Spectrophotometry in the Visible Region 851—

Copper stock solution and Standard solutions—Prepare as directed in the test for Content of total copper.

Test solution— Transfer about 100 mg of Chlorophyllin Copper Complex Sodium, accurately weighed, to a 150-mL conical flask, add 75 mL of water, and shake by mechanical means for about 3 minutes. Adjust with 1 N hydrochloric acid to a pH of 3.0, transfer the suspension thus obtained to a 100-mL volumetric flask, dilute with water to volume, and mix. Filter this suspension, discarding the first 10 mL of the filtrate. Use the clear filtrate as the Test solution.

Procedure— Proceed as directed for Procedure in the test for Content of total copper. Calculate the percentage of ionic copper in the portion of Chlorophyllin Copper Complex Sodium taken by the formula: in which the terms are as defined therein: not more than 0.25% is found, calculated on the dried basis.

Copper stock solution— Transfer 1.000 g of copper to a 1000-mL volumetric flask, dissolve in 20 mL of nitric acid, dilute with 0.2 N nitric acid to volume, and mix. This solution contains 1000 µg of copper per mL. Store in a polyethylene bottle.

Standard solutions— Pipet 5.0 mL of Copper stock solution into a 500-mL volumetric flask, dilute with water to volume, and mix. Transfer 5.0, 10.0, 15.0, and 20.0 mL, respectively, of this solution to separate 100-mL volumetric flasks, dilute the contents of each flask with water to volume, and mix. These Standard solutions contain 0.5, 1.0, 1.5, and 2.0 µg of copper per mL, respectively.

Test solution— Transfer about 100 mg of previously dried Chlorophyllin Copper Complex Sodium, accurately weighed, to a Kjeldahl flask. Add 2.0 mL of sulfuric acid, 1.0 mL of nitric acid, and 1.0 mL of hydrogen peroxide, and carefully heat under a fume hood until a light green color is obtained. [note—If the solution has any hint of a brown tint, continue to add 0.5-mL portions of nitric acid until a green color is obtained.] Cool, transfer the contents quantitatively to a 1000-mL volumetric flask with several portions of water, dilute the contents of the flask with water to volume, and mix. Transfer 10.0 mL of this solution to a 50-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard solutions and the Test solution at the copper emission line of 324.8 nm with an atomic absorption spectrophotometer (see Spectrophotometry and Light-Scattering 851) equipped with a copper hollow-cathode lamp and an air–acetylene flame, using water as the blank. Plot the absorbances of the Standard solutions versus the concentration, in µg per mL, of copper, 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 copper in the Test solution. Calculate the percentage of copper in the portion of Chlorophyllin Copper Complex Sodium taken by the formula: in which W is the weight, in mg, on the dried basis, of Chlorophyllin Copper Complex Sodium taken to prepare the Test solution: not less than 4.25% is found.

Standard stock solution— Dissolve 254.2 mg of sodium chloride, previously dried at 105 for 2 hours, in 50 mL of water, transfer to a 1000-mL volumetric flask, dilute with water to volume, and mix. Each mL of this solution contains 100 µg of sodium.

Standard solutions— Transfer to each of five 100-mL volumetric flasks 10 mL of a solution of a nonionic wetting agent (1 in 500). Dilute the contents of one of the flasks with water to volume to provide a blank. To the remaining flasks add, respectively, 2.5, 5.0, 10.0, and 15.0 mL of the Standard stock solution, dilute with water to volume, and mix. These Standard solutions contain 2.5, 5.0, 10.0, and 15.0 µg of sodium per mL, respectively.

Test solution— Transfer about 100 mg of Chlorophyllin Copper Complex Sodium, accurately weighed, to a 1000-mL volumetric flask, add 100 mL of a solution of nonionic wetting agent (1 in 500) and 400 mL of water, and shake by mechanical means for about 5 minutes. Dilute with water to volume, and mix.

Standard graph— Set the flame photometer for maximum transmission at a wavelength of about 589 nm. Adjust the instrument to zero transmittance with the blank. Adjust the instrument to 100% transmittance with the most concentrated of the Standard solutions. Read the percentage of transmittance of the other Standard solutions, and plot the percentage of transmittance versus the concentration, in µg per mL, of sodium.

Procedure— Adjust the instrument as directed under Standard graph, read the percentage of transmittance of the Test solution, and from the Standard graph read the concentration, C, in µg per mL, of sodium in the Test solution. Calculate the percentage of sodium in the portion of Chlorophyllin Copper Complex Sodium taken by the formula: in which W is the weight, in mg, of Chlorophyllin Copper Complex Sodium taken to prepare the Test solution: between 5% and 7%, calculated on the dried basis, is found.