Hello,
I got a partial answer from another forum, but I thought I would double check by asking again on sciencemadness...
Let's first mention that I am not planning any actual chemistry here, this is simply in a perpsective of risk assessment.
So, I'd have a question in regard of potential hazardous chemical reaction between Potassium Ferrocyanide K4[Fe(CN)6] and acids, leading to evolution
of HCN.
Please note that this is not about potassium ferricyanide (a different chemical).
My question would be, is there any risk of evolving HCN when adding concentrated HCl, or H2SO4 or HNO3 to Potassium Ferrocyanide at room temperature
(20-30°C) and WITHOUT applying heat?
Besides, with any of the above concentrated acids, at what temperature would you risk evolving HCN? Any risk at 50°C ?
My question would be, is there any risk of evolving HCN when adding concentrated HCl, or H2SO4 or HNO3 to Potassium Ferrocyanide at room temperature
(20-30°C) and WITHOUT applying heat?
Yes, differences may be associated with the amount of HCN released rather than its complete or absent presence.
Besides, with any of the above concentrated acids, at what temperature would you risk evolving HCN? Any risk at 50°C ?
Pyrolysis typically yields potassium cyanide, not HCN, but 50°C is below even the lowest decomposition temperatures reported.Lithine - 30-3-2026 at 04:53
Quote:
Yes, differences may be associated with the amount of HCN released rather than its complete or absent presence.
Thanks.
To : Precipitates
So just to be sure:
Even at room temp, a large quantity of the ferrocyanide (say a hundred grams) mixed with an excess of concentrated strong acid would definitely yield
a hazardous amount of HCN within a realtively short amount of time (i.e. that the reaction would have completed within a day or two)?
Same would be true for the potassium ferricyanide, correct?
Besides, I assume the HCN would be released as a gas but also that a significant part of it would remain in solution rather than being off-gased?
Ps: however, that is probably untrue for highly diluted HCL, given the very low acute toxicity of both the ferro and ferricyanides orally (highly
diluted HCl in the stomach not being able to break the ligands, or it does react but just too slowly to be a hazard within the few hours of stomach
digestion?)
[Edited on 30-3-2026 by Lithine]
[Edited on 30-3-2026 by Lithine]
[Edited on 30-3-2026 by Lithine]
[Edited on 30-3-2026 by Lithine]DraconicAcid - 30-3-2026 at 08:08
FerrOcyanide is non-labile. Fe(II) is d6, and is low-spin when coordinated by cyanide. Cotton & Wilkinson claim that the free acid H4[Fe(CN)6]
can be isolated from strongly acidic solution.
FerrIcyanide is much more labile. Acidify it at your peril.Lithine - 30-3-2026 at 11:57
Quote:
the free acid H4[Fe(CN)6] can be isolated from strongly acidic solution.
To Draconic acid
Are you saying that the ferrocyanide would rather acidify into ferrocyanic acid rather than break into HCN ?(!)
PS: chemistry was a long time ago and I more or less dropped it at the spin concept. Please remind me, in what regard is the low spin relevant here?
(all I can remember is that electron's spin impacted magnetism)
[Edited on 30-3-2026 by Lithine]
[Edited on 30-3-2026 by Lithine]DraconicAcid - 30-3-2026 at 13:34
In an octahedral coordination complex (so, a metal with six monodentate ligands, or equivalent), three of the d orbitals get stabilized, and two of
them get destabilized. If the three stable orbitals (and none of the unstable ones) are either filled or contain exactly 1 electron, then that
complex is going to be non-labile (meaning its only going to exchange ligands very slowly, regardless of the magnitude of the equilibrium constant).
This is true for Cr(III) species- if you want to change the ligands on a Cr(III) complex, you often have to add a reducing agent such as zinc as a
catalyst. The Zn changes the Cr to Cr(II), which is labile, and then it changes back to (III) after doing its ligand exchange.
Fe(II) and Co(III) are both d6 species. Most Co(III) species are low spin (so there is a big difference between the stabilized and destabilized d
orbitals, so all the electrons are filling the low-energy ones, giving a non-labile complex. For Fe(II), only a few of the complexes are low spin.
Fe(H2O)6(2+) will be high-spin and labile. Ferrocyanide will be low spin, and those cyanides will be VERY slow to come off.
Ferricyanide contains Fe(III), which is d5, so that's going to be labile. The cyanides will be exchanged with other ligands much more rapidly, so I
would never acidify a solution of ferricyanide.
Then again, I'd be very nervous about acidifying a solution of ferrocyanide, just in case C&W were wrong.....Pumukli - 31-3-2026 at 07:41
Spins aside, mid-range sulfuric acid generates HCN from ferrocyanide at room temperature. Lithine - 31-3-2026 at 10:18
