Скачать или смотреть Marsh Test : Detect the presence of Arsenic in a given compound

This video features the test used to detect the presence of arsenic.
I thought I will just give a small writeup on the classical Marsh test for detecting arsenic ions in a given substance.

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.



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