Marsh's sample

Marsh's sample: a black arsenic mirror forms.

An arsenic mirror on a porcelain surface is washed off with an ammoniacal hydrogen peroxide solution. It is therefore not antimony.

Structure from 1921

The Marsh probe is a classic detection reaction in chemistry and forensic medicine for arsenic , antimony and germanium . It was developed in 1836 by the English chemist James Marsh . Before the Marsh test was discovered, arsenic (III) oxide (As ₂ O ₃ ) was a popular murder poison because it was difficult to detect. After 1836, murders by arsenic trioxide (arsenic) became increasingly rare.

reaction

Arsenic is reduced to gaseous arsine by nascent hydrogen . This metastable compound breaks down in the heat to black, elemental arsenic and hydrogen and can thus be detected.

For this purpose, hydrogen in statu nascendi is generated in a vessel by the action of sulfuric acid on zinc , which is dried through a calcium chloride tube and ignited at the outlet after performing an oxyhydrogen gas test . The arsine produced in the vessel after an arsenic-containing sample has been added decomposes in the flame and forms a black so-called arsenic mirror on a piece of porcelain held in it. The detection limit of this detection reaction is 1 µg arsenic.

{\ displaystyle \ mathrm {6 \ Zn + 12 \ H_ {3} O ^ {+} \ longrightarrow 6 \ Zn ^ {2 +} + 12 \ H_ {nasc.} + 12 \ H_ {2} O}}

{\ displaystyle \ mathrm {As_ {2} O_ {3} +12 \ H_ {nasc.} \ longrightarrow 2 \ AsH_ {3} +3 \ H_ {2} O}}

An arsenic (III) compound reacts with the hydrogen formed in acidic solution with zinc to form arsine

Antimony and the rare germanium also form gaseous hydrides, which form such a mirror when they decompose. However, like arsenic, they do not dissolve in an ammoniacal hydrogen peroxide solution, which means that they can be differentiated.

{\ displaystyle \ mathrm {2 \ As + 5 \ H_ {2} O_ {2} +6 \ NH_ {3} \ longrightarrow 2 \ AsO_ {4} ^ {3 -} + 6 \ NH_ {4} ^ {+ } +2 \ H_ {2} O}}

Social reception

Marsh's test was described by R. Austin Freeman in the 1923 novel The Cat's Eye , where detective John Evelyn Thorndyke proves that chocolate was poisoned (1951, child detective Kalle Blomquist also used Marsh's test in Astrid Lindgren's Kalle Blomquist Dangerously Live). In 1929 Dorothy L. Sayers had Lord Peter Wimsey and his servant Bunter carry out the test in the variant modified by Berzelius in the crime novel Strong Poison . Even the Studer by Friedrich Glauser , who had begun to study chemistry, led in his third event , the Chinese an arsenic detection by Marsh.

literature

Primary literature

CF Mohr: Additions to the method indicated by Marsh of excreting the arsenic from every liquid immediately in the regulated state. In: Annals of Chemistry and Pharmacy . 23, 1837, pp. 217-225 doi : 10.1002 / jlac.18370230218 .
JE Herberger: About the application of Marsh's sample in forensic chemical investigations. In: Archives of Pharmacy . 1841 doi : 10.1002 / ardp.18410750131 .
J. Marsh: Description of a new process to separate small quantities of arsenic from the substances with which it is mixed. In: Annals of Chemistry and Pharmacy. 23, 1837, p. 207 doi : 10.1002 / jlac.18370230217 .

Secondary literature

Stewart H. Webster: The Development of the Marsh Test for Arsenic, Journal of Chemical Education , Volume 24, 1947, pp. 487-490.
William B. Jensen: The Marsh Test for Arsenic Oesper Museum Note 26 May / June 2014.

Web links

Wikibooks: Practical Inorganic Chemistry / Marsh's Sample - Learning and Teaching Materials

History of the Marsh sample and laboratory demonstration

Individual evidence

^ S. Ebel, HJ Roth (Ed.): Lexicon of Pharmacy. Thieme, 1987, ISBN 3-13-672201-9 , p. 412.
^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 91st – 100th, improved and greatly expanded edition. Walter de Gruyter, Berlin 1985, ISBN 3-11-007511-3 , pp. 673-674.
↑ Blasius, Ewald., Jander, Gerhart .: Inorganic Chemistry. 1, introduction and qualitative analysis: with ... 79 tables. 17., completely reworked. Edition volume 1 . Hirzel, Stuttgart 2012, ISBN 978-3-7776-2134-0 , pp. 416 .

[Ebel-1] S. Ebel, HJ Roth (Ed.): Lexicon of Pharmacy. Thieme, 1987, ISBN 3-13-672201-9 , p. 412.

[2] AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 91st – 100th, improved and greatly expanded edition. Walter de Gruyter, Berlin 1985, ISBN 3-11-007511-3 , pp. 673-674.

[3] Blasius, Ewald., Jander, Gerhart .: Inorganic Chemistry. 1, introduction and qualitative analysis: with ... 79 tables. 17., completely reworked. Edition volume 1 . Hirzel, Stuttgart 2012, ISBN 978-3-7776-2134-0 , pp. 416 .