I need some arsenic (soluble) compounds for testing a qualititative scheme Im devising.
The only source I got is 3gr Orpiment stone (As2S3).
What I need are soluble compounds of arsenic as As (III) and As (V) as neutral as possible (to test acidic and basic reactions).
.
The main idea is make solution of diferent elements and add an arsenic compound, and then test separation and precipitation (using different reagents)
(solution should be 0,1 molar or less, and no more than 5 cm3)
My (beta) plan is to:
Ground the orpiment stone, separate 1/3 to keep (or maybe purifying to As2S3)
but the other 2/3 to make As(III) / As(v) soluble compounds.
1) dissolve the Orpiment stone in nitric acid (excess) ->arsenic acid
2) neutralized it with ammonium hydroxide and recrystallize.
(or pass NH3 to te arsenic acid.
How do I reduce it to As(III)? (high heating? to decompose to the oxide? or I would get arsenic pentoxide?)
How would you proceed?
I (think) undersand the risk of Arsenic chem., this is why I would ask first, to have a plan and try to be as methodical an tidy as possible.
It would be done outside.
"If one decides to experiment with arsenic or its compounds, then one has to be very careful with strong reducing agents in combination with acids
(e.g. zinc metal in acid, magnesium powder, borane-based reductors). Under such conditions, both arsenic(III) and arsenic(V) compounds can easily be
reduced beyond elemental arsenic to arsenic(-III) as arsine, which has formula AsH3. This is a colorless flammable gas, which is outrageously
poisonous. A few ml of this gas can kill a grown-up person. This is a very serious risk, which makes the use of arsenic compounds in a home-lab a real
no-go, unless very strict safety measures are taken."-woelen
Regretfully arsenic is present in a lot of mineral rocks, and I would like to learn to identify it.
Grounding is better done with some water? (to avoid/reduce dust) or is not mandatory - or will complicate things?
Orpiments dissolves in solutions of NaOH quite well. Take a moderately concentrated solution of NaOH, crunch the orpiment to small pieces and add
these to the solution of NaOH. You'll get a solution with AsO3(3-) ions, AsO2S(3-), AsOS2(3-) and AsS3(3-) ions, with O : S = 1.5 : 1.5 on average.
Here you have your arsenic(III) solution, but its pH is quite high.
If you want to simulate the presence of arsenic in leached out minerals, then a dilute solution of this form may be a nice starting point for you.
I am not fond of AliExpress at all, but this seller is trustworthy. It is the real stuff. I purchased both the red and yellow variation some years
ago. At the moment, the price is amazingly low, much lower than when I purchased the stuff.
[Edited on 2-11-24 by woelen]Bedlasky - 2-11-2024 at 21:26
How do I reduce it to As(III)? (high heating? to decompose to the oxide? or I would get arsenic pentoxide?)
I wouldn't heat arsenic compounds. They are quite volatile, they like to sublime (especially As2O3).RU_KLO - 5-11-2024 at 05:19
Thanks,
after reading some material (and spooking out about hazzards) I think that the best to do is to store as orpiment (lightly crushed).
As orpiment oxidizes over time to As2O3, when needed I would add dH2O to the storing test tube, pipette out and use this solution for testing.
If not enough material, will take a grain of orpiment and:
1) if alkaline is needed, use NaOH to dissolve it.
2) if Acidic, use nitric acid + 1 drop of HCl to dissolve it. or concentrated H2SO4 at 120°C)
