ZnSO4·7H2O in 100.0 mL of water. [NOTE—This solution contains the equivalent of 1000 µg of Zn per mL.]

Zinc standard solutions— Dilute a volume of the Zinc standard stock solution quantitatively, and stepwise if necessary, with water to obtain a solution having a known concentration of about 25 µg of zinc per mL. Transfer 1.0-mL, 2.0-mL, and 4.0-mL portions of this solution to separate 100-mL volumetric flasks. Dilute the contents of each flask with water to volume, and mix to obtain solutions having known concentrations of about 0.25 µg of zinc per mL, 0.5 µg of zinc per mL, and 1.0 µg of zinc per mL, respectively.

Test solution— To 10.0 g of Sodium Sulfite add 25 mL of water. Shake until mostly dissolved, and carefully and slowly add 15 mL of hydrochloric acid. Heat to boiling. Cool, and dilute with water to 100.0 mL. Transfer 2.0 mL of this solution to a 10-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Zinc standard solutions and the Test solution at the zinc emission line at 213.9 nm with an atomic absorption spectrophotometer (see Spectrophotometry and Light-Scattering 851), equipped with a zinc hollow-cathode lamp and an air–acetylene flame. Plot the absorbances of the Zinc standard solutions versus concentration of zinc, in µg per mL, and draw the straight line best fitting the plotted points. From the graph so obtained, determine the concentration of zinc, in µg per mL, in the Test solution: not more than 25 µg per g is found.

(Official January 1, 2007)