Sciencemadness Discussion Board

NaCN/HCN preparation plan

rememe - 12-1-2014 at 09:23

Outside in the freezing cold. I planned to place a beaker 100g potassium ferricyanide (iii) and some amount of cheap hardware store 31.45% HCl. Double boil in water with a thermometer in the cork to see gas stays around 26C/79F. Other hole of cork going to copper condenser coil about 10inches long probably packed with snow going to another beaker in ice water. Output from that to 3rd beaker with some NaOH solution to bubble through.

Is there any danger of HCN gas leak assuming my hoses and rubber corks are tight? No ground glassware.

What amount of HCl should I add to 1st beaker? I haven't worked out molar amounts but I assumed excess would be fine like 200ml.

Will HCl even work? The youtube video showed phosphoric acid but thats a huge pain to get ahold of here. I read that any weak acid will work to free HCN from ferricyanide.

I assume the HCN will mostly condense into the 2nd beaker and then I can mix it with concentrated NaOH in 40% ethanol solution to crash it out. Is that enough ethanol or will it stay dissolved? I can't get ahold of pure ethanol but I could try pouring the HCN liquid into concentrated NaOH in methanol.

Would NaOH/40% ethanol work?
Would NaOH/methanol work?

Otherwise if I have to use concentrated NaOH aqueous I'll have to work out the molar amounts of NaOH to somehow be nearly the same as the amount of HCN produced so I dont get an excess of NaOH when exaporated. How the hell can I do that.

I wanted to condense the HCN first instead of running it straight into NaOH solution to prevent polymerization that I've read about.

I'm going to use the NaCN for trying to recover electroplating gold if anyone's curious. All done outside of course.

[Edited on 12-1-2014 by rememe]

[Edited on 12-1-2014 by rememe]

[Edited on 12-1-2014 by rememe]

[Edited on 12-1-2014 by rememe]

TheChemiKid - 12-1-2014 at 10:37

The copper tube should be longer. Copper, even though a great conductor of heat, will still let lots of gas through. The rubber stoppers worry me as well, I feel they may let some gas through.

HCl should work, it is a stronger acid than HCN.

Not sure about this, but suckback may occur when HCN production starts to slow. Do you have a good way to prevent that?
Here is a good video on preventing this, but attaching a funnel may be difficult.

Just making sure, but you now how toxic HCN is, right? less than 350ppm (300mg/m3) can kill you in 10 minutes!

EDIT: If you do this, have some Sodium Thiosulfate solution nearby, it is an antidote to HCN.

[Edited on 1-12-2014 by TheChemiKid]

testimento - 12-1-2014 at 11:07

Any strong acid will work and with sodium or potassium even weak acids like acetic and citric acid will do. HCN is lighter than air. Use fume hood or well ventilated area, preferably outside. Use fan to blow the fumes off yourself. Normal glass app with ground joints is very enough to keep the fumes in. Prepare dropper funnel with HCl and drop in solution and lead the gases into concentrated water/NaOH solution made accordingly to molar amounts to make NaCN. As long as the water solution of cyanide salts is basic, at least 8-9, it will not give HCN fumes, since it would instantly form NaCN and the reaction ceases. If it gets basic or slightly acidic during bubbling HCN into it, add little more NaOH. The HCN will dissolve miscibly into cold water(keep in ice water bath), so you can add the NaOH when you're done.

When you're done, pour into the beaker a 2-3 volumes of ethanol, that will instantly crystallize the NaCN out of the liquid and you can filter it out. Neutralize the remaining liquids with hypochlorite.

rememe - 12-1-2014 at 11:11

Sorry I meant to say 120 inches copper 1/4" tubing instead of 10 inches. I think that should be long enough, if its packed with snow.

I'm not really sure if polymerization of HCN is a real problem but I read that it happened to someone on another thread. It is supposed to occur above 50C but maybe his ambient temps were high. It's freezing winter temps where I am now.

I know HCN is very poisonous that's why I'm posting my plan here to see what everyone thinks. In case the tubing suddenly breaks off I guess I should have a bucket of water to dump all the apparatus in.

I hadn't thought about suckback so I'll have to research that.

