Arsenic Trioxide - ACS Reagent Chemicals (ACS Publications)

Monograph pubs.acs.org/doi/book/10.1021/acsreagents

Arsenic Trioxide (Arsenic(III) Oxide, Reductometric Standard)

Downloaded by CORNELL UNIV on June 17, 2017 | http://pubs.acs.org Publication Date (Web): February 28, 2017 | doi: 10.1021/acsreagents.4041

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 Arsenic Trioxide 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, Residue after Ignition, Insoluble in Dilute Hydrochloric Acid, Chloride, Sulfide, Antimony, Lead, and Iron. Special caution is advised for the handling or testing of this substance.

As2O3

Formula Wt 197.84

CAS No. 1327-53-3

GENERAL DESCRIPTION Typical appearance . . . . . . . . Applications . . . . . . . . . . . . . Change in state (approximate) . Aqueous solubility . . . . . . . . .

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white solid reductometric standard melting point, 655 °C crystalline forms, 2 g in 100 mL at 20 °C; amorphous forms, 3.3 g in 100 mL at 20 °C

SPECIFICATIONS Assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99.95–100.05% As2O3 Maximum Allowable Residue after ignition. . . . . . . . . . . . . . . . . . 0.02% Insoluble in dilute hydrochloric acid . . . . . . . . 0.01% Chloride (Cl) . . . . . . . . . . . . . . . . . . . . . . . 0.005% Sulfide (S) . . . . . . . . . . . . . . . . . . . . . . . . . Passes test Antimony (Sb) . . . . . . . . . . . . . . . . . . . . . . 0.05% Lead (Pb) . . . . . . . . . . . . . . . . . . . . . . . . . 0.002% Iron (Fe) . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ppm

© 2017 American Chemical Society

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ACS Reagent Chemicals ACS Reagent Chemicals; American Chemical Society: Washington, DC, 2017.

DOI:10.1021/acsreagents.4041 ACS Reagent Chemicals, Part 4

ACS Reagent Chemicals

Monograph

pubs.acs.org/doi/book/10.1021/acsreagents

TESTS Assay (By oxidation–reduction titration of arsenic). Place 2.00 g of sample in a weighing bottle, dry to constant weight at 110 C, and cool for 2 hr in a desiccator. Accurately weighed portions of the arsenic trioxide are reacted with accurately weighed portions of the NIST potassium dichromate of such size as to provide a small excess of the latter. The excess potassium dichromate is determined by titration with standardized ferrous ammonium sulfate solution, using a potentiometrically determined end point.


C a u t i o n : Because of the small tolerance in assay limits for this reductometric standard, extreme care must be observed in the weighing, transferring, and titrating operations in the following procedure. Strict adherence to the specified sample weights and final titration volumes is absolutely necessary. It is recommended that the titrations be run at least in duplicate. Duplicate values for the assay of this material should agree within 2 parts in 5000 to be acceptable for averaging.

Pr o c e d u r e . Place 2.00 g of sample in a weighing bottle, dry for 1 h at 105 °C, and cool for 2 h in a desiccator. Weigh accurately to within 0.1 mg, and transfer to a 400 mL beaker. Weigh the bottle again, and determine by difference to the nearest 0.1 mg the weight of arsenic trioxide. Weigh accurately to within 0.1 mg an amount of NIST potassium dichromate equivalent to a slight excess of the weight of arsenic trioxide taken, and transfer it to a 50 mL beaker. Dissolve the arsenic trioxide in 20 mL of 20% sodium hydroxide solution, add 100 mL of water and 10 mL of dilute sulfuric acid (1:1), and stir. Transfer the potassium dichromate to the solution, using water to loosen any crystals adhering to the beaker. Stir until all of the potassium dichromate is dissolved, and let stand for 10 min. Add 20 mL of dilute sulfuric acid (1:1), dilute with water to 300 mL, and stir. Titrate the excess potassium dichromate with standardized 0.02 N ferrous ammonium sulfate solution (described below), using a platinum indicator electrode and a calomel reference electrode for the measurement of the potential difference in millivolts. The end point of the titration is determined potentiometrically, using the second derivative method [Part 2: Titrimetric Methods; Potentiometric Titrations; Second Derivative Method for Determination of Equivalence Point]. A correction for “blank” oxidants or reductants must be applied by titrating an accurately weighed 40-mg portion of the NIST potassium dichromate with the standardized ferrous ammonium sulfate in an equal volume of solution containing the quantities of reagents used in this assay. The “blank” is equal to the calculated milliequivalents of potassium dichromate minus the calculated milliequivalents of ferrous ammonium sulfate. A positive blank correction results from other oxidants and a negative correction from other reductants.

