Marsh test : classical test for arsenic detection

I thought I will just give a small writeup on the classical Marsh test for detecting arsenic ions in a given substance.

If you don't want to continue reading you can directly go into the test
Link below
https://youtu.be/Udi8686GAHE

Or continue reading

Marsh test had been in use since a long time
Here's a little bit of history
Arsenic is perhaps history’s most prolific poison, and for good reason: it has been historically easy to obtain, is odorless and tasteless, can be introduced quietly over time in small unassuming doses, and in the end its symptoms mimic those of any number of ordinary diseases. For most of history there was no reliable way to detect it, and so arsenic was a lurking threat, with deaths both common and under-reported
In 1832 police arrested John Bodle for lacing his grandfather's coffee with poison. Chemist James Marsh tested the drink in his laboratory, and confirmed the presence of arsenic by producing a yellow precipitate of arsenic sulfide. But the precipitate was unstable and, by the time of trial, had deteriorated. Without forensic proof, Bodle was acquitted. Stung by the verdict, Marsh devised a test that could better stand up in court. His 1836 "Marsh Test" won worldwide acclaim and became a standard procedure.
Principle
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.

Procedure of how I did it
Materials required
0.05g arsenic trioxide ( minimal the better)
Don't try to be generous here.
Approximately 1g of zinc metal
(I used a small piece of zinc metal turning)
5ml of dilute sulfuric acid (10%)
Always take care with mixing chemicals that generate noxious gases. Use the minimum amounts .

Procedure
Take 0.05g of arsenic trioxide in a 100ml Erlenmeyer flask and add the piece of zinc metal into it .
Now add 5ml of dilute sulfuric acid into it and mix well
Immediately close the flask with a stopped inserted to a glass tubing
Ignite the end of the glass tubing
Keep a piece of cold porcelain plate at the end of flame
Observe the black deposition of arsenic.

Inference
As2O3 + 6 Zn + 6 H+ → 2 As3− + 6 Zn2+ + 3 H2O
In an acidic medium, As3−
is protonated to form arsine gas (AsH3), so adding sulphuric 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

Now one last thing
Antimony also gives a similar reaction with marsh test. So there is a simple method to differentiate between them. Pour some hypochlorite solution over this porcelain. The metal dissolves in it forming arsenous acid.
But antimony does not dissolve in it.

I hope this gives you an idea about marsh test.
Now to watch my video please do check out the link below
https://youtu.be/Udi8686GAHE

Thank you

Quote: Originally posted by AJKOER

... Also, commentary in Bretherick, Volume 1, page 23-24, to quote: "A student mixed aluminium foil and drain cleaner in a soft drink bottle, which started emitting gas. Another student carried the bottle outside and was claimed to have been overcome by the toxic fumes [1]. Most drain cleaners are alkalis, so that aluminium will dissolve to produce hydrogen. The bleach that it is is suggested may have been present will produce no toxic fumes in alkali, and one would be surprised to find arsenic or antimony compounds present. If the collapse was not purely hysterical, the remaining, though remote, possibility would be phosphine. The soft drink the bottle had contained was one of the many perhaps best described as impure dilute phosphoric acid [2]. " A related comment attributing the formation of gases like arsine and stibine from nascent hydrogen at https://pubs.acs.org/doi/pdf/10.1021/ed011p308 Education on nascent hydrogen interaction is important so as to avoid toxic gases that may be inadvertently and unexpectedly created in significant amounts.