I don't even have a balanced equation for the reaction yet so I'm not sure how much HCl solution I need to use to be in excess.

If I need 2-3 volumes of ethanol to crash out it might be a real problem since I can only get everclear here and its expensive. Can I use methanol?

[Edited on 12-1-2014 by rememe]

cyanureeves - 12-1-2014 at 11:13

i cannot find those back flow valves on e-bay only flow control valves.what are they called?i did this bubbling hcn into cold methanol using sulfuric acid and gold plated with the cyanate.i never got the cyanide to dissolve gold because i knew very little about cyanide and how it turns to cyanate in water.i even bought phosphoric acid but never did it again.i never thought about back flow either and it eventually happened afterwards when i was bubbling ammonia into water.i used a vacuum coffee maker and a garden hose going directly into the cold methanol/hydroxide mix.the ammonia generator that sucked back cold water happened only when i stopped the heat and the reaction vessel dropped just a few degrees.the explosion was instantly and had it been cyanide i never would have known about back flow or many other things i have learned since.Myfanwy's cyanide was in the condensate dripping into the hydroxide but i mostly just bubbled the gas into the solution. Mario840 talked to the maker of that particular video right here on this forum and decided phosphoric acid was better than sulfuric acid for this method.Mario840 told me to use phosphoric acid but i lost my confidence altogether now.take much care and if possible keep your distance while you are making the stuff.

[Edited on 1-12-2014 by cyanureeves]

[Edited on 1-12-2014 by cyanureeves]

Bot0nist - 12-1-2014 at 12:01

Make sure to search for and read the cyanide threads on this very site, and take all nessasary precautions. Synthesis of Na or K cyanide is definatly in the capabilities of the amatuer chemist, but never underestimate its abilitly to do you and others harm. Always imagine the worst case scenario when doing these reactions and plan accordingly. That's what I have done whenever working with energetic materials, and it has kept me safe and intact, even in the event of an unexpected detonation. In my opinion, any synth involving HCN is just as risky as a sensitive EM synthesis, so please use caution. Remember, failure to plan is planning to fail, and with these compounds, a single failure could be your last.

I hope this doesn't come across as fear mongering, discouraging, or chemophobic. I only want to help prevent another tragic and early loss of an up and coming amateur chemist, like myfanwy.

Good luck, and stay safe...
The Bot.

[Edited on 12-1-2014 by Bot0nist]

testimento - 12-1-2014 at 13:17

HCN is more dangerous than general explosive synthesis. When a synth goes wrong, it generally just sits and one can dump it into water, but with HCN as a gas, it's totally another story. Blowing up apparatus with 100 grams of HCN gas into 100 liters is not enough that you just pour water on it, it will kill everything within certain range.

Check plastic tubing and HCN gas compatibility, I remember that normal materials withstand it, so you can make two bottles row setup to prevent suckback into reaction.

Ethanol and methanol both work fine. In technical grade very well too.

Use gas mask. Its cheap insurance. I use one always when performing with anything hazardous. Theyre not expensive. Most of this forum people waste tons of money into something useless like drinking alcohol but cannot spare 50 bucks for a decent protective gear.

Pyro - 12-1-2014 at 13:37

drinking isn't useless, it makes us forget our worries (for good in a high enough dose :D)

having isopropyl nitrate on hand isn't bad either.

You don't really need a condenser, if you use a good bubbler and steady gas flow

rememe - 12-1-2014 at 14:15

A small fish tank bubbler on the end of silicone tubing?

I was concerned about using a bubbler at all because of the back pressure on the tubing. I wanted no back pressure to cause some accidental hose popping off or cork popping out of the flask.

I sure hope I don't catch the HCN on fire or something and cause an explosion. As for it leaking out, since the ambient temps outside are so cold I suppose a leak wouldn't get very far before condensing and dropping to the ground. Gas mask sounds like a really good idea.

[Edited on 12-1-2014 by rememe]

Pyro - 12-1-2014 at 14:17

I meant bubbler for HCN.
you have a tube with a fritted glass thing on the end to disperse the bubbles and get complete absorbtion

DJF90 - 12-1-2014 at 16:02

Quote: Originally posted by Pyro  


having isopropyl nitrate on hand isn't bad either.


