Marsh test
Encyclopedia
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.
Arsenic, in the form of white arsenic trioxide
As2O3, 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. In France, it came to be known as poudre de succession ("inheritance powder"). For the untrained, arsenic poisoning
would have symptoms similar to cholera
.
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
.
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
:
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.
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.
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 demonstrate the presence of arsenic. Taking Scheele's work as a basis, he constructed a simple glass apparatus capable of not only detecting minute traces of arsenic but also measuring its quantity. Adding a sample of tissue or body fluid to a glass vessel with zinc and acid would produce arsine gas if arsenic was present, in addition to the hydrogen that would be produced regardless by the zinc reacting with the acid. Igniting this gas mixture would oxidize any arsine present into arsenic and water vapor. This would cause a cold ceramic bowl held in the jet of the flame to be stained with a silvery-black deposit of arsenic, physically similar to the result of Metzger's reaction. The intensity of the stain could then be compared to films produced using known amounts of arsenic. Not only could minute amounts of arsenic be detected (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 dissolve in a solution of sodium hypochlorite (NaOCl), while arsenic would.
Overall, we have this reaction:
In an acidic medium, is protonated to form arsine
gas (AsH3), so adding sulfuric acid (H2SO4) to each side of the equation we get:
As the As3− combines with the H+ to form arsine:
By eliminating the common ions:
was ever introduced — was in Tulle
, France
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 had mixed a white powder into 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 detect arsenic. Mathieu Orfila
, the renowned toxicologist retained by the defence and an acknowledged authority of the Marsh test examined the results. He performed the test again and demonstrated that the Marsh test was not at fault for the misleading results but rather those who performed it did it incorrectly. Orfila thus proved the presence of arsenic in LaFarge's body using the test. As a result of this, Marie was found guilty and sentenced to life imprisonment.
The existence of the Marsh test also served a deterrent effect: deliberate arsenic poisonings became rarer because of the fear of discovery became more present.
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
, especially useful in the field of forensic toxicology
Forensic toxicology
Forensic toxicology is the use of toxicology and other disciplines such as analytical chemistry, pharmacology and clinical chemistry to aid medical or legal investigation of death, poisoning, and drug use...
when arsenic was used as a poison
Poison
In the context of biology, poisons are substances that can cause disturbances to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism....
. It was developed by the chemist James Marsh and first published in 1836.
Arsenic, in the form of white arsenic trioxide
Arsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...
As2O3, 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. In France, it came to be known as poudre de succession ("inheritance powder"). For the untrained, arsenic poisoning
Arsenic poisoning
Arsenic poisoning is a medical condition caused by increased levels of the element arsenic in the body. Arsenic interferes with cellular longevity by allosteric inhibition of an essential metabolic enzyme...
would have symptoms similar to cholera
Cholera
Cholera is an infection of the small intestine that is caused by the bacterium Vibrio cholerae. The main symptoms are profuse watery diarrhea and vomiting. Transmission occurs primarily by drinking or eating water or food that has been contaminated by the diarrhea of an infected person or the feces...
.
Precursor methods
The first breakthrough in the detection of arsenic poisoning was in 1775 when Carl Wilhelm ScheeleCarl Wilhelm Scheele
Carl Wilhelm Scheele was a German-Swedish pharmaceutical chemist. Isaac Asimov called him "hard-luck Scheele" because he made a number of chemical discoveries before others who are generally given the credit...
discovered a way to change arsenic trioxide to garlic-smelling arsine
Arsine
Arsine is the chemical compound with the formula AsH3. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic...
gas (AsH3), by treating it with nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
(HNO3) and combining it with zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
.
- 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
Charcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...
, a shiny black powder (arsenic mirror) would be formed over it. This is the reduction of As2O3 by carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
:
- 2 As2O3 + 3 C → 3 CO2 + 4 As
In 1806, Valentin Rose
Valentin Rose (pharmacologist)
Valentin Rose was a German pharmacologist from Berlin, Margraviate of Brandenburg. Son of Valentin Rose the Elder.He found the inuline and sodium carbonate. And he invented a way to detect arsenic.-Children:...
took the stomach of a victim suspected of being poisoned and treated it with potassium carbonate
Potassium carbonate
Potassium carbonate is a white salt, soluble in water , which forms a strongly alkaline solution. It can be made as the product of potassium hydroxide's absorbent reaction with carbon dioxide. It is deliquescent, often appearing a damp or wet solid...
(K2CO3), calcium oxide
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
(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
Samuel Hahnemann
Christian Friedrich Samuel Hahnemann was a German physician, known for creating an alternative form of medicine called homeopathy.- Early life :Christian Friedrich Samuel Hahnemann was born in Meissen, Saxony near Dresden...
. It would involve combining a sample fluid with hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
(H2S) in the presence of hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
(HCl). A yellow precipitate, arsenic trisulfide
Arsenic trisulfide
Arsenic trisulfide is the inorganic compound with the formula As2S3. This bright yellow solid is a well known mineral orpiment , has been used as a pigment, and has played a role in the analysis of arsenic compounds. This chalcogenide material is a group V/VI, intrinsic p-type semiconductor and...
