Arsenic Tests

Somewhere around the 8th century, an Arab alchemist produced white arsenic trioxide (As₂O₃) from realgar, a naturally occurring arsenic sulfide mineral, As₄S₄:
As₄S₄ + 7O₂ → 2As₂O₃ + 4SO₂ Arsenic trioxide became one of the most widely administered poisons in history, and therefore, it became necessary to find reliable tests to show whether or not arsenic was present in a sample of food or drink, or whether it was present in a corpse.

In 1775, Carl Wilhelm Scheele treated arsenic trioxide with nitric acid and zinc which resulted in arsine gas (AsH₃), zinc nitrate and water:
As₂O₃ + 6Zn + 12HNO₃ → 2AsH₃ + 6Zn(NO₃)₂ + 3H₂O Arsenic was said to be present if an odour of garlic was produced because arsine gas smells a lot like garlic.

In 1787, Johann Metzger used a carbon reduction method to produce carbon dioxide gas and solid arsenic from arsenic trioxide and carbon:
2 As₂O₃ + 3 C → 3 CO₂ + 4 As As the arsenic trioxide is heated with charcoal, an "arsenic mirror" forms on the charcoal.

In 1785, Samuel Hahnemann produced a bright yellow precipitate of arsenic trisulfide, As₂S₃, by passing a stream of hydrogen sulfide gas, H₂S, through an acidified arsenic solution.

In 1836, James Marsh designed an apparatus that would detect and measure arsenic.
The sample is placed in a flask with arsenic-free zinc and sulfuric acid.
Arsine gas forms:

As₂O₃ + 6 Zn + 6 H₂SO₄ → 2 AsH₃ + 6 ZnSO₄ + 3 H₂O

The arsine gas is fed through a drying tube to a glass tube which is heated.
Arsine deomposes on heating, forming elemental arsenic which is a shiny black substance:

2 AsH₃ → 3 H₂ + 2 As

By placing a cold surface at the end of the heated tube it is possible to condense this arsenic, which forms a black "mirror".

http://www.youtube.com/watch?v=-vUZdAwgl2g

Today, these "wet" methods of chemical detection have been superceded by instrumental analysis. e Atomic absorption spectroscopy (AAS)  can now be used to determine not only the presence of arsenic, but also how much arsenic is present.

Further Reading:
Solubility Rules
Writing Precipitation Equations
Gravimetric Analysis
AAS
Oxidation and Reduction 
Carbon Reduction Methods

Suggested Study Questions:

1. Give the oxidation state (number) for arsenic in each of the following:
   • As₂O₃
   • As₄S₄
   •  AsH₃
   • As
2. For the reaction: As₄S₄ + 7O₂ → 2As₂O₃ + 4SO₂ which element, or elements, have been oxidized?
3.  Consider the reaction: 2 As₂O₃ + 3 C → 3 CO₂ + 4 As
   Is arsenic likely to be more or less active than carbon?
4. Write a possible net ionic equation for the reaction in which arsenic ions react with  hydrogen sulfide gas to produce a precipitate of arsenic trisulfide.
5. Consider the reaction: 2 AsH₃ → 3 H₂ + 2 As which results in the formation of a shiny black "mirror" of arsenic.
   • Has arsenic been oxidized or reduced?
   • Has hydrogen been oxidized or reduced?
   • If 1 mole of arsine gas decomposes completely, how many moles of hydrogen gas would be produced?
   • At 0^oC and 100 kPa, what volume of gas would be produced by the thermal decomposition of 25 moles of arsine?
   •  At 0^oC and 100 kPa, what mass of arsenic would be deposited after  the thermal decomposition of 150 mg of arsine.
   • At 25^oC and 100 kPa, what mass of arsine will decompose to produce 0.05 L of hydrogen gas?
   • A sample of gas containing arsine produces an "arsenic mirror" containing 0.02 g of arsenic. What mass of arsine was present in the gas sample?