Gravimetric analysis can be used to determine the quantity of an ion present in a solution.
This can be done by adding a reagent that causes the ion under investigation to form an insoluble compound (a precipitate) that precipitates out of the solution.
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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".
Today, these "wet" methods of chemical detection have been superceded by instrumental analysis.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.
Give the oxidation state (number) for arsenic in each of the following:
As2O3
As4S4
AsH3
As
For the reaction: As4S4 + 7O2 → 2As2O3 + 4SO2
which element, or elements, have been oxidized?
Consider the reaction: 2 As2O3 + 3 C → 3 CO2 + 4 As Is arsenic likely to be more or less active than carbon?
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.
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 0oC and 100 kPa, what volume of gas would be produced by the thermal decomposition of 25 moles of arsine?
At 0oC and 100 kPa, what mass of arsenic would be deposited after the thermal decomposition of 150 mg of arsine.
At 25oC 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?
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