Monograph pubs.acs.org/doi/book/10.1021/acsreagents
Potassium Dichromate 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 Potassium Dichromate 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, Loss on Drying, Chloride, Sulfate, Calcium, Iron, and Sodium.
K2Cr2O7
Formula Wt 294.18
CAS No. 7778-50-9
GENERAL DESCRIPTION Typical appearance . . . . . . . . Applications . . . . . . . . . . . . . Change in state (approximate) . Aqueous solubility . . . . . . . . .
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orange-red solid oxidizing agent; redox standard melting point, 398 °C 12.3 g in 100 mL at 20 °C
SPECIFICATIONS Assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . ≥99.0% K2Cr2O7 Maximum Allowable Insoluble matter . . . . . . . . . . . . . . . . . . . . . 0.005% Loss on drying . . . . . . . . . . . . . . . . . . . . . . 0.05% Chloride (Cl) . . . . . . . . . . . . . . . . . . . . . . . 0.001% Sulfate (SO4) . . . . . . . . . . . . . . . . . . . . . . . 0.005% Calcium (Ca) . . . . . . . . . . . . . . . . . . . . . . . 0.003% Iron (Fe) . . . . . . . . . . . . . . . . . . . . . . . . . . 0.001% Sodium (Na) . . . . . . . . . . . . . . . . . . . . . . . . 0.02%
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4286 ACS Reagent Chemicals, Part 4
ACS Reagent Chemicals
Monograph
pubs.acs.org/doi/book/10.1021/acsreagents
TESTS Assay (By iodometric oxidation–reduction titration). Weigh, to the nearest 0.1 mg, about 0.2 g of sample, transfer to a 500 mL glass-stoppered iodine flask, and dissolve with 125 mL of water. Add 5 g of potassium iodide and 5 mL of 6 M hydrochloric acid with constant swirling. Carefully wash down the sides of the flask with water so that a layer of water is formed on top of the solution, stopper the flask, and allow to stand in the dark for 10 min. Add about 160 mL of water, and titrate with 0.1 N sodium thiosulfate volumetric solution, adding 5 mL of starch indicator solution near the end point. One milliliter of 0.1 N sodium thiosulfate corresponds to 0.004903 grams of K2Cr2O7.
Insoluble Matter [Part 2: Gravimetric Methods; Insoluble Matter]. Dissolve 20 g in 200 mL of water.
Loss on Drying Weigh accurately about 2 g, and dry in a tared, preconditioned container to constant weight at 105 °C.
Chloride Dissolve 1.0 g in 10 mL of water, filter if necessary through a small 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. Add 25 mL of a solution containing 1 g of barium chloride and 2 mL of hydrochloric acid per 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 is formed, 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 (~100 °C) until 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, and 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 of barium sulfate obtained in a complete blank test. If the original precipitate of barium sulfate weighs less than the specification allows, the fusion with sodium carbonate is not necessary.
Calcium, Iron, and Sodium (By flame AAS, [Part 2: Trace and Ultratrace Elemental Analysis; Atomic Absorption Spectroscopy; Analysis; Procedure for Flame AAS]).
© 2017 American Chemical Society
B
DOI:10.1021/acsreagents.4286 ACS Reagent Chemicals, Part 4
ACS Reagent Chemicals
Monograph
pubs.acs.org/doi/book/10.1021/acsreagents
For the Determination of Calcium, Iron, and Sodium
S a m p l e S t o c k S o l u t i o n f o r C a l c i u m a n d S o d i u m . Dissolve 5.0 g of sample in 20 mL of water, transfer the solution to a 100 mL volumetric flask, and dilute to the mark with water (1 mL = 0.05 g). S a m p l e S t o c k S o l u t i o n f o r I r o n . Dissolve 10.0 g of sample in 20 mL of water, transfer the solution to a 100 mL volumetric flask, and dilute to the mark with water (1 mL = 0.10 g).
For the Determination of Calcium, Iron, and Sodium
Element
Wavelength (nm)
Sample Wt (g)
Standard Added (mg)
Flame Type*
Background Correction
Ca
422.7
0.50
0.015; 0.03
N/A
No
Fe
248.3
2.0
0.04; 0.08
A/A
Yes
Na
589.0
0.05
0.01; 0.02
A/A
No
*A/A is air/acetylene; N/A is nitrous oxide/acetylene.
© 2017 American Chemical Society
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DOI:10.1021/acsreagents.4286 ACS Reagent Chemicals, Part 4
