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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.

Arsenic, in the form of white arsenic trioxide, was a highly favored poison, for it is odorless, easily incorporated into food and drink, and before the advent of the Marsh test, untraceable in the body. For the untrained, arsenic poisoning would have symptoms similar to cholera. Indeed, in France it came to be known as poudre de succession, "inheritance powder" for obvious reasons.

Precursor methods

The first breakthrough in the detection of arsenic poisoning was in 1775 when Carl Wilhelm Scheele discovered a way to change arsenic trioxide to arsine gas (AsH₃), a garlic-smelling gas by treating it with nitric acid (HNO₃) and combining it with zinc.

As_{2O3 + 6 Zn + 12 HNO3 → 2 AsH3 + 6 Zn(NO3)2 + 3 H2O}

Later on, Johann Metzger discovered that if arsenic trioxide was heated in the presence of charcoal, a shiny black powder (arsenic mirror) would be formed over it. This is the reduction of As_{2O3 by carbon:}

2 As_{2O3 + 3 C → 3 CO2 + 4 As}

In 1806, Valentine 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.

However, 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 were present.

Circumstances and methodology behind the Marsh test

Even so, these tests have proven not 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 British Arsenal in Woolwich was called by the prosecution to try to detect its presence. He performed the standard test by passing hydrogen sulfide through the suspect fluid. While Marsh was able to detect arsenic, the yellow precipitate did not keep very well, and by the time it was presented to the jury it deteriorated. The jury was not convinced, and John Bodle was acquitted.

Angered and frustrated by this, especially when John Bodle confessed later that he indeed killed his grandfather, Marsh decided to devise a better test to demonstate the presence of arsenic. Taking Scheele's method as a basis, he constructed a simple glass apparatus capable of not only detecting minute traces of it but also measure its quantity. While the Scheele test used nitric acid, in Marsh's case the suspect fluid would be mixed with sulfuric acid (H₂SO₄) and passed through a U-shaped tube with a piece of arsenic-free zinc at the end. If even a trace of arsenic was present, arsine gas would result. When he ignited this gas, it gas would decompose into arsenic and hydrogen. When he held a cold ceramic bowl, the arsenic would form a silvery-black deposit on the bowl, a result similar to that of Metzger's test. Not only could minute amounts of arsenic be detected (for as little as 0.02 mg), the test was very specific for arsenic. Although antimony (Sb) could give a false-positive test by forming a similar black deposit, it would not react with sodium hypochlorite (NaOCl), while arsenic would.

Specific reactions involved with the Marsh test

The Marsh test treats the sample with sulfuric acid and arsenic-free zinc. Even if there are minute amounts of arsenic present, the zinc reduces the As. Here are the two half-reactions:

Oxidation: Zn → Zn

Reduction: As₂O₃ + 12e + 6H → 2As + 3 H₂O

Overall, we have this reaction:

As₂O₃ + 6 Zn + 6 H → 2 As + 6 Zn + 3 H₂O

But in an acidic medium, As_-3 actually forms arsine gas (AsH₃), so adding sulfuric acid (6 H₂SO₄) to each side of the equation we get:

As₂O₃ + 6 Zn + 6 H + 6 H₂SO₄ → 2 As + 6 H₂SO₄ + 6 Zn + 3 H₂O,

or as the As combines with the H to form arsine:

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

or by eliminating the common ions:

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

First notable application

Despite the efficacy of the Marsh test, it first notable case was in 1840 with the celebrated LaFarge poisoning case. Charles LaFarge, a foundry owner, was suspected of being poisoned with arsenic by his wife Marie. The circumstantial evidence was great: it was shown that she brought arsenic trioxide from a local chemist, supposedly to kill rats which infested their home. In addition, their maid swore that she mixed adding a white powder in his drink. Although the food was found to be positive for the poison using the old methods as well as the Marsh test, when the husband's body was exhumed and tested, the chemists assigned to the case were not able to do so. Mathieu Orfila, the renowned toxicologist retained by the defence and an expert of the test examined the results and showed that the Marsh test was not at fault for the negative results but rather that of those who performed it. They have bungled the test. Orfila performed the test again and this time proved the presence of the poison in LaFarge's body. Marie was found guilty and sentenced to death, but later comunted to life imprisonment.