Monograph pubs.acs.org/doi/book/10.1021/acsreagents
Molybdic Acid, 85% Part 4, Monographs for Reagent Chemicals: General Descriptions, Specifications, and Tests eISBN: 9780841230460 Tom Tyner Chair, ACS Committee on Analytical Reagents James Francis Secretary, ACS Committee on Analytical Reagents
ABSTRACT This monograph for Molybdic Acid, 85% provides, in addition to common physical constants, a general description including typical appearance, applications, and aqueous solubility. The monograph also details the following specifications and corresponding tests for verifying that a substance meets ACS Reagent Grade specifications including: Assay, Insoluble in Dilute Ammonium Hydroxide, Chloride, Arsenate, Phosphate, Silicate, Phosphate, Sulfate, and Heavy Metals.
CAS No. 7782-91-4
Note: This reagent consists largely of an ammonium molybdate.
GENERAL DESCRIPTION Typical appearance . . . . . . . . . . . . . . . . . . . white or slightly yellowish solid Applications . . . . . . . . . . . . . . . . . . . . . . . . determination of phosphate Aqueous solubility . . . . . . . . . . . . . . . . . . . . slightly soluble
SPECIFICATIONS Assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . ≥85.0% MoO3 Maximum Allowable Insoluble in dilute ammonium hydroxide. . . . . . 0.01% Chloride (Cl) . . . . . . . . . . . . . . . . . . . . . . . 0.002% Arsenate phosphate, and silicate (as SiO2) . . . . 0.001% Phosphate (PO4) . . . . . . . . . . . . . . . . . . . . . 5 ppm Sulfate (SO4) . . . . . . . . . . . . . . . . . . . . . . . 0.2% Heavy metals (as Pb) . . . . . . . . . . . . . . . . . . 0.003%
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4232 ACS Reagent Chemicals, Part 4
ACS Reagent Chemicals
Monograph
pubs.acs.org/doi/book/10.1021/acsreagents
TESTS Assay (By precipitation titration of molybdate). Weigh accurately about 0.6 g, and dissolve in 100 mL of 2.5% ammonium hydroxide reagent solution. Adjust the pH of the solution to 4.0 (using a pH meter) with 10% nitric acid reagent solution. Add saturated hexamethylenetetramine reagent solution to a pH of 5–6. Heat the solution to 60 °C, and titrate with 0.1 M lead nitrate volumetric solution. Using 0.1% PAR indicator solution, titrate from yellow to the first permanent pink end point. One milliliter of 0.1 M lead nitrate corresponds to 0.01439 g of MoO3.
Insoluble in Dilute Ammonium Hydroxide Dissolve 10 g in 100 mL of dilute ammonium hydroxide (1:9), heat to boiling, and digest in a covered beaker on a hot plate (~100 °C) for 2 h. Filter through a tared, preconditioned filtering crucible, wash thoroughly, and dry at 105 °C.
Chloride Dissolve 1.0 g in 4 mL of ammonium hydroxide, heat on a hot plate (~100 °C) until completely dissolved, and evaporate to dryness. Dissolve the residue in 20 mL of water, add 4 mL of nitric acid, and dilute with water to 30 mL. To 15 mL, add 1 mL of silver nitrate reagent solution. Any turbidity should not exceed that produced by 0.01 mg of chloride ion (Cl) in an equal volume of solution containing the quantities of reagents used in the test. The comparison is best made by the general method for chloride in colored solutions [Part 2: Colorimetry and Turbidimetry; Chloride].
Arsenate, Phosphate, and Silicate Dissolve 2.5 g in 60 mL of water and enough silica-free ammonium hydroxide to effect dissolution (about 4 mL) in a platinum dish. For the control, dissolve 0.5 g in 60 mL of water and enough silica-free ammonium hydroxide to effect dissolution, and add 0.02 mg of silica (SiO2). Heat on a hot plate (~100 °C) until the solutions are neutral as determined with an external indicator (at least 30 min). Cool, and adjust the pH to between 3 and 4 with an external indicator. Transfer to beakers, and dilute with water to 80 mL. Add enough bromine water to impart a distinct yellow color to the solutions. Adjust the pH of each solution to 1.8 with dilute hydrochloric acid (1:9) (using a pH meter). Heat just to boiling, and allow to cool to room temperature. (If a precipitate forms, it will dissolve when the solution is acidified in the next operation.) Add 10 mL of hydrochloric acid, and dilute with water to 100 mL. Transfer the solutions to separatory funnels, and add 30 mL of 4-methyl2-pentanone and 1 mL of butyl alcohol. Shake vigorously, and allow to separate. Draw off and discard the aqueous phase. Wash the ketone phase three times with 10 mL portions of dilute hydrochloric acid (1:99), discarding the aqueous phase each time. To the washed ketone phase, add 10 mL of dilute hydrochloric acid (1:99), to which has just been added 0.2 mL of a freshly prepared 2% stannous chloride reagent solution. Any blue color in the solution of the sample should not exceed that in the control.
Phosphate [Part 2: Colorimetry and Turbidimetry; Phosphate; Procedure for Phosphate, Method 3 (Precipitation)]. Use a 20 g sample.
Sulfate Boil 1.0 g with 15 mL of dilute nitric acid (1:2) for 5 min. Cool thoroughly, dilute with water to 100 mL, and filter. Evaporate 2 mL of the filtrate to dryness on a hot plate (~100 °C), dissolve the residue in 4 mL of water plus 1 mL of dilute hydrochloric acid (1:19), and filter if necessary through a small filter. Wash with two 2 mL portions of water, dilute with water to 10 mL, and add 1 mL of 12% barium chloride reagent solution. Any turbidity should not exceed that produced by 0.04 mg of sulfate
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4232 ACS Reagent Chemicals, Part 4
ACS Reagent Chemicals
Monograph
pubs.acs.org/doi/book/10.1021/acsreagents
ion (SO4) in an equal volume of solution containing the quantities of reagents used in the test. Compare 10 min after adding the barium chloride to the sample and standard solutions.
Heavy Metals Dissolve 1.3 g in 15 mL of 10% sodium hydroxide reagent solution, add 2 mL of ammonium hydroxide, and dilute with water to 40 mL. For the control, add 0.02 mg of lead ion (Pb) to 10 mL of the solution, and dilute with water to 40 mL. For the sample, dilute the remaining portion with water to 40 mL. Add 5 mL of freshly prepared hydrogen sulfide water to each, and mix. Any color in the solution of the sample should not exceed that in the control.
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4232 ACS Reagent Chemicals, Part 4
