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Author: Subject: Cyanide purification- prevent phase separation from salt
walruslover69
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[*] posted on 25-6-2019 at 08:06
Cyanide purification- prevent phase separation from salt


I recently carried out a modification of Nurdrage's Potassium Cyanide synthesis. The result of which is a large amount ~100g of rock hard salt deposited on the sides of my reaction container consisting of KOH, KOCN, KCN and probably some K2CO3.

This was dissolved in water to remove from the stainless steal container. I then saturated the water solution with NaCl with the goal of precipitating out any unreduced cyanate as NaOCN is only soluble at ~12g/100ml @ 25C. I filtered the solution to remove NaOCN and excess NaCl.

I then attempted to precipitate out most of the KCN by adding IPA to the aqueous solution. I should have foreseen it but this failed, resulting in complete phase seperation with a Clear IPA layer sitting ontop of the aqueous.

I then decided to just remove all of the water and IPA bringing me back to the thick hard salt deposited on the walls of my beaker, this time with the cyanate removed but a substantial amount of NaCl mixed in.

I attempted to extract all KOH with 2X100ml hot IPA and 1X100ml hot ethanol. Then extract KCN with 4X50ml hot methanol. This somewhat worked but I only managed to extract ~2.5g of KCN. Each aliquot of methanol contained roughly the same amount so there is still probably lots of KCN in the salt crust. I think one problem is the salt forms a very thick and hard 1/4 layer on the bottom of the flask, my stirbar doesn't really work due to the unevenness of the salt layer so I doubt the methanol is penetrating and extracting very well. Doing this many methanol extractions is a huge pain for the amount of product I am extracting, Does anyone have any recommendations?

The easiest way seems to be my initial approach of adding ethanol, IPA, or something else to a aqueous solution to crash out the KCN, but the phase separation is a problem. Is there any simple way to prevent the alcohol from being salted out?
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happyfooddance
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[*] posted on 25-6-2019 at 23:45


Wait, did you take a small aliquot of both the aqueous and ipa layers and compare them? Drop them in an evaporating dish or test tube in the (not food) freezer

You may have stumbled upon an effective way of separating the bulk of your components cleanly, but you will never know unless you check...

In general, sodium cations phase separate ipa/water, and potassium cations phase separate etOH/water (although fair bits of hydroxides solubilize in either alcohol well)

I feel like if you might get better results perhaps if you try the same thing but not add any excess salt, just use a proportionately smaller volume of water. In other words, dissolve your cake in nearly the minimum amount of water (it may be fused in places and will probably need to be broken up; obviously in this case everything we are talking about requires extreme vigilance, the general safety disclaimer that attends this subject is magnified), and then take 3 samples of this aqueous rxn mixture in 3 test tubes, and test each of your 3 alcohols dropwise, with an observant eye and go from there.

I don't know exactly what your rxn mixture is though, but hope this helps. Also, do be safe and clean up with a light bleach soln
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walruslover69
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[*] posted on 26-6-2019 at 08:11


I didn't test the alcohol layer, but normally KCN is only soluble in IPA at less than 1g/100ml while its water solubility is ~70g/100ml. I would be tremendously suprized it there was any KCN in the IPA. I did take an aliquot of the aques solution and trying Methanol instead of IPA and the the same effect occured.

The initial reactants were Urea, KOH and Magnesium metal. There was an excess of KOH to keep the reaction fluid(ish) and not solidifying. So the salt is probably 95% or more KOH, KCN and NaCl.

For waste I have been washing everything with water, storing it in a big jug that I plan on mixing with either bleach or peroxide to oxidize it it.
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[*] posted on 26-6-2019 at 17:41


Well you're still alive to talk, so I will let off on safety warnings.

I only mentioned the bit about IPA because it is an underestimated non-polar solvent in sodium-laden aqueous solutions, and not many people seem to realize that (valuable) fact.

What I said about the cations and alcohols is unequivocally true; so I would say, try to exclude either K or Na as a cation, and if you have a K mix, use IPA, or if an Na mix use etOH. Or use methanol, but I don't see any actual usefulness of using NaCl at any point, if you are just trying to keep your solution saturated, use less water.
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walruslover69
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[*] posted on 27-6-2019 at 05:54


The point of the NaCl was to precipitate out the NaOCN which is much less soluble than KOCN. I didn't have any other great idea on how to seperate the cyanate from the cyanide otherwise. In the future I can try doing it with KOH or KCl instead of NaCl and seeing If it will separate.

Thats a really useful/interesting difference in properties between the potassium and sodium ions that I wasn't aware of.

I am considering just acidifying and distilling the HCN, bubbling the distillate through a saturated solution of KOH in IPA. KCN should precipitate. I realize there is a fair/large amount of danger involved but with the glassware setup I have in mind it seems very doable. I have an adapter that has glass joints but on the inside is a 6 inch pipette tip that goes most of the way down the flask it connects to. I plan on leading the HCN through that to bubble it through the IPA and having the 2nd neck of the flask connected to either a long drying tube filled with carbonate or bubble through a tall graduated cylinder filled with bleach.
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Mabus
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[*] posted on 7-7-2019 at 07:40


I tried to separate crude cyanide from NaOH a while ago using isopropanol and I failed miserably. The excess salt, regardless what salt it is kept separating the isopropanol from the solution. Isopropanol is not good for this. I'd try ethanol, since last time it doesn't form a phase (though I still didn't manage to get any CN, must have done something wrong).



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