Monograph pubs.acs.org/doi/book/10.1021/acsreagents
Sodium Dichromate Dihydrate 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 Sodium Dichromate Dihydrate provides, in addition to common physical constants, a general description including typical appearance, applications, change in state (approximate), 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 Matter, Chloride, Sulfate, Calcium, Magnesium, Potassium, and Aluminum.
Na2Cr2O7 • 2H2O
Formula Wt 298.00
CAS No. 7789-12-0
GENERAL DESCRIPTION Typical appearance . . . . . . . . . . . . . . . . . . . orange solid Applications . . . . . . . . . . . . . . . . . . . . . . . . preparation of titrant in redox titrations Change in state (approximate) . . . . . . . . . . . . becomes anhydrous on prolonged heating at 100 °C; melting point, anhydrous salt, 357 °C Aqueous solubility . . . . . . . . . . . . . . . . . . . . 180 g in 100 mL
SPECIFICATIONS Assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99.5–100.5% Na2Cr2O7 · 2H2O Maximum Allowable Insoluble matter . . . . . . . . . . . . . . . . . . . . . 0.005% Chloride (Cl) . . . . . . . . . . . . . . . . . . . . . . . 0.005% Sulfate (SO4) . . . . . . . . . . . . . . . . . . . . . . . 0.01% Calcium (Ca) . . . . . . . . . . . . . . . . . . . . . . . 0.003% Magnesium (Mg) . . . . . . . . . . . . . . . . . . . . . 0.005% Potassium (K) . . . . . . . . . . . . . . . . . . . . . . . 0.01% Aluminum (Al) . . . . . . . . . . . . . . . . . . . . . . 0.002%
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4344 ACS Reagent Chemicals, Part 4
ACS Reagent Chemicals
Monograph
pubs.acs.org/doi/book/10.1021/acsreagents
TESTS Assay (By titration of oxidizing power). Weigh accurately about 0.2 g, and dissolve in 200 mL of water. Add 7 mL of hydrochloric acid and 3 g of potassium iodide, mix, and allow to stand in the dark for 10 min. Titrate the liberated iodine with 0.1 N sodium thiosulfate volumetric solution, using starch indicator solution near the end point, to a greenish blue color. One milliliter of 0.1 N sodium thiosulfate corresponds to 0.004967 g of Na2Cr2O7 · 2H2O.
Insoluble Matter [Part 2: Gravimetric Methods; Insoluble Matter]. Dissolve 20.0 g in 200 mL of water.
Chloride Dissolve 0.2 g in 10 mL of water, filter if necessary through a chloride-free filter, and add 1 mL of ammonium hydroxide and 1 mL of silver nitrate reagent solution. Prepare a standard containing 0.01 mg of chloride ion (Cl) in 10 mL of water, and add 1 mL of ammonium hydroxide and 1 mL of silver nitrate reagent solution. Add 2 mL of nitric acid to each. The comparison is best made by the general method for chloride in colored solutions, [Part 2: Colorimetry and Turbidimetry; Chloride].
Sulfate Dissolve 10 g in 250 mL of water, filter if necessary, and heat to boiling in a dish. Add 25 mL of a solution containing 1 g of barium chloride and 2 mL of hydrochloric acid in 100 mL of solution. Digest in a covered beaker on a hot plate (~100 °C) for 2 h, and allow to stand for at least 8 h. If a precipitate forms, filter, wash thoroughly, and ignite. Fuse the residue with 1 g of sodium carbonate. Extract the fused mass with water, and filter off the insoluble residue. Add 5 mL of hydrochloric acid to the filtrate, dilute with water to about 200 mL, heat to boiling, and add 10 mL of ethyl alcohol. Digest in a covered beaker on the hot plate until the reduction of chromate is complete, as indicated by the change to a clear green or colorless solution. Neutralize the solution with ammonium hydroxide, and add 2 mL of hydrochloric acid. Heat to boiling, add 10 mL of 12% barium chloride reagent solution, digest in a covered beaker on a hot plate (~100 °C) for 2 h, and allow to stand for at least 8 h. Filter, wash thoroughly, and ignite. Correct for the weight obtained in a complete blank test. If the original precipitate of barium sulfate weighs less than the requirement permits, the fusion with sodium carbonate is not necessary.
Calcium, Magnesium, and Potassium (By flame AAS, [Part 2: Trace and Ultratrace Elemental Analysis; Atomic Absorption Spectroscopy; Analysis; Procedure for Flame AAS]).
For the Determination of Calcium, Magnesium, and Potassium
S a m p l e S t o c k S o l u t i o n . Dissolve 5.0 g in 50 mL of water, transfer to a 100 mL volumetric flask, and dilute to the mark with water (1 mL = 0.05 g).
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4344 ACS Reagent Chemicals, Part 4
ACS Reagent Chemicals
Monograph
pubs.acs.org/doi/book/10.1021/acsreagents
For the Determination of Calcium, Magnesium, and Potassium
Element
Wavelength (nm)
Sample Wt (g)
Standard Added (mg)
Flame Type*
Background Correction
Ca
422.7
1.0
0.03; 0.06
N/A
No
Mg
285.2
0.20
0.005; 0.01
A/A
Yes
K
766.5
0.20
0.01; 0.02
N/A
No
*A/A is air/acetylene; N/A is nitrous oxide/acetylene.
Aluminum Dissolve 20.0 g in 140 mL of water, filter, and add 5 mL of glacial acetic acid to the filtrate. Make alkaline with ammonium hydroxide. Digest for 2 h on a hot plate (~100 °C), filter, wash, ignite, and weigh.
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4344 ACS Reagent Chemicals, Part 4
