211_ARSENIC,USP31《砷盐检查》中英对照

211 ARSENIC

This procedure is designesigned to determine the presence of trace amo

arsenic (As) by convertingerting the arsenic in a substance under test to

which is then passed throu through a solution of silver diethyldithiocarbam

a red complex. The red coed color so produced is compared, either visu

spectrophotometrically, to ly, to the color produced similarly in a control c

an amount of arsenic equi equivalent to the limit given in the individual m

Limits are stated in terms erms of arsenic (As). The content of arsenic do

exceed the limit given in thn in the individual monograph.

Two methods are providedovided, the methods differing only in the prelim

treatment of the test substsubstance and the standard. Generally, Metho

for inorganic materials, whs, while Method II is used for organic mater

Apparatus—

The apparatus (see ) consists of an arsine generator (

scrubber unit (c) and an ab an absorber tube (e) with standard-taper or gr

ball-and-socket joints (b and d) between the units. However, any ot

apparatus, embodying theg the principle of the assembly described and

may be used.

Arsenic Test Apparatus

Arsenic Trioxide Stock So

previously dried at 105

hydroxide solution (1 in 5)

solution with 2 N sulfuric a

recently boiled and cooled

Standard Arsenic Solution

Solution to a 1000-mL volu

add recently boiled and co

Arsenic Solution contains

solution in an all-glass con

Standard Preparation—

generator flask, and dilute

Test Preparation— Unless

transfer to the generator fl

calculated by the formula:

in which L is the arsenic lim

35 mL.

Procedure— Treat the Sta

as follows. Add 20 mL of 7

mL of stronger acid stanno

mix. Allow to stand at room

tube (c) with two pledgets

acetate solution, freed fromvacuum at room temperatuic trioxide, n 5 mL of sodium tralize the acid, then add xide Stock uric acid, then h mL of Standard s). Keep this ution into a monograph, bstance ute with water to paration similarly , 0.5 pyl alcohol, and the scrubber saturated lead d dried in the two pledgets.

Lubricate the joints (b and d) with a suitable stopcock grease designed for use with organic solvents, and connect the scrubber unit to the absorber tube (e). Transfer 3.0 mL of to the absorber tube. Add 3.0 g of granular zinc (No. 20 mesh) to the mixture in the flask, immediately connect the assembled scrubber unit, and allow the evolution of hydrogen and the color development to proceed at room temperature for 45 minutes, swirling the flask gently at 10-minute intervals. Disconnect the absorber tube from the generator and scrubber units, and transfer the absorbing solution to a 1-cm absorption cell. Any red color produced by the Test Preparation does not exceed that produced by the Standard Preparation. If necessary or desirable, determine the absorbance at the wavelength of maximum absorbance

between 535 and 540 nm, with a suitable spectrophotometer or colorimeter, using as the blank.

Interfering Chemicals— Metals or salts of metals, such as chromium, cobalt, copper, mercury, molybdenum, nickel, palladium, and silver, may interfere with the evolution of arsine. Antimony, which forms stibine, produces a positive interference in the color development with ; when the presence of antimony is suspected, the red colors produced in the two silver diethyldithiocarbamate solutions may be compared at the

wavelength of maximum absorbance between 535 and 540 nm, with a suitable colorimeter, since at this wavelength the interference due to stibine is negligible.

METHOD II

NOTES—

(1) Caution—Some substances may react with explosive violence when digested with hydrogen peroxide. Exercise safety precautions at all times.

(2) If halogen-containing compounds are present, use a lower temperature while heating the test specimen with sulfuric acid, avoid boiling the mixture,

and add the hydrogen peron peroxide with caution, before charring beginsegins, to prevent loss of trivalent arsenic.nic.

(3) If the test substance rece reacts too rapidly and begins charring withg with 5 mL of sulfuric acid before heatingeating, use instead 10 mL of cooled dilute sulfe sulfuric acid (1 in 2), and add a few dropsrops of the hydrogen peroxide before heatingeating.

Standard Preparation—— Pipet 3.0 mL of Standard Arsenic Solutionution into a

generator flask, add 2 mL 2 mL of sulfuric acid, mix, and add the total amal amount of 30 percent hydrogen peroxideroxide used in preparing the Test Preparationation. Heat the mixture to strong fuming, cing, cool, add cautiously 10 mL of water, and and again heat to strong fumes. Repeat theat this procedure with another 10 mL of wate water to remove any traces of hydrogen peen peroxide. Cool, and dilute with water to 35 mo 35 mL. Test Preparation— Unlessnless otherwise directed in the individual mon monograph, transfer to a generator flasor flask the quantity, in g, of the test substancetance calculated by the formula:

3.0 / L

in which L is the arsenic limnic limit in ppm. Add 5 mL of sulfuric acid and d and a few glass beads, and digest in a fuma fume hood, preferably on a hot plate and at nd at a

temperature not exceedingeding 120, until charring begins. (Additional sonal sulfuric acid may be necessary to wet s wet some specimens completely, but the totae total volume added should not exceed ceed 10 mL.) Cautiously add, dropwise, 30 per30 percent hydrogen peroxide, allowinllowing the reaction to subside and again hean heating

between drops. Add the firthe first few drops very slowly with sufficient ment mixing, in order to prevent a rapid repid reaction. Discontinue heating if foaming beng becomes excessive. When the reac reaction h …… 此处隐藏：5619字，全部文档内容请下载后查看。喜欢就下载吧 ……