| Revision as of 22:07, 20 July 2005 edit143.166.255.11 (talk)No edit summary← Previous edit | Revision as of 00:46, 21 July 2005 edit undoRashBold (talk | contribs)Extended confirmed users1,101 edits total rewriteNext edit → | ||
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| Later on, ] discovered that if arsenic trioxide was heated in the presence of ], a shiny black powder (arsenic mirror) would be formed over it. This is the reduction of As<sub>2<sub>O<sub>3<sub> by ]: | Later on, ] discovered that if arsenic trioxide was heated in the presence of ], a shiny black powder (arsenic mirror) would be formed over it. This is the reduction of As<sub>2<sub>O<sub>3<sub> by ]: | ||
| :::::::::: |
::::::::::2 As<sub>2<sub>O<sub>3<sub> + 3 C → 3 CO<sub>2<sub> + 4 As | ||
| In ], ] took the stomach of a victim suspected of being poisoned and treated it with ] (K<sub>2</sub>CO<sub>3</sub>), ] (CaO) and nitric acid (HNO<sub>3</sub>. Any arsenic present would appear as arsenic trioxide and then could be subjected to Metzger's test. This was essentially the same methodology as that |
In ], ] took the stomach of a victim suspected of being poisoned and treated it with ] (K<sub>2</sub>CO<sub>3</sub>), ] (CaO) and nitric acid (HNO<sub>3</sub>. Any arsenic present would appear as arsenic trioxide and then could be subjected to Metzger's test. This was essentially the same methodology as that discovered by ], using ] (HCl)and ] (HS) as reagentswhich would result in a yellow precipitate containing arsenic. As a matter of fact, prior to the Marsh test this became the standard way to detect it. | ||
| ==Circumstances and methodology behind the Marsh test== | ==Circumstances and methodology behind the Marsh test== | ||
| Even so, these tests have proven not be sensitive enough. In ], a certain John Bodle was brought to trial for poisoning his grandfather by putting arsenic in his coffee. ], a chemist working at the ] in ] 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. | |||
| Here are the two half-reactions: | |||
| 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 ] (H<sub>2</sub>SO<sub>4</sub>) 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 ] (Sb) could give a false-positive test by forming a similar black deposit, it would not react with ] (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<sup>+3</sup>. Here are the two half-reactions: | |||
| :::::::::Oxidation: Zn → Zn<sup>+2<sup> + 2e | :::::::::Oxidation: Zn → Zn<sup>+2<sup> + 2e | ||
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| Overall, we have this reaction: | Overall, we have this reaction: | ||
| :::::::::As<sub>2</sub>O<sub>3</sub> + 6 Zn + |
:::::::::As<sub>2</sub>O<sub>3</sub> + 6 Zn + 6 H<sup>+1</sup> → 2 As<sup>-3</sup> + 6 Zn<sup>+2</sup> + 3 H<sub>2</sub>O | ||
| But in an acidic medium, As<sub>-3</sub> actually forms arsine gas (AsH<sub>3</sub>), so adding sulfuric acid (6 H<sub>2</sub>SO<sub>4</sub>) to each side of the equation we get: | |||
| :::::::As<sub>2</sub>O<sub>3</sub> + 6 Zn + 6 H<sup>+1</sup> + 6 H<sub>2</sub>SO<sub>4</sub> → 2 As<sup>-3</sup> + 6 H<sub>2</sub>SO<sub>4</sub> + 6 Zn<sup>+2</sup> + 3 H<sub>2</sub>O, | |||
| or as the As<sup>+3</sup> combines with the H<sup>+</sup> to form arsine: | |||
| :::::::As<sub>2</sub>O<sub>3</sub> + 6 Zn + 6 H<sup>+1</sup> + 6 H<sub>2</sub>SO<sub>4</sub> → 2AsH<sub>3</sub> + 6 ZnSO<sub>4</sub> + 3 H<sub>2</sub>O + 6 H<sup>+1</sup>, | |||
| or by eliminating the common ions: | |||
| :::::::::As<sub>2</sub>O<sub>3</sub> + 6 Zn + 6 H<sub>2</sub>SO<sub>4</sub> → 2AsH<sub>3</sub> + 6 ZnSO<sub>4</sub> + 3 H<sub>2</sub>O | |||
| But in an acidic medium | |||
Revision as of 00:46, 21 July 2005
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, a garlic-smelling gas by treating it with nitric acid and combining it with zinc.
- As2O3 + 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 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 (HNO3. Any arsenic present would appear as arsenic trioxide and then could be subjected to Metzger's test. This was essentially the same methodology as that discovered by Samuel Hahnemann, using hydrochloric acid (HCl)and hydrogen sulfide (HS) as reagentswhich would result in a yellow precipitate containing arsenic. As a matter of fact, prior to the Marsh test this became the standard way to detect it.
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