Mailinmypocket
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Synthesis of sodium monothioarsenate
Recently upon reading on lambdasyn.org, I noticed a synthesis for sodium monothioarsenate, which looked rather simple. Seeing as this forum doesn't
have any real photos of syntheses involving arsenic (probably a good thing) I decided to do a scaled down synthesis of this compound to show a little
bit of As chemistry.
It goes without saying that arsenic compounds are very toxic and one must be extremely careful to not contaminate the work area or air.
Repeating this synthesis is not encouraged at all unless you have the experience and ability to perform this safely and dispose of everything
appropriately.
The synthesis on lambdasyn (http://lambdasyn.org/synfiles/natriumthioarsenat.htm) was originally derived from Brauer, I have attached the synthesis from the PDF version
available in the SciMad library.
The process in itself is very simple. I scaled it down to 1/10 of the original preparation.
2g of As2O3 were weighed out, this is an extremely fine free-flowing powder that is very "staticky" and adheres to everything. To this was added 25 ml
of distilled water. Notice how the powder is somewhat hydrophobic and does not mix well with water.
 
To this was added 2.4g of NaOH, and magnetic stirring started. The As2O3 went into solution quite quickly as it is soluble in alkaline conditions.
Once this was done, 0.65g of sulfur powder was added and a reflux condenser attached. Brought to a boil with stirring the sulfur very slowly dissolved
into solution. This took about an hour despite Brauer stating to reflux only 30 minutes.
 
Finally a pale yellow solution was obtained. This was hot filtered and placed into a storage vial overnight.
 
Now, Brauer says to evaporate the solution on a steam bath... I didn't like the idea of doing that and opted to crash out the product with 12ml of
chilled ethanol instead. Immediately after adding the ethanol to the yellow solution a white crystalline precipitate materialized. This was vacuum
filtered and the resulting compound can be seen in the filter. It is currently drying in a vacuum desiccator so I will have to post the final yield
later on. I am not dealing with a large quantity and so I won't be recrystallizing this just to avoid messing around with As more than necessary.
Purity is not a major concern as I won't be using this for experiments, disposal is a pain and it's an all-around dangerous material.
 
I will just keep it as is as a curiosity.  And now I am done experimenting with
As... Its simply too stressful and uncomfortable. Not to mention the planning that goes into making sure not one mistake is made etc.
*EDIT* the final dried yield is 4.07g
[Edited on 30-1-2013 by Mailinmypocket]
[Edited on 30-1-2013 by Mailinmypocket]
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blogfast25
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"2g of As2O3 were weighed out, this is an extremely fine free-flowing powder that is very "staticky" and adheres to everything. To this was added
25 ml of distilled water. Notice how the powder is somewhat hydrophobic and does not mix well with water."
Luvvlyjubly: poisonous, stick to everything and isn't wetted easily. I thing I might pass.
Well done ethough...
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chemrox
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Now I have to review the capitalization conventions for technical German. Aside from normal poison precautions was there anything specific for
thioarsenates? Like not using nitrile gloves?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Mailinmypocket
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Quote: Originally posted by chemrox  | | Now I have to review the capitalization conventions for technical German. Aside from normal poison precautions was there anything specific for
thioarsenates? Like not using nitrile gloves? |
I only had nitrile gloves during the weighing of the reagents, during the filtration and drying steps, as well as wrapping up the waste and washing
the glassware. I didnt research the specific hazards associated to skin contact with thioarsenates though...
[Edited on 30-1-2013 by Mailinmypocket]
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Mailinmypocket
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It seems unstable...
Upon looking for a reagent I noticed that this chemical seems to be unstable. It has went from being pure white to yellow... I'm having a bit of a
hard time finding information about this compounds' stability though. I'm assuming that the yellow is elemental sulfur. More reading is in order!
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Boffis
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Nice preparation.
Have you thought of trying it with antimony oxide or natural arsenic and antimony sulphides?
Many years ago I tried a similar reaction with antimony sulphide (natural stibnite but antimony sulphide is available from various ebay suppliers) and
orpiment (natural As2S3), sulphur and either sodium hydroxide or sodium sulphide. The tri sodium tetrathioantimonate (schlipp's salt) is particularly
impressive because is crystallises easily into large yellow tetrahedrons and is much less toxic. The arsenic analogue is white.
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AndersHoveland
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It seems similar to the reaction of sulfur dioxide with sodium hydroxide and sulfur. First sulfite is formed, and then the solution of sodium sulfite
is able to dissolve elemental sulfur to form thiosulfate.
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Boffis
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I have been looking through my collection of bottles of things I prepared in the past and found a jar of schlipps salt (about 35-40g), the preparation
date on the label has almost faded away but you can still read "mar 1974" the pale yellow crystals have turned brownish on the outside but are still
yellow inside. I'll have to recrystallise it sometime. I also found the sodium (tetra) thioarsenate, its still fresh, white, crystalline solid and the
prep date on the label is aug 1976! so its pretty stable.
I can't find my notes from that for back but the basic idea for the preparations came from an old inorganic chemistry book. I do remember that the
preparation of schlipps salt seemed very wasteful because much of the antimony ended up in an insoluble sodium antimonate. I attempted to reduce this
loss by using sodium sulphide instead of sodium hydroxide with some success.
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Mailinmypocket
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I will maybe try with antimony since I have a lot of elemental Sb kicking around. I would like to know for sure what is going on with the
(mono)thioarsenate salt though. Being a smart guy(lol) I smelled the compound carefully and it has a distinctive strong sulfurous smell, which
probably means hydrogen sulfide or sulfur...
Either way I'm not getting into this until I have time to, but neat nonetheless. All it said in Brauer was that the salt was efflorescent, but no
mention of instability. Wishing tomorrow was a day off to really delve into this, dissolve some in water and run some tests 
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