Does anyone have any experience with the preparation of potassium cyanide from potassium ferrocyanide via $\ce{HCN}$ method? I did some math and am going to proceed eventually, any advice would be welcomed. I have access to well equipped lab, respirator with filter rated for $\ce{HCN}$ and I know what I'm doing, I just never previously worked with $\ce{HCN}$, so I'm looking for "holes" in my preparation to do it as safely as possible.

\begin{align} \ce{K4[Fe(CN)6] + 3 H2SO4 &-> FeSO4 + 2 K2SO4 + 6 HCN} \\ \ce{2 K4[Fe(CN)6] + 3 H2SO4 &-> 3 K2SO4 + 6 HCN + K2Fe[Fe(CN)6]} \end{align}

Then we pass $\ce{HCN (g)}$ through absolute ethanol medium to precipitate $\ce{KCN}$. An aqueous solvent not feasible due to polymerization.

$$\ce{HCN (aq) + KOH (aq) -> KCN (aq) + H2O (l)}$$

My question is: Why doesn't first reaction work under usual conditions?

There are some major mistakes in the above equations: both $\ce{H2SO4}$ and potassium (III) ferrocyanide couldn't be used, thus should be swapped for concentrated nitric acid and potassium (II) ferricyanide, which make them not feasible.

I did have access to $\ce{KCN}$ for years now, which was given to me by a fellow chemist, now I'm running low on supply and I'm about to prepare some myself. I am aware of how does this thread may sound like, so I can provide any proof of my word upon request.

  • 1
    $\begingroup$ I don't know if it is wise to answer, because HCN is one of the worst toxic gases. It was used by the Nazis during WWII to kill hundreds of Jews in a couple of minutes. So why are you so interested in producing HCN ? Why ? I will not develop too much this field, as for example the first reaction does not work in usual conditions. $\endgroup$
    – Maurice
    Apr 1 '20 at 12:40
  • 1
    $\begingroup$ Since you can buy potassium cyanide and it is far easier to handle than HCN, why the heck would you choose this route to it? $\endgroup$
    – matt_black
    Apr 1 '20 at 12:43
  • 1
    $\begingroup$ @THF. No. I will not help you synthesize HCN. I will not tell you anything about your reaction for producing it. $\endgroup$
    – Maurice
    Apr 1 '20 at 12:58
  • 2
    $\begingroup$ If one needs and does not have a licence to buy and manage KCN, it is a very bad idea to want to synthetize it via the more feared HCN, and even worse idea would be providing any advice for that, except not doing so. $\endgroup$
    – Poutnik
    Apr 1 '20 at 13:52
  • 2
    $\begingroup$ Related: How to prepare potassium cyanide from potassium ferrocyanide $\endgroup$
    – user7951
    Apr 3 '20 at 16:46

I will quote some words from an experienced chemist per this source:

Ferricyanide and ferrocyanide have their cyanide ions very tightly bound to the iron core and 6M HCl will have difficulty releasing the cyanide as HCN. Maybe you get a little HCN if you heat the solution, but even then do not expect a smooth and satisfactory reaction. Ferricyanide also has the added disadvantage that the iron(III) is somewhat oxidizing and in practice, the yield will be even further reduced, due to internal oxidation of the ion. You'll get other decomposition products.

Another problem with your suggestion is that you should not bubble the gas through aqueous KOH. The aqueous solution of KCN is quite unstable and this solution quickly turns brown with formation of polymeric species. Better results are obtained with solutions of KOH in absolute ethanol. KCN will precipitate from such solutions if HCN is passed through them.

which should answer your question as to why your first reaction will not work under usual conditions.

  • 3
    $\begingroup$ Remember you are helping also people who want to obtain KCN for malicious reasons. Not very wise. The is no guarantee the OP does not belong to them either. $\endgroup$
    – Poutnik
    Apr 3 '20 at 2:21
  • $\begingroup$ I got KCN for years in my lab, which was given to me. Now Im running low on supply and want to actually extract some gold from processor pins which I have too much for what I got now. $\endgroup$
    – THF
    Apr 3 '20 at 13:44
  • 1
    $\begingroup$ @THF You state to be in Germany, so I speculate the processors in question equally are. Just type «processor recycling germany» into a search engine and you find multiple companies which already have the know-how and equipment in place to do this for you (even now) and offer money in return. By reading their lists, you'll see how little Au still is used today (e.g., precious-metal-services.com/…). Think twice, not only about your safety and your neighbours. Let professionals do this instead of you. $\endgroup$
    – Buttonwood
    Apr 3 '20 at 20:01
  • $\begingroup$ Dear Buttonwood, didn't you think that everything I do got a reason behind? First off: Im already into reprocessing other metals like silver, platinum or palladium so its gold time now. Im not doing it for money because gold recycling became way less profitable in recent years. My final objectives are always to explore the science myself and learn. I also got every bit of equipment and now I got complete know-how too so? $\endgroup$
    – THF
    Apr 3 '20 at 20:12
  • $\begingroup$ "Let professionals do this instead of you" sounds like you telling me that doing anything by yourself instead of sending it to someone who does it at large scale is nonsense, especially when you are not a "professional" whatever it means. My neighbours are too far away for them to be affected by anything I do, and my own safety is my own concern. What do you think is the difference betwen self-learning and studying it on university? Someone tells you what to do to do it safely. You know why? Because like 90% of students are complete morons, just a bit educated to help them elaborate. $\endgroup$
    – THF
    Apr 3 '20 at 20:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.