April 2017

# Redox and murder

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Jean Hein

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Have your students read Andrea Sella’s engaging article, Marsh’s Mirror in Chemistry World online1, which has both murder and redox. Students will enjoy the interesting history of James Marsh who created a sensitive test in 1835 for arsenic poisoning. The test determines the presence of arsenic oxide — found in rat poison at the time — in the stomach contents of the victim.

Marsh based his test on a 1775 experiment by Carl Wilhelm Scheele. Scheele had produced the highly toxic gas “arseniuretted hydrogen” or AsH3 by treating zinc with arsenic dissolved in sulfuric acid. In the Chemistry World article, Marsh’s test is described as follows: A piece of zinc was suspended in the short side [of a miniature Kipp’s apparatus] and the device was then filled with sulfuric acid. When a sample was added to the other arm and stoppered, hydrogen and arsine would build up in short arm of the tube. The gas could then be released through the nozzle and ignited. On playing a piece of glass into the whitish flame, a brownish/black spot of arsenic metal would appear.

There are two half reactions you can discuss with your students or better yet, have the students balance the redox reaction.2

Oxidation: Zn --> Zn2+ + 2e−

Reduction: As2O3 + 12e− + 6H+ --> 2As3− + 3H2O

Overall, we have this reaction:

As2O3 + 6Zn + 6H+ --> 2As3− + 6Zn2+ + 3H2O

As2O3 + 6Zn + 6H2SO4 --> 2AsH3 + 6ZnSO4 + 3H2O

There is much more to the story, so I recommend you have students read the entire article.

1. Marsh’s Mirror, Chemistry World, February 28, 2017 www.chemistryworld.com/opinion/marshs-mirror/2500415.article
2. Reactions taken from “Marsh test” on Wikipedia [JLH]

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