Limit test for Arsenic

• The limit test for arsenic is a qualitative test used to determine if the concentration of arsenic in a sample is below a predefined limit.
• The test is based on the reduction of arsenic (As) to arsine gas (AsH₃), which reacts with silver nitrate to form a yellow to brown stain on filter paper.

Chemical Reactions:

Reduction of arsenic to arsine gas:

• As3+(aq) + 3Zn(s) + 6H+(aq) → AsH₃(g) + 3Zn2+(aq) + 3H₂(g)

Reaction of arsine gas with silver nitrate:

• AsH₃(g) + 6AgNO₃(aq) → 3Ag₂As(s) + 6HNO₃(aq) + 3H₂(g)

Procedure:

Prepare Reference Standard Solution:

• Prepare a reference solution with a known concentration of arsenic, usually from sodium arsenite, representing the acceptable arsenic limit.

Prepare Test Solution:

• Prepare a test solution from the sample, ensuring any arsenic present is in a soluble form.

Set Up Test Apparatus:

• Place the test solution and reference standard solution in separate test tubes connected to a hydrogen sulfide (H₂S) generator containing zinc and dilute hydrochloric acid (HCl).

Add Silver Nitrate Solution:

• Add a few drops of silver nitrate solution to the upper part of both test tubes and cover each tube with a moistened filter paper containing silver nitrate.

Generate H₂S Gas:

• Generate H₂S gas by adding dilute HCl to the zinc in the generator. The gas passes through the solutions, reducing arsenic to arsine gas (AsH₃).

Formation of Stain:

• Arsine gas reacts with the silver nitrate on the filter paper, producing a yellow to brown stain due to the formation of silver arsenide (Ag₂As).

Compare Stains:

• Compare the intensity of the stain on the test solution’s filter paper with the stain on the reference standard’s filter paper.
• If the test solution’s stain is less intense, the arsenic concentration in the sample is below the acceptable limit.

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