Marsh test - Precursor methods, Circumstances and methodology behind the Marsh test, Specific reactions involved with the Marsh test

A test for arsenic and antimony involving the reduction of their compounds to volatile AsH₃ and SbH₃, which deposit the metals as a mirror on a glass surface. It is named after British chemist James Marsh (1794–1846), assistant to Michael Faraday at the Royal Military Academy, London.

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.

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.

As2O3 + 6 Zn + 12 HNO3 → 2 AsH3 + 6 Zn(NO3)2 + 3 H2O

In 1787, 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 As2O3 by carbon:

2 As2O3 + 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. 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 to be sensitive enough.

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. 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.

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 trivalent arsenic (As+3). Here are the two half-reactions:

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

Overall, we have this reaction:

As2O3 + 6 Zn + 6 H + 6 Zn+2 + 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 + 6 Zn+2 + 3 H2O,

or as the As to form arsine:

As2O3 + 6 Zn + 6 H,

or by eliminating the common ions:

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

First notable application

Although the Marsh test was efficacious, its first publicly documented use — as the matter of fact the first time evidence from forensic toxicology was introduced — was in Tulle, France in 1840 with the celebrated LaFarge poisoning case. 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. The French press covered the trial and gave the test the publicity it needed to give the field of forensic toxicology the legitimacy it deserved, although in some ways it trivialized it: Marsh test assays were actually done in salons, public lectures and even in some plays that recreated the LaFarge case.

The existence of the Marsh test also served a deterrent effect: deliberate arsenic poisonings became rarer because of the fear of discovery became present.