Water, Method I 
921
(where labeled as the dried form):
between 3.0% and 9.0%, the determination being performed by stirring the mixture containing the
Test preparation, maintained at a temperature of 50
, for 30 minutes before titrating with the
Reagent: where labeled as the dihydrate it is between 6.0% and 9.0%.
Limit of lead—
[NOTE—For the preparation of all aqueous solutions and for the rinsing of glassware before use, employ water that has been passed through a strong-acid, strong-base, mixed-bed ion-exchange resin before use. Select all reagents to have as low a content of lead as practicable, and store all reagent solutions in containers of borosilicate glass. Cleanse glassware before use by soaking in warm 8 N nitric acid for 30 minutes and by rinsing with deionized water.
]
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 Manganese 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%).
Reducing substances—
Transfer 1.0 g to a 250-mL conical flask, dissolve in 10 mL of water, and add 25 mL of
alkaline cupric citrate TS. Cover the flask, boil gently for 5 minutes, accurately timed, and cool rapidly to room temperature. Add 25 mL of 0.6 N acetic acid, 10.0 mL of 0.1 N iodine VS, and 10 mL of 3 N hydrochloric acid, and titrate with 0.1 N sodium thiosulfate VS, adding 3 mL of
starch TS as the endpoint is approached. Perform a blank determination, omitting the specimen, and note the difference in volumes required. Each mL of the difference in volume of 0.1 N sodium thiosulfate consumed is equivalent to 2.7 mg of reducing substances (as dextrose): the limit is 1.0%.
Assay—
Dissolve about 700 mg of Manganese Gluconate, accurately weighed, in 50 mL of water. Add 1 g of ascorbic acid, 10 mL of ammonia-ammonium chloride buffer TS, and 0.1 mL of
eriochrome black TS, and titrate with 0.05
M edetate disodium VS until the solution is deep blue in color. Each mL of 0.05
M edetate disodium is equivalent to 22.26 mg of C
12H
22MnO
14.
;)
