Introduced in the Grand Illusions article was the Marsh test. This test is a highly sensitive method for detecting arsenic compounds. When samples containing arsenic (As+3) are treated with sulfuric acid and arsenic-free zinc, the arsenic is reduced by the zinc and due to the presence of sulfuric acid arsine gas (AsH3) is produced.
Here are the two half reactions:
Oxidation: Zn à Zn+2 + 2e
Reduction: As2O3 + 12e + 6H+1 à 2As-3 + 3 H2O
Overall reaction:
As2O3 + 6 Zn + 6 H2SO4 à2 AsH3 + 6 ZnSO4 + 3 H2O
The sample, zinc, and sulfuric acid are placed in a flask that has a drying tube connected to it. The water vapor and arsine gas pass through the tube. As the arsine is heated, it decomposes and forms arsenic. The resulting product is a black and shiny powder, referred to as an arsenic mirror. The concentration of arsenic in the sample is directly proportional to the length of the powdery line. Quantitative measures can be made by running standards of known concentrations of arsenic.
Previous, less sensitive, methods for arsenic detection started in 1775 by Carl Scheele. He converted arsenic trioxide (As+3) to arsine gas by treating it with nitric acid and zinc. Then, in 1787, Johann Metzger discovered that a shiny black powder is formed over a piece of charcoal when arsenic trioxide is heated due to its reduction by carbon. Also, in 1806, Valentine Rose discovered that treating arsenic with potassium carbonate, calcium oxide, and nitric acid would form a yellow precipitate. However, none of these tests were as sensitive or specific for arsenic as the Marsh test.
http://encyclopedia.stateuniversity.com/pages/14469/Marsh-test.html
http://www.emsb.qc.ca/laurenhill/science/forensic.html
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment