Ammonium Molybdate
(NH4)6Mo7O24·4H2O 1235.86

Molybdate (Mo7O246), hexaammonium, tetrahydrate.
Hexaammonium molybdate tetrahydrate [12054-85-2].
» Ammonium Molybdate contains not less than 99.3 percent and not more than 101.8 percent of (NH4)6Mo7O24·4H2O.
Packaging and storage— Preserve in tight containers.
Identification— Dissolve 0.6 g of it in 1.4 mL of water and 1.45 mL of ammonium hydroxide. Cool this mixture, and add slowly, with mixing, 7.2 mL of a well-cooled mixture of 3.2 mL of nitric acid and 4 mL of water. Allow to stand for 24 to 48 hours, and filter through a sintered-glass filter. To 5 mL of the filtrate add 2 mL of dibasic sodium phosphate TS: a yellow precipitate is formed, and it is soluble in an excess of 6 N ammonium hydroxide.
Insoluble substances— Dissolve 20 g in 200 mL of water in a beaker, heat to boiling, cover, and heat on a steam bath for 1 hour. Filter the hot solution through a tared filtering crucible, wash the insoluble residue with hot water, and dry at 105 for 2 hours: the weight of the residue so obtained does not exceed 1 mg (0.005%).
Chloride 221 A 0.5-g portion shows no more chloride than 0.30 mL of 0.001 N hydrochloric acid (0.002%).
Limit of nitrate— Dissolve 1 g in 10 mL of water containing 5 mg of sodium chloride, and add 0.10 mL of a 1 in 1000 solution of indigo carmine in 3.6 N sulfuric acid: the blue color is not completely discharged in 5 minutes.
Sulfate 221 A 0.25-g portion shows no more sulfate than corresponds to 1.0 mL of 0.001 N sulfuric acid (0.02%).
Arsenate, phosphate, and silicate— Dissolve 2.5 g in 70 mL of water in a container other than one of glass. Adjust with 1.2 N hydrochloric acid to a pH of between 3 and 4, transfer to a glass container, add 2 mL of bromine TS, and adjust with 1.2 N hydrochloric acid to a pH of 1.8 ± 0.1. Heat almost to boiling, and cool to room temperature. Dilute with water to 90 mL, add 10 mL of hydrochloric acid, and transfer to a separator. Add 1 mL of butyl alcohol and 30 mL of 4-methyl-2-pentanone, shake vigorously, and allow the phases to separate. Discard the aqueous phase, and wash the ketone phase with three successive 10-mL portions of 1.2 N hydrochloric acid, discarding the washings. To the washed ketone phase add 10 mL of 1.2 N hydrochloric acid to which has just been added 0.2 mL of a freshly prepared 1 in 50 solution of stannous chloride in hydrochloric acid. Similarly treat a control solution prepared by dissolving 0.5 g of specimen in 70 mL of water and adding 2 mL of sodium silicate solution (1 in 20,000): any blue color in the test solution does not exceed that in the control solution.
Phosphate— Dissolve 20 g in 100 mL of 3 N ammonium hydroxide, add 3.5 mL of ferric nitrate solution (1 in 10), and allow to stand for about 15 minutes. Warm gently to coagulate the precipitate, and filter. Wash the filter several times with 1.5 N ammonium hydroxide. Then wash the filter with 60 mL of warm 4 N nitric acid to dissolve the residue on the filter, collecting the filtrate in a glass-stoppered, 250-mL conical flask. Add 13 mL of ammonium hydroxide, warm to 40, add 50 mL of ammonium molybdate TS, shake for 5 minutes, and allow to stand at 40 for 2 hours. Similarly treat 100 mL of a Standard solution prepared by dissolving 143.3 mg of dried monobasic potassium phosphate in water to make 1000 mL, and then diluting 1.0 mL of this solution with 3 N ammonium hydroxide to 100 mL: any yellow precipitate formed from the test solution does not exceed that obtained from the Standard solution (5 ppm).
Magnesium and alkali salts— Dissolve 5.0 g in 50 mL of water, and filter. To the filtrate add 0.5 g of sodium carbonate and 25 mL of 2.5 N sodium hydroxide. Boil the solution gently for 5 minutes, cool, and filter through an ignited and tared filter. Wash the filter with 1 N ammonium hydroxide. Ignite the filter at 800 ± 25 for 30 minutes: the weight of the residue so obtained does not exceed 1 mg (0.02%).
Heavy metals— Dissolve 2.0 g in about 20 mL of water, add 10 mL of 2.5 N sodium hydroxide and 2 mL of ammonium hydroxide, and dilute with water to 40 mL. To 10 mL of this stock solution add 1.0 mL of Standard Lead Solution, prepared as directed under Heavy Metals 231, and dilute with water to 40 mL (Control solution). Dilute the remaining 30-mL portion of the stock solution with water to 40 mL (Test solution). To both solutions add 1.2 mL of thioacetamide-glycerin base TS and 2 mL of pH 3.5 Acetate Buffer, and allow to stand for 5 minutes: any color in the Test solution does not exceed that in the Control solution. The limit is 10 µg per g.
Assay— Dissolve about 1 g of Ammonium Molybdate, accurately weighed, in a mixture of 10 mL of water and 1 mL of ammonium hydroxide in a 250-mL volumetric flask, dilute with water to volume, and mix. Filter the solution, and transfer 50.0 mL of the filtrate to a 600-mL beaker. Add 250 mL of water, 20 g of ammonium chloride, 15 mL of hydrochloric acid, and 0.15 mL of methyl orange TS, heat nearly to boiling, and add 18 mL of lead acetate TS. To the hot solution add, slowly and with constant stirring, a saturated solution of ammonium acetate until the color turns yellow, and then add 15 mL of lead acetate TS. Cover the beaker, and heat just below the boiling temperature until the precipitate has settled. Filter through a tared, porous porcelain crucible, wash with seven or eight successive portions of a mixture of water, saturated ammonium acetate solution, and nitric acid (890:100:10), and finally wash with three successive portions of hot water. Ignite to constant weight at 560 to 625. Each mg of the lead molybdate so obtained is equivalent to 0.4809 mg of (NH4)6Mo7O24·4H2O.
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