(As2S3) would be formed if arsenic were present.
Circumstances and methodology
Even so, these tests have 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 ArsenalRoyal Arsenal
The Royal Arsenal, Woolwich, originally known as the Woolwich Warren, carried out armaments manufacture, ammunition proofing and explosives research for the British armed forces. It was sited on the south bank of the River Thames in Woolwich in south-east London, England.-Early history:The Warren...
in Woolwich
Woolwich
Woolwich is a district in south London, England, located in the London Borough of Greenwich. The area is identified in the London Plan as one of 35 major centres in Greater London.Woolwich formed part of Kent until 1889 when the County of London was created...
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 demonstrate the presence of arsenic. Taking Scheele's work as a basis, he constructed a simple glass apparatus capable of not only detecting minute traces of arsenic but also measuring its quantity. Adding a sample of tissue or body fluid to a glass vessel with zinc and acid would produce arsine gas if arsenic was present, in addition to the hydrogen that would be produced regardless by the zinc reacting with the acid. Igniting this gas mixture would oxidize any arsine present into arsenic and water vapor. This would cause a cold ceramic bowl held in the jet of the flame to be stained with a silvery-black deposit of arsenic, physically similar to the result of Metzger's reaction. The intensity of the stain could then be compared to films produced using known amounts of arsenic. Not only could minute amounts of arsenic be detected (as little as 0.02 mg), the test was very specific for arsenic. Although antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
(Sb) could give a false-positive test by forming a similar black deposit, it would not dissolve in a solution of sodium hypochlorite (NaOCl), while arsenic would.
Specific reactions involved
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 (As3+ ). Here are the two half-reactions:- Oxidation: Zn → Zn2+ + 2 e−
- Reduction: As2O3 + 12 e− + 6 H+ → 2 As3− + 3 H2O
Overall, we have this reaction:
- As2O3 + 6 Zn + 6 H+ → 2 As3− + 6 Zn2+ + 3 H2O
In an acidic medium, is protonated to form arsine
Arsine
Arsine is the chemical compound with the formula AsH3. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic...
gas (AsH3), so adding sulfuric acid (H2SO4) to each side of the equation we get:
- As2O3 + 6 Zn + 6 H+ + 6 H2SO4 → 2 As3− + 6 H2SO4 + 6 Zn2+ + 3 H2O
As the As3− combines with the H+ to form arsine:
- As2O3 + 6 Zn + 6 H+ + 6 H2SO4 → 2 AsH3 + 6 ZnSO4 + 3 H2O + 6 H+
By eliminating the common ions:
- As2O3 + 6 Zn + 6 H2SO4 → 2 AsH3 + 6 ZnSO4 + 3 H2O
First notable application
Although the Marsh test was efficacious, its first publicly documented use — in fact, the first time evidence from forensic toxicologyForensic toxicology
Forensic toxicology is the use of toxicology and other disciplines such as analytical chemistry, pharmacology and clinical chemistry to aid medical or legal investigation of death, poisoning, and drug use...
was ever introduced — was in Tulle
Tulle
Tulle is a commune and capital of the Corrèze department in the Limousin region in central France. It is also the episcopal see of the Roman Catholic Diocese of Tulle...
, France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
in 1840 with the celebrated LaFarge poisoning case
Marie Lafarge
Marie-Fortunée Lafarge, née Capelle was a Frenchwoman who was convicted of murdering her husband by arsenic poisoning in 1840. Her case became notable, because it was one of the first trials to be followed by the public through daily newspaper reports, and because she was the first person...
. 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 had mixed a white powder into 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 detect arsenic. Mathieu Orfila
Mathieu Orfila
Mathieu Joseph Bonaventure Orfila was a Spanish-born French toxicologist and chemist, the founder of the science of toxicology.- Role in Forensic Toxicology :...
, the renowned toxicologist retained by the defence and an acknowledged authority of the Marsh test examined the results. He performed the test again and demonstrated that the Marsh test was not at fault for the misleading results but rather those who performed it did it incorrectly. Orfila thus proved the presence of arsenic in LaFarge's body using the test. As a result of this, Marie was found guilty and sentenced to life imprisonment.
Effects of the Marsh test
The case proved to be controversial, for it divided the country into factions who were convinced or otherwise of Mme. LaFarge's guilt; nevertheless, the impact of the Marsh test was great. 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 more present.
See also
- James Marsh, who discovered the reaction in 1836
- Nascent hydrogenNascent hydrogenAtomic hydrogen is the species denoted by H , contrasted with dihydrogen, the usual 'hydrogen' commonly involved in chemical reactions. It is claimed to exist transiently but long enough to effect chemical reactions...
- Devarda's alloyDevarda's alloyDevarda's alloy , is an alloy of aluminium , copper and zinc ....
- ArsineArsineArsine is the chemical compound with the formula AsH3. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic...
- StibineStibineStibine is the chemical compound with the formula SbH3. This colourless gas is the principal covalent hydride of antimony and a heavy analogue of ammonia. The molecule is pyramidal with H–Sb–H angles of 91.7° and Sb–H distances of 1.707 Å...