á211ñARSENIC
This procedure is designed to determine the presence of trace amounts of arsenic (As)by converting the arsenic in a substance under test to arsine,which is then passed through a solution of silver diethyldithiocarbamate to form a red complex.The red color so produced is compared,either visually or spectrophotometrically,to the color produced similarly in a control containing an amount of arsenic equivalent to the limit given in the individual monograph.Limits are stated in terms of arsenic (As).The content of arsenic does not exceed the limit given in the individual monograph.
Two methods are provided,the methods differing only in the preliminary treatment of the test substance and the standard.Generally,Method Iis used for inorganic materials,while Method IIis used for organic materials.
Apparatus—
The apparatus (see illustration)consists of an arsine generator (a)fitted with a scrubber unit (c)and an absorber tube (e)with standard-taper or ground glass ball-and-socket joints (band d)between the units.However,any other suitable apparatus,embodying the principle of the assembly described and illustrated,may be used.
;)
Arsenic Test Apparatus
Arsenic Trioxide Stock Solution— Dissolve 132.0mg of arsenic trioxide,previously dried at 105
for 1hour and accurately weighed,in 5mLof sodium hydroxide solution (1in 5)in a 1000-mLvolumetric flask.Neutralize the solution with 2Nsulfuric acid,add 10mLmore of 2Nsulfuric acid,then add recently boiled and cooled water to volume,and mix.
Standard Arsenic Solution— Transfer 10.0mLof Arsenic Trioxide Stock Solutionto a 1000-mLvolumetric flask,add 10mLof 2Nsulfuric acid,then add recently boiled and cooled water to volume,and mix.Each mLof Standard Arsenic Solutioncontains the equivalent of 1µg of arsenic (As).Keep this solution in an all-glass container,and use within 3days.
METHOD I
Standard Preparation—
Pipet 3.0mLof Standard Arsenic Solutioninto a generator flask,and dilute with water to 35mL.
Test Preparation—
Unless otherwise directed in the individual monograph,transfer to the generator flask the quantity,in g,of the test substance calculated by the formula:
3.0/L,
in which Lis the arsenic limit in ppm,dissolve in water,and dilute with water to 35mL.
Procedure—
Treat the Standard Preparationand the Test Preparationsimilarly as follows.Add 20mLof 7Nsulfuric acid,2mLof potassium iodide TS,0.5mLof stronger acid stannous chloride TS,and 1mLof isopropyl alcohol,and mix.Allow to stand at room temperature for 30minutes.Pack the scrubber tube (c)with two pledgets of cotton that have been soaked in saturated lead acetate solution,freed from excess solution by expression,and dried in vacuum at room temperature,leaving a 2-mm space between the two pledgets.Lubricate the joints (band d)with a suitable stopcock grease designed for use with organic solvents,and connect the scrubber unit to the absorber tube (e).Transfer 3.0mLof silver diethyldithiocarbamate TSto the absorber tube.Add 3.0g of granular zinc (No.20mesh)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 45minutes,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 Preparationdoes not exceed that produced by the Standard Preparation.If necessary or desirable,determine the absorbance at the wavelength of maximum absorbance between 535and 540nm,with a suitable spectrophotometer or colorimeter,using silver diethyldithiocarbamate TSas 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 silver diethyldithiocarbamate TS;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 535and 540nm,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 peroxide with caution,before charring begins,to prevent loss of trivalent arsenic.
(3)If the test substance reacts too rapidly and begins charring with 5mLof sulfuric acid before heating,use instead 10mLof cooled dilute sulfuric acid (1in 2),and add a few drops of the hydrogen peroxide before heating.
Standard Preparation—
Pipet 3.0mLof Standard Arsenic Solutioninto a generator flask,add 2mLof sulfuric acid,mix,and add the total amount of 30percent hydrogen peroxide used in preparing the Test Preparation.Heat the mixture to strong fuming,cool,add cautiously 10mLof water,and again heat to strong fumes.Repeat this procedure with another 10mLof water to remove any traces of hydrogen peroxide.Cool,and dilute with water to 35mL.
Test Preparation—
Unless otherwise directed in the individual monograph,transfer to a generator flask the quantity,in g,of the test substance calculated by the formula:
,until charring begins.(Additional sulfuric acid may be necessary to wet some specimens completely,but the total volume added should not exceed 10mL.)Cautiously add,dropwise,30percent hydrogen peroxide,allowing the reaction to subside and again heating between drops.Add the first few drops very slowly with sufficient mixing,in order to prevent a rapid reaction.Discontinue heating if foaming becomes excessive.When the reaction has abated,heat cautiously,rotating the flask occasionally to prevent the specimen from caking on glass exposed to the heating unit.Maintain oxidizing conditions at all times during the digestion by adding small quantities of the hydrogen peroxide solution whenever the mixture turns brown or darkens.Continue the digestion until the organic matter is destroyed,gradually raising the temperature of the hot plate until fumes of sulfur trioxide are copiously evolved,and the solution becomes colorless or retains only a light straw color.Cool,add cautiously 10mLof water,mix,and again evaporate to strong fuming,repeating this procedure to remove any trace of hydrogen peroxide.Cool,add cautiously 10mLof water,wash the sides of the flask with a few mLof water,and dilute with water to 35mL.
3.0/L,
in which Lis the arsenic limit in ppm.Add 5mLof sulfuric acid and a few glass beads,and digest in a fume hood,preferably on a hot plate and at a temperature not exceeding 120,until charring begins.(Additional sulfuric acid may be necessary to wet some specimens completely,but the total volume added should not exceed 10mL.)Cautiously add,dropwise,30percent hydrogen peroxide,allowing the reaction to subside and again heating between drops.Add the first few drops very slowly with sufficient mixing,in order to prevent a rapid reaction.Discontinue heating if foaming becomes excessive.When the reaction has abated,heat cautiously,rotating the flask occasionally to prevent the specimen from caking on glass exposed to the heating unit.Maintain oxidizing conditions at all times during the digestion by adding small quantities of the hydrogen peroxide solution whenever the mixture turns brown or darkens.Continue the digestion until the organic matter is destroyed,gradually raising the temperature of the hot plate until fumes of sulfur trioxide are copiously evolved,and the solution becomes colorless or retains only a light straw color.Cool,add cautiously 10mLof water,mix,and again evaporate to strong fuming,repeating this procedure to remove any trace of hydrogen peroxide.Cool,add cautiously 10mLof water,wash the sides of the flask with a few mLof water,and dilute with water to 35mL.
Procedure—
Proceed as directed for Procedureunder Method I.
Interfering Chemicals—
See Interfering ChemicalsunderMethod I.