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

1) Reduction of arsenic to arsine gas

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

2) Reaction of arsine gas with silver nitrate

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

Procedure of Limit test for Arsenic

1) Prepare Reference Standard Solution

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

2) Prepare Test Solution

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

3) 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).

4) 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.

5) 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).

6) 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).

7) 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.