Rather you than me. Personally I'd have amyl *nitrite* on hand, but even then its just to keep your heart beating until the paramedics get to you. Its not an antidote, per se. Supposedly medical services use cobalamine (like vitamin B12...) as an antidote.

Pyro - 12-1-2014 at 16:50

amyl nitrite is a lot harder to come by.
anything is better than nothing...

DJF90 - 12-1-2014 at 23:25

You missed the *point*

woelen - 13-1-2014 at 03:10

Do not get surprised about the difficulty you will have getting HCN out of the ferricyanide and HCl. The iron binds the cyanide ions very strongly and even strong HCl does not simply release the HCN. You need to heat the liquid very strongly (boiling) and then you get impure HCN (mixed with HCl if you use hydrochloric acid).

If you use 50% sulphuric acid or phosphoric acid, then you at least do not have contamination with acid vapor. But with these you also need to heat strongly to drive off HCN and do not expect to have a good yield. Maybe you drive off 2 or so of the 6 cyanide ligands before total decomposition occurs.

It is exactly for this reason why ferrocyanides and ferricyanides are so easy to obtain and are not considered suspect at all. They are only weakly toxic and making free cyanide from these chemicals is not easy.

If you really want to try this, then I warn you not to isolate pure HCN. This is very volatile and even if you work outside on a breezy day, you most likely kill yourself if you play around with a beaker, filled with a few tens of ml of HCN which you want to pour into the alcohol. Don't do that if you don't want to kill yourself or your neighbours.

rememe - 13-1-2014 at 10:43

Double boiler is a pointless then? I forget where I got the idea that I'd need to keep the reaction vessel under 100C.

Someone said that almost any acid will work. I get the point about HCl vapor contamination. What about citric acid? Could I mix the ferricyanide with maybe 200g citric acid and enough water and get any HCN gas out of it?

5% acetic acid?

On the other giant cyanide thread someone mentioned getting a lungful of HCN and it didnt kill him, and that he has done the phosphoric acid reaction on a benchtop without special ventilation. I'm obviously not going to do something stupid if this is really as dangerous as it sounds, but other experienced chemists seem to think that some of the warnings are exaggerated.

About isolating pure HCN, I planned to start the 2nd beaker with some water so that it wouldnt end up as pure HCN. I forget the reason that that was recommended, maybe to prevent polymerization. Is that what you mean by not isolating pure HCN?

Honestly I dont want HCN at all but NaCN or KCN or some other salt that I can electrolyze. I read about another reaction with chloroform and ammonia gas in a hot vessel but it sounded harder and chloroform costs a lot to make.

Someone mentioned heating zinc powder and urea to produce HCN. Has anyone tried that?

[Edited on 13-1-2014 by rememe]

Pyro - 13-1-2014 at 13:41

DJF90; Id rather crap my pants than die :)

Woelen: if it's very very cold it shouldn't be a problem to have a beaker (or flask) should it? anyway, killing my neighbours, well, ok. but me? no thanks :)

why not wait until you have a full glass apparatus, then you are a lot safer than with a slipshod setup.

Bot0nist - 13-1-2014 at 15:00

I think he was pointing out that you said nitrate instead of nitrite...

DJF90 - 13-1-2014 at 23:36

Thanks B0tonist.

woelen - 14-1-2014 at 01:03

Getting HCN from ferricyanide or ferrocyanide only is possible with strong acids, so forget the use of acetic acid, citric acid, boric acid and other weak stuff. You really need 50% H2SO4 or 80% H3PO4.

Nicodem - 14-1-2014 at 09:54

Woelen, are you sure that citric acid would not work? Citric acid is quite a strong ligand for Fe(II&III) (see Chem. Zvesti 36, 333—334 for a table of stability constants). The various stability constants might be comparable to the first few stability constants of ferricyanide/ferrocyanide, but for some reason I can't find the step-wise stability constants for cyanide (every source just lists the cumulative constants for this ligand vs. Fe). It is hard to say much about the displacement ability, because the citrate demands chelation while the cyanide can be displaced step-wisely. I guess this requires comparison with the third ferricyanide/ferrocyanide stability constants which I just can't find anywhere.

