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.