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Concept
Marsh test
Summary
The Marsh test is a highly sensitive method in the detection of arsenic, especially useful in the field of forensic toxicology when arsenic was used as a poison. It was developed by the chemist James Marsh and first published in 1836. The method continued to be used, with improvements, in forensic toxicology until the 1970s.
Arsenic, in the form of white arsenic trioxide As2O3, was a highly favored poison, being odourless, easily incorporated into food and drink, and before the advent of the Marsh test, untraceable in the body. In France, it came to be known as poudre de succession ("inheritance powder"). For the untrained, arsenic poisoning will have symptoms similar to cholera.
The first breakthrough in the detection of arsenic poisoning was in 1775 when Carl Wilhelm Scheele discovered a way to change arsenic trioxide to garlic-smelling arsine gas (AsH3), by treating it with nitric acid (HNO3) and combining it with zinc.
As2O3 + 6 Zn + 12 HNO3 → 2 AsH3 + 6 Zn(NO3)2 + 3 H2O
In 1787, German physician Johann Metzger (1739-1805) discovered that if arsenic trioxide were heated in the presence of carbon, the arsenic would sublime. This is the reduction of As2O3 by carbon:
2 As2O3 + 3 C → 3 CO2 + 4 As
In 1806, Valentin Rose took the stomach of a victim suspected of being poisoned and treated it with potassium carbonate (K2CO3), calcium oxide (CaO) and nitric acid. Any arsenic present would appear as arsenic trioxide and then could be subjected to Metzger's test.
The most common test (and used even today in water test kits) was discovered by Samuel Hahnemann. It would involve combining a sample fluid with hydrogen sulfide (H2S) in the presence of hydrochloric acid (HCl). A yellow precipitate, arsenic trisulfide (As2S3) would be formed if arsenic was present.
Though precursor tests existed, they had sometimes proven not to be sensitive enough. In 1832, a certain John Bodle was brought to trial for poisoning his grandfather by putting arsenic in his coffee. James Marsh, a chemist working at the Royal Arsenal in Woolwich, was called by the prosecution to try to detect its presence.
Official source
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