According to different pharmacopeia, procedures to perform the limit test for arsenic are different. Here is only the limit test for arsenic as per USP. But first, need to understand the principle of arsenic.
What is the Principle of Arsenic?
Arsenic’s limit test depends on hydrogen ions reacting with arsenic gas in presence of reducing agents such as potassium iodide to form yellow stains on mercuric chloride paper.
Introduction of Arsenic.
Trace amounts of arsenic (As) are determined 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 specification.
Limits are stated in terms of arsenic (As).
The content of arsenic does not exceed the limit given in the individual specification. Two methods are provided to determine the arsenic in a substance. The methods differ only in the preliminary treatment of the test substance and the standard.
Generally, Method I is used for inorganic materials, while Method II is used for organic materials.
Sodium Hydroxide Solution (1 in 5): Dissolve 2 g of sodium hydroxide in 10 mL of water.
2N Sulfuric acid: Add slowly, with stilling, 60 mL of sulfuric acid to about 1020 mL of water, and allow to cool to 25 0C.
7N Sulfuric acid: Add slowly, with stilling, 210 mL of sulfuric acid to about 1020 mL of water, and allow to cool to 25 0 C.
Potassium Iodide TS: Dissolve 16.5 g of potassium iodide in water to make 100 mL. Store in light-resistant containers.
Stronger acid Stannous Chloride, TS: Dissolve 8 g of stannous chloride in 20 mL of hydrochloric acid. Store in glass containers, and use within 3 months.
Silver diethyldithiocarbamate TS: Dissolve 1 g of silver diethyldithiocarbamate in 200 mL of pyridine from a freshly opened bottle or that which has been recently distilled. Store in light-resistant containers, and use within 30 days.
Arsenic Trioxide Stock Solution: Dry arsenic trioxide at 105 0C for 1 hour. Weigh accurately 132.0 mg into a 1000 mL volumetric flask and dissolve in 5 mL of Sodium hydroxide solution (l in 5). Neutralize the solution with 2N Sulfuric acid, add 10 mL more of 2N sulfuric acid, then add recently boiled and cooled water to volume, and mix.
Standard Arsenic Solution: Transfer 10.0 mL of Arsenic Trioxide Stock Solution to a 1000 mL volumetric flask, add 10 mL of 2N Sulfuric acid, then add recently boiled and cooled water to volume, and mix. Each mL of Standard Arsenic solution contains the equivalent of 1 g of arsenic (As). Keep this solution in an all-glass container, and use it within 3 days. Apparatus
The apparatus (see figure-I) 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 (b and d) between the units.
However, any other suitable apparatus, embodying the principle of the assembly described and illustrated, may be used.
How to perform the Limit test for Arsenic?
Limit test for Arsenic Method – I
Standard preparation: Pipette 3.0 mL of Standard Arsenic Solution into a Generator flask, and dilute with water to 35 mL.
Test preparation: Unless otherwise directed in the individual STP, transfer to the generator flask the quantity, in g, of the test substance calculated by the formula:
In which L is the arsenic limit in ppm, dissolve in water, and dilute with water to 35ml.
Procedure for Limit test for Arsenic:
Treat the Standard Preparation and the Test Preparation similarly as follows:
Add 20 mL of 7N sulfuric acid, 2 mL of potassium iodide TS, 0.5 mL of stronger acid stannous chloride TS, and I mL of isopropyl alcohol and mix.
Allow to stand at room temperature for 30 minutes.
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 a vacuum at room temperature, leaving a 2-mm space between the two pledgets. Lubricate the joints (b and d) with suitable stopcock grease designed for use with organic solvents, and connect the scrubber unit to the absorber tube (e). Transfer 3.0 mL of silver diethyldithiocarbamate TS 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 silver diethyldithiocarbamate TS 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 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 535 and 540 mn, with a suitable colorimeter, since at this wavelength the interference due to stibine is negligible.
Limit Test for Arsenic Method-II
Precautionary note while performing a limit test for arsenic:
- Caution: Sometimes substances may react with explosive violence when digested with hydrogen peroxide. Exercise safety precautions at all times.
- 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.
- If the substance reacts too rapidly and begins charring with 5ml of sulfuric acid before heating, use instead 10ml of cooled dilute sulfuric acid (1 in 2) and add a few drops of the hydrogen peroxide before heating.
Standard preparation: Pipette 3.0 mL of Standard Arsenic Solution into a generator flask, add 2 mL of sulfuric acid, mix, and add the total amount of 30 percent hydrogen peroxide used in preparing the Test preparation.
Heat the mixture to strong fuming, cool, add cautiously 10 mL of water, and again heat to strong fumes.
Repeat this procedure with another 10 mL of water to remove any traces of hydrogen peroxide. Cool, and dilute with water to 35 mL.
Test Preparation: Unless otherwise directed in the individual STP, transfer to a generator flask the quantity, in g, of the test substance calculated by the formula:
3.0 / L
In which L is the arsenic limit in ppm.
Add 5 mL of 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°C, until changing begins.
Note: Additional sulfuric acid may be necessary to wet some specimens completely, but the total volume added should not exceed 10 mL
Cautiously add, dropwise, 30 percent 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 tums brown or darken.
Continue the digestion until the organic matter is destroyed, gradually raise 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 10 mL of water, mix, and again evaporate to strong fuming repeating this procedure to remove any trace of hydrogen peroxide.
Cool, add cautiously 10 mL of water, wash the sides of the flask with a few mL of water, and dilute with water to 35 mL.
Procedure for limit test for arsenic: Proceed as directed for Procedure under Method I.
Interfering Chemicals: See Interfering Chemicals under Method I.
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This may help you to understand the procedure for limit test for arsenic as per USP.