vibbzlab
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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
Amateur chemist. Doctor by profession
Have a small cute home chemistry lab.
Please do check out my lab in YouTube link below
This is my YouTube channel |
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woelen
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Nice experiment and nice demo.
I have one remark. Assure that sufficient hydrogen gas is produced and that all air in the erlenmeyer is displaced by the hydrogen gas, before
igniting the gas. It is safer to use a system with just a little space of air above the liquid (e.g. a small test tube), because less air needs to be
displaced by the hydrogen gas. If the mix of hydrogen is too dilute and still quite some air is mixed with the gas, then the flame may go inside the
system and the gas mix may explode, rupturing the glass vessel, or at least popping the stopper and having all arsine released into the air at once.
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SWIM
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The old analytical apparatus for the Marsh test was so prone to explosions that it was recommended in some lab manuals to throw a towel or blanket
over it to catch any flying glass if it blew up.
I don't think the gas was ignited in this apparatus, but just heated to decomposition by a Bunsen burner.
(EDIT: As described below!)
The positive result of this test was described as an arsenic mirror where the burner heated the tube.
[Edited on 18-2-2020 by SWIM]
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Bedlasky
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Great experiment!
I know this test little differently - arsine decomposes in glass tube which is heated by Bünsen burnen and hydrogen ignites on the end of glass tube.
But procedures may be different and result is the same.
As I know, Marsh's test also work for bismuth. According to the wiki, there is simple distinction: As dissolves in NaOCl, Sb dissolves in ammonium
polysulfide and bismuth resists both reagents. Did you ever try this test for Sb or Bi?
Thanks for this experiments. I love classical analyitcal chemistry.
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vibbzlab
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Thank you woelen for the advise. I will note it down . And bedlasky no ,I don't have antimony or bismuth compound with me. Maybe I will get it next
month or so. If then I will do the video
Amateur chemist. Doctor by profession
Have a small cute home chemistry lab.
Please do check out my lab in YouTube link below
This is my YouTube channel |
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chornedsnorkack
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How would Te act on Marsh test? Hydrogen telluride is liable to pyrolysis, too.
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tahallium
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Can I try this on lead from car battery cuz I'm nervous and I don't want arsenic contamination
How to stay safe arsine is sooOoOOOOoOo toxIXc
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woelen
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That would be an interesting test. Just put a little of the acid in a test tube and add some zinc metal to this. You need to stopper the test tube and
have a thin tube going through a tight hole in the stopper. Let the reaction run for a while (so that air is replaced by the hydrogen) and then ignite
the gas and proceed as demonstrated here. If you have a little glass tube, then you could also lead the gas through this thin tube and keep the tube
in a flame.
if you do this test with zinc, then be sure that the zinc itself is free from arsenic. You can test this by adding the zinc metal to some acid, which
is known not to contain any arsenic.
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AJKOER
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As to mechanics, in an old text, it has been noted that nascent hydrogen generated in situ by an acid on zinc was able to partially reduce phosphorous
acid, H3PO3, forming some PH3. A quote:
""...it is known that phosphorous acid reduced with nascent hydrogen, yields phosphine and hypophosphorous acid"
Source: "investigation of sodamide and of its reaction-products with phosphorus" William Phillips Winter p42-43
In the current context, introduce any As ions, and similarly, expect some AsH3.
I personally view generating hydrogen atom radicals (aka nascent hydrogen) in a heterogeneous mix containing miscellaneous ions, like P or As or Sb,
creating a potentially toxic cloud, is, from a safety perspective, something I would never recommend for poorly equipped home chemists.
Here is a related comment on SM where the cited reference below also mentions the Marsh test:
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.
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Expected reactions:
Zn -> Zn(II) + 2 e-
H+ + e-(aq) = •H
As(III)2O3 + 6 (e-, H+) -> 2 As + 3 H2O
As + 3 •H -> AsH3
[Edited on 27-2-2020 by AJKOER]
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woelen
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What you quote might be true, but then the bottle must have contained a lot of phosphate/phosphoric acid. I hardly can believe this. I think it is due
to hysteria.
But I'll try in practice. I have H3PO4. I'll add a few drops of H3PO4 to a highly concentrated and excess amount of solution of NaOH and then I'll add
some Al. Let's see if I can smell any PH3 once there is good bubbling of H2 (the reaction usually does not set in immediately, it slowly speeds up,
while the oxide layer on the Al-foil is destroyed). I expect no smell of PH3 at all. I'll come back on this.
From personal experience I know that phosphate is VERY hard to reduce in aqueous solution. Posphite is reduced more easily, despite the phosphorus
having a lower oxidation state. A similar observation can be made with sulfate vs. sulfite and perchlorate vs. chlorate.
In general, production of hydrogen from metals and acids indeed can be risky. I once read an anecdotal story from the 1890's or so. This was about a
young man, who wanted to demonstrate the effect on his voice of inhaling a light gas. He used zinc metal and sulfuric acid to make a lot of H2. He
produced a lot of gas, cleaned this gas in order to remove any acid droplets from it by bubbling it through a column of water and collecting it in a
balloon, made from the skin of an pig's intestine (at that time there were no balloons as we can buy them). He then inhaled a lot of the gas and
demonstrated the effect. One day later he became very ill and several days later he died. Investigations showed he died of arsenic poisoning. In those
days, both zinc metal and technical grade sulfuric acid could contain some arsenic.
I however, do not think that if you do little experiments on test tube scale you run any risk of poisoning yourself. You must see these things in
perspective.
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