Oxalic acid is also a relatively strong ligand for Fe, particularly for Fe(III) (logK1=9.4) and might work better as it displaces only two cyanides. Salicylic acid is even stronger (logK1 = 16.5 for Fe(III) and 6.6 for Fe(II)). I think these carboxylic acids coordinate Fe(III) more strongly that phosphoric acid and all are relatively non-volatile (at least from solutions). So I would not discard the possibility that citric and oxalic acid could be applicable. If nothing else, I would guess they have an even better chance of working than hydrochloric acid. Chloride is a lousy Fe ligand: logK1=1.5 for Fe(III) and 0.4 for Fe(II), insignificant when compared to cyanide.

Stability constants are cited from Lange's Handbook of Chemistry (15th Edition), chapter 8.2.2

DJF90 - 14-1-2014 at 13:30

I was curious about this, considering the chelate effect that would be afforded by e.g. citrate, and the large entropic gain afforded by such a ligand substitution. So I found the attached paper which states the Log K of ferricyanide is 43.5. After searching for comparitive numbers, I came up with (for Fe (III)):
Citric acid Log k = 11.85
Salicylic acid Log k = 16.35
EDTA Log k - 25.7

So on first glance, it looks as if we don't have a chance, but this doesn't account for the gain in entropy of the system. It is also ambiguous in that the value for salicylic acid is not specified as an overall Log k or not.

The plus side is that the paper states
Quote:
because strong complexes such as iron (II) cyanide dissociate only under strongly acidic conditions (pH < 2) and are considered relatively stable and nontoxic.


Bear in mind that a 10% citric acid solution has a pH of 0-1, so it should be sufficient for the liberation of hydrogen cyanide.

Attachment: 6760-AU147_LPN1570.pdf (159kB)
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woelen - 15-1-2014 at 01:36

Nicodem, your remark about coordinating capabilities of citric acid and oxalic acid are quite interesting. This is a nice reason to take out the test tubes and try this. I'll try on a small scale with K4Fe(CN)6 and oxalic acid and do the same with citric acid. Most likely a lot of heating is required. HCN can easily be detected, because it can easily be ignited (I once made a warm solution from which HCN was bubbling much like CO2 is bubbling from fresh soft drinks and when a flame was kept near the open end of the test tube I had a nice flame of a few cm height).

I, still, however, do not expect much from this, but let experimental outcome decide. I'll come back on this the day after tomorrow or next weekend, when I have done the experiment.

phlogiston - 15-1-2014 at 02:00

Considering that you have to use improvised equipment (no ground glass joints etc), and also seem relatively unexperienced at this kind of work (did not consider flashback etc), are you sure you want to go this route?
There are alternative methods to recover gold that are a lot safer.

One possible option for avoiding the need to handle HCN directly and still make sure you have little remaining NaOH even if the yield is unpredictable is to use a train of wash bottles each with a small amount of NaOH solution to absorb the HCN. As the reaction proceeds, the first bottle will be neutralized first, and then the next one, etc. At the end, you check the pH of all the solutions, pool those that are nearly neutral and dispose of the rest.

[Edited on 15-1-2014 by phlogiston]

Nicodem - 15-1-2014 at 09:31

Quote: Originally posted by DJF90  
So I found the attached paper which states the Log K of ferricyanide is 43.5.

That is a cumulative stability constant which is of little value here (essentially, it is the constant for the: Fe(CN)63- -> Fe3+ + 6 CN-). What is needed are the first few step-wise stability constants (K1, K2...) as these are the ones that determine the strength of the coordinative bond. Unfortunately, I can't find any such data for Fe-CN complexes. Most likely, the two factors that most significantly influence the formation of HCN from ferricyanides/ferrocyanides is the K1 and the pKa's constants of both, the cyanide and the competing ligand. The stronger the ligand, the more CN- is substituted. The stronger the acid, the less CN- can there exist in the reaction media (and also less competing ligand). Unfortunately, there is very little oxalate/citrate/salicylate present at low pH values, so the two factors act antagonistically (oxalic acid being the most acidic of these three). This problem does not occur with phosphoric and sulfuric acid (the first is not that well dissociated, but this must be compensated by it being a good Fe ligand, while the second is a very strong acid and quantitatively protonates any liberated cyanide ligand).

