A: Dissolve about 100 mg in 2 mL of water in a test tube, and add 1 mL of nitric acid: a brown precipitate of iodine is formed. Transfer 1 drop of this mixture to another test tube, add 1 mL of solvent hexane, and shake: the solvent hexane takes on a pink color.

B: Heat the remainder of the brown reaction mixture from Identification test A over a flame until a colorless solution remains (about 3 minutes). Cool, and dilute with water to about 10 mL: a 2-mL portion of this solution responds to the tests for Phosphate 191.

C: A 2-mL portion of the solution obtained in Identification test B responds to the tests for Sulfate 191.

(Official January 1, 2007)

pH 12 phosphate buffer— Transfer 5.44 g of anhydrous dibasic sodium phosphate to a 100-mL volumetric flask, and add a volume of 1 N sodium hydroxide VS that contains 36.5 mEq of sodium hydroxide. Add about 40 mL of water, shake to dissolve the sodium phosphate, dilute with water to volume, and mix.

0.004 N Iodine— Dilute 0.1 N iodine with water to 0.004 N, and standardize the solution on the day of use as follows. Weigh accurately about 150 mg of arsenic trioxide, and dissolve in 20 mL of 1 N sodium hydroxide, by warming if necessary, in a 500-mL volumetric flask. Dilute with 40 mL of water, add 2 drops of methyl orange TS, then add 3 N hydrochloric acid until the yellow color is changed to pink. Add 2 g of sodium bicarbonate, add water to volume, and mix. Transfer 5.0 mL of this solution to a titration vessel, and add 50 mL of pH 7 phosphate buffer (see under Solutions in the section Reagents, Indicators, and Solutions). Titrate with 0.004 N iodine, determining the endpoint potentiometrically, using platinum and silver-silver chloride electrodes. Calculate the normality. Each 197.8 µg of arsenic trioxide is equivalent to 1 mL of 0.004 N iodine.

Procedure— Dissolve about 125 mg of Echothiophate Iodide, accurately weighed, in about 50 mL of water in a 100-mL volumetric flask, add water to volume, and mix. Transfer 10.0 mL of this solution to a titration vessel containing 30 mL of water. Add 10.0 mL of pH 12 phosphate buffer, mix, cover, and allow to stand for 20 minutes at 25 ± 3. Add 2 mL of glacial acetic acid, rapidly and with mixing. Titrate with 0.004 N iodine, determining the endpoint potentiometrically, using platinum and silver-silver chloride electrodes. Correct for the amount of free thiol sulfur by repeating the procedure but adding the glacial acetic acid first, then the pH 12 phosphate buffer, and titrating immediately. Subtract the volume of 0.004 N iodine used in the second titration from that used in the first. Each mL of 0.004 N iodine is equivalent to 1.533 mg of C9H23INO3PS.