Ammonium molybdate reagent— Dissolve 6.4 g of ammonium molybdate in 40 mL of water, and add 50 mL of 10 N sulfuric acid. Mix, and dilute with water to 100 mL. This solution is stable for about 2 weeks.

Stannous chloride reagent— Dissolve 1 g of stannous chloride in 5 mL of hydrochloric acid. Just prior to use, dilute 1 mL of this solution with water to 100 mL.

Standard preparation— Weigh accurately about 275 mg of anhydrous dibasic sodium phosphate (Na2HPO4) into a 1000-mL volumetric flask. Dissolve in and dilute with water to volume. Dilute 10 mL of this solution with water to 100 mL.

Test preparation— Pipet 1 mL of the well-shaken Injectable Suspension into a 25-mL volumetric flask, add 0.1 mL of 10 N sulfuric acid, mix, dilute with water to volume, and again mix.

Procedure— Into separate 100-mL volumetric flasks pipet duplicate 10-mL portions of Test preparation, duplicate 10-mL portions of Standard preparation, and 10 mL of water to provide a blank. Treat each flask as follows. Dilute the contents with water to 60 mL, then add 10 mL of 10 N sulfuric acid, and mix. Add 10 mL of Ammonium molybdate reagent, mix, add 10 mL of water, and again mix. Add slowly, with mixing, 5 mL of Stannous chloride reagent, dilute with water to volume, and mix. Measure the absorbances of the solutions at 10 minutes, accurately timed, after the first addition of the Stannous chloride reagent, at a wavelength of 710 nm, with a suitable spectrophotometer, relative to the blank. Calculate the quantity, in µg, of anhydrous dibasic sodium phosphate in the portion of Injectable Suspension taken by the formula: in which C is the concentration, in µg per mL, of Na2HPO4 in the Standard preparation; and AU and AS are the absorbances of the solutions from the Test preparation and the Standard preparation, respectively.