woelen - 16-1-2014 at 12:52

Nicodem, I tried the experiment with K4Fe(CN)6 and oxalic acid (I surprisingly do not have any citric acid anymore, I really need to buy that again). And to my surprise, this reaction does give HCN! I did not expect anything of it, but I got sufficient HCN to ignite it. The experiment was very simple and is described below:

Take a test tube and add a fairly big spatula of solid K4Fe(CN)6.3H2O. To this add a similar big spatula full of crystalline H2C2O4.2H2O. Then add a little water and swirl for a while. Part of the solids dissolve, a mix of pale yellow K4Fe(CN)6.3H2O and white H2C2O4.2H2O remains settled at the bottom. The resulting liquid is very pale yellow.
Next, put a plug of cotton wadding at the open end of the test tube and then carefully heat the liquid with the solid in it. All of the solid material dissolves and the liquid becomes yellow and also slightly turbid. Keep heating, such that the liquid does not boil, but becomes quite hot (80 C, maybe 90 C). All the time, keep the wadding on the open end of the test tube. The liquid becomes more turbid while it is heated and a bluish/green precipitate is formed. There are no visible gas bubbles. Keep on heating for two minutes or so, until the liquid is totally opaque and greenish/blue.
Finally, take away the wadding 'stopper' and immediately keep the open end of the hot test tube near an open flame. When this is done, then a clearly visible orange flame moves inwards, into the test tube. The test tube contains a mix of a flammable gas with air.

The presence of the flame can only be explained by formation of HCN, I see no option for any other flammable gas. The green/blue solid most likely is some complicated iron cyanide/oxalate complex.

I do not consider this as a viable method for making cyanides, the yield is way too low and the reaction is slow, but from a theoretical point of view it is interesting.

Refinery - 18-2-2014 at 10:55

Apologies for my limited knowledge on chemical properties of acids, but I have a question regarding citric acid and sodium cyanide. Since sodium cyanide is a base of weak acid and citric acid readily makes citrates from CaCO3, I am in impression that citric acid and sodium cyanide will cause formation of hydrogen cyanide. Am I right, and what would be the balanced equation for this reaction?

Secondly I have heard that HCN may form polymers when liquefied/solidified. If I condense HCN fumes with salt ice bath, would it stay as pure liquid that can be stored in suitable, pressure-rated glass container or would it eventually polymerize into room temperature solid? Is there any risk that the possible polymer would act similar next to SO3 polymers that may decompose violently when heated?

Oscilllator - 18-2-2014 at 13:30

Quote: Originally posted by woelen  
Do not get surprised about the difficulty you will have getting HCN out of the ferricyanide and HCl. The iron binds the cyanide ions very strongly and even strong HCl does not simply release the HCN. You need to heat the liquid very strongly (boiling) and then you get impure HCN (mixed with HCl if you use hydrochloric acid).

If you use 50% sulphuric acid or phosphoric acid, then you at least do not have contamination with acid vapor. But with these you also need to heat strongly to drive off HCN and do not expect to have a good yield. Maybe you drive off 2 or so of the 6 cyanide ligands before total decomposition occurs.

Woelen I feel you may have spoken to hastily. In the other enormous cyanide thread zan divine describes getting an 80% yield of cyanide, much more that the 33% you suggested:
Quote: Originally posted by Zan Divine  
Typical run:
422 g (1 mol) K4[Fe(CN)6] added to 2 L round bottom with stir-bar. Add a solution of conc. H2SO4 (200 mL) in H2O (600 mL) in one portion. Attach distillation head cooled with ice-water. Reaction appratus vented by tubing running out the window. Slowly heat the stirred blue suspension. You don't need to go much above 50 - 60 degrees C. Foaming occurs in the first hour or longer as cyanide is distilled out and collected in an ice-cooled flask containing 5 mL H2O and a couple mL H2SO4. Foaming subsides after a couple hours and then the suspension thickens. Adjust stirring as necessary. After 4 hours, 147 g HCN (80%) has been collected as a water-white liquid.

Dispose of residue in accordance with local laws.