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Author: Subject: Cyanide from Prussian Blue

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[*] posted on 24-1-2008 at 09:12
Cyanide from Prussian Blue

I have some prussian blue. How would I go about safely isolating a cyanide salt like the sodium or potassium cyanide from it?

I also suppose if you dissolve it in phosphoric acid that it liberates cyanide gas? Does someone know how violent/quick the release would be?
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[*] posted on 24-1-2008 at 10:02

The release is not violent at all. The cyanide ligands are attached to the iron cores very tightly. If you want to release HCN from this compound, then you need to add it to dilute sulphuric acid (30% or so) and heat. In that case, HCN is released, but even then is it going slowly. A high concentration of phosphoric acid most likely will release HCN as well, but this also will require heating.

Bubbling the HCN through an aqueous solution of NaOH or KOH does not give you the cyanide when the solution is evaporated to dryness. Much of the HCN is released again under such conditions and at best you get a highly alkaline NaCN/NaOH mix. There also will be side reactions, in which HCN/CN(-) is oxidized and you most likely get brown crap.

You could try getting NaCN from ethanol or methanol solution of NaOH.

Beware, working with HCN is EXTREMELY dangerous. Something like 50 mg of the gas can knock you down! If you don't have extensive experience with complex chemical apparatus, then DON'T MAKE HCN as you suggested.

Keep an antidote kit nearby (amyl nitrate, IIRC sodium thiosulfate with some nitrite also works) and never work alone with HCN. Also, be sure how to apply the antidote if necessary. Study this before doing any work with HCN, both you and the other person, who is nearby.

[Edited on 24-1-08 by woelen]

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[*] posted on 24-1-2008 at 16:30

Turning HCN into a cyanide salt is best done in alcoholic solution, as an aqueous solution of cyanide indeed decomposes very easily (not into HCN, but into formate and ammonia, by breakdown of the cyanide ion itself).
Adding HCN to an alcoholic KOH solution directly precipitates KCN which can be filtered off.

[Edited on 25-1-2008 by garage chemist]
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