fluorescence
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First test for a safe KCN synthesis
Hi,
As some of you might know I am working a lot with Cyanides and Cyano-complexes of transition metals. Usually I just buy KCN or NaCN as precursor but
some Cyanides I still have on my to-do list can't be made that way as they won't precipitate or fall apart if kept for too long in aqueous conditions.
Another way to make them would be using the corresponding Hydroxides and bubble HCN through them. The problem here is not so much the HCN as I guess
every step is dangerous enough but rather the fact, that especially main group metal-cyanides have the formation of (hydr)oxides as side reaction
which is usually irreversible if for example basic oxides are formed. So they either fall apart because they are too long in water or they fall apart
because they react with the rest of the Hydroxides present...
A third way I though of was to use Calcium- oder Bariumcyanide and Metal sulfate. Precipitate the Bariumsulfate and quickly decant or filter the
solution and add a lot of Ethanol, as Ethanol precipitates many cyanides.
I checked the literature for stuff like Thallium(I)Cyanide and although it decomposes in water it is said be stable for some time...so I expect a
similar behaviour for many other main group metals...I guess some like Aluminium won't form at all but it gives a bigger spectrum of possible products
once I am able to make Ba(CN)2 in a larger quantity.
Now I planned on making it by bubbling HCN through Ba(OH)2 not sure about the stability here, I will have to check that, as Calciumcyanide
isn't stable at all and nearly impossible to isolate clean.
But I won't focus on that part here but rather the setup.
A good friend of mine uses this method quite often to make KCN when he needs it. Usually he distills the HCN from Ferri(o)cyanide and acid into KOH
and adds a lot of Ethanol to yield a pure product. And this is where I would like to continue in a probably safer method.
Lately many videos appeared on the net on how to make cyanide. Some are quite creative but many rely on leaching the stuff from a crude mixture and I
find this is unnecessary contact with KCN. For some reason people avoid just distilling it off as this seems to be the safest method, the product
easily precipitates, Ethanol can be easily get rid of, etc.
Obviously it's because of the setup....so I though why can't I take this quite easy method and convert it into something you can either throw away
later or clean but operate without any contact to the chemicals..
To be honest I didn't come up with it, it was also my friends idea, but he recommended the use of so called Fermentation locks. For me, who makes wine
himself this is no big problem, the shop for all wine related stuff is quite close to where I live and I think even IKEA sells them for like 1-2€
(plastic) or if you buy them on the net also made from glass.
I plan on ordering some made from glass for stuff like Brominations and Chlorinations, soon. They are super useful as you can bubble almost anything
through them without buying a lot of equipement and they don't build up any pressure.
For the setup I bought a rubber stopper which had a hole in the size of the lock....and that's pretty much all. The test tube or flask, depending on
size was filled with Ferrocyanide (although Ferricyanide or Prussian blue should work as well) and diluted H2SO4 was added.
Usually nothing happens when they are mixed until you start heating it. But you could also pierce the stopper with a syringe and add the acid that
way. The lock is filled with a very dilute KOH.
Now there are two possible reactions:
conc. H2SO4 reacts according to:
K4[Fe(CN)6] + 6 H2SO4 + 6 H2O -> 6 CO + 2 K2SO4 + 3
(NH4)2SO4 + FeSO4.
This would only produce CO gas, so you need to keep the conc. low.
You could add an additional hose leading outside so the gas can just escape, then pour your product into another container and fill the lock with
compounds that destroy cyanides and flip it back over so you can get rid of all the cyanide before you open it.
That way it seems like the safest way to me as you never get into contact with either HCN or KCN in the process. Of course this is something for small
scale only !
By now the problem is, that the overall reaction just won't work as I wanted it and I am not sure why this is the case as this reaction is usually
carried out in larger scales by a friend. The reaction mixture switches to blue which seems to be a good sign as this indicates the formation of
smaller Fe-fragments like FeSO4 which oxidizes and reacts with the Ferrocyanide to Prussian blue...so at least something is happening here.
Also some gas is produced. I noticed that the HCN will not mix with pure water. At least it doesn't show any reaction with Iron salts.
The KOH-version worked a lot better although I think I used too much KOH to start with.
I tested it with Fe(II), Ni(II) and Cu(II).
- The Fe(II)/Fe(III) turned brown (hydroxides) first but then after a day it was all blue
- The Ni(II) showed a small orange layer indicating the presence of [Ni(CN)4]2- and the Cu(II) one produced small amounts of a
brownish precipitate although likely 90% of the rest was pure Hydroxide.
So a lot of this has to be improved but it's showing some tendencies already that this method could work. I will keep you updated on the process as I
believe this could be a quite easy, cheap and safe method to make small amounts of cyanides in a home lab. If you have any ideas or suggestions there
are all welcome.
[Edited on 4-2-2017 by fluorescence]
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unionised
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Airlocks are not designed to ensure that gases mix well with the water so they may not be very efficient traps.
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nezza
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You can make mixed KCN/NaCN by heating DRY ferrocyanide with Sodium and extracting with water. I suspect using Potassium would give KCN only, but the
reaction may be violent.
If you're not part of the solution, you're part of the precipitate.
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fluorescence
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You could also heat it directly till it decomposes but as I mentioned above I don't really like these leaching methods and it's not about the KCN or
NaCN, I can just buy them if I need them, it's for more instable Cyanides. One way, which has been tried by some russian chemists in the 60s I think
was to set the desired cation as counterion for a ferricyanide and thermally decompose it. Problem many of these cyanides decompose, too. They tried
to make Rare-Earth Cyanides that way (even today they are really hard to make). So yes this could be an idea for some stable ones...maybe even for the
barium one but I am looking for a completely different way.
[Edited on 5-2-2017 by fluorescence]
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