where M1 = number of milliequivalents of potassium dichromate equivalent to arsenic trioxide; W1 = weight of potassium dichromate in milligrams; V = volume of ferrous ammonium sulfate solution in milliliters; N = normality of ferrous ammonium sulfate solution in equivalents per liter; B = blank correction in milliequivalents (may be either positive or negative); and F = assay value of the NIST standard in percent divided by 100. (The equivalent formula weight of potassium dichromate is 49.0307, and the conversion factor for air to vacuum weight of potassium dichromate is 1.00032.) Calculate the assay value of the arsenic trioxide from

where W2 = weight of arsenic trioxide in milligrams and 1.00017 is the conversion factor for air to vacuum weight of arsenic trioxide.


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Transfer a 40-mg portion of the potassium dichromate, accurately weighed to within 0.1 mg, to a 400 mL beaker. Add 300 mL of dilute sulfuric acid (1:19), and stir to dissolve the salt. Titrate the solution with the 0.02 N ferrous ammonium sulfate solution, using a potentiometrically determined end point. The concentration of the ferrous ammonium sulfate, in equivalents per liter (N), is calculated from the following equation:


where N = normality of ferrous ammonium sulfate; W = weight of potassium dichromate in grams; and V = volume of ferrous ammonium sulfate solution in milliliters. F e r r o u s A m m o n i u m S u l f a t e S o l u t i o n , 0 . 0 2 N . Weigh 8.0 g of ferrous ammonium sulfate hexahydrate, and transfer to a 1 L flask. Add 200 mL of dilute sulfuric acid (1:19), and after the salt is dissolved, dilute to the mark with the dilute sulfuric acid. Mix thoroughly.

Residue after Ignition [Part 2: Gravimetric Methods; Residue after Ignition]. Ignite 5.0 g in a tared, preconditioned platinum dish. Retain the residue for the test for iron.

Insoluble in Dilute Hydrochloric Acid To 10 g, add 90 mL of dilute hydrochloric acid (3:7) and 10 mL of hydrogen peroxide. Cover with a watch glass and allow to stand until the sample goes into solution, heating if necessary. Heat to boiling, digest in a covered beaker on a hot plate (~100 °C) for 1 h, and filter through a tared, preconditioned filtering crucible. Retain the filtrate for the antimony and lead test, wash the insoluble matter thoroughly, and dry at 105 °C.

Chloride [Part 2: Colorimetry and Turbidimetry; Chloride]. Dissolve 1.0 g in 10 mL of dilute ammonium hydroxide (1:2) with the aid of gentle heating, and dilute with water to 100 mL. Neutralize 20 mL with nitric acid.

Sulfide Dissolve 1.0 g in 10 mL of 10% sodium hydroxide reagent solution, and add 0.05 mL of 10% lead acetate reagent solution. The color should be the same as that of an equal volume of sodium hydroxide solution to which only the lead acetate is added. (Limit about 0.001%.)

Antimony and Lead (By flame AAS, [Part 2: Trace and Ultratrace Elemental Analysis; Atomic Absorption Spectroscopy; Analysis; Procedure for Flame AAS]).

For the Determination of Antimony and Lead

S a m p l e S t o c k S o l u t i o n . Dilute the filtrate retained from the insoluble in dilute hydrochloric acid test with water to the mark in a 200 mL volumetric flask (1 mL = 0.05 g).


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For the Determination of Antimony and Lead

Element

Wavelength (nm)

Sample Wt (g)

Standard Added (mg)

Flame Type*

Background Correction

Sb

217.6

1.0

0.5; 1.0

A/A

Yes

Pb

217.0

1.0

0.05; 0.10

A/A

Yes

*A/A is air/acetylene.

Iron


[Part 2: Colorimetry and Turbidimetry; Iron; Procedure for Iron, Method 1 (Ammonium Thiocyanate)]. To the residue from the test for residue after ignition, add 3 mL of dilute hydrochloric acid (1:1), and warm. Add 5 mL of hydrochloric acid, dilute with water to 125 mL, and use 50 mL of this solution without further acidification.


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Arsenic Trioxide - ACS Reagent Chemicals (ACS Publications)

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