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I have read that Hydrogen Cyanide is emitted if Potassium Ferricyanide is combusted, but would this be the same case for Potassium Ferrocyanide? I am preparing to perform some case hardening on steel using a compound that contains 30-50% Potassium Ferrocyanide, so I need to be sure what measures are needed for protection from the fume.

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    $\begingroup$ I thought "combustion" only applies to something that can burn. Let's say "heating" instead. Also, heating or no heating, K3Fe(CN)6 contains no H, hence it can't possibly give off HCN. (Unless reacted with something else, that is.) $\endgroup$ Apr 28 at 21:18
  • $\begingroup$ See my answer here: chemistry.stackexchange.com/questions/32189/… $\endgroup$ Apr 29 at 2:24
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Potassium ferrocyanide cannot burn, because it is the product of a combustion. When meat or blood is overheated, carbonized and burnt, the remaining ashes contain potassium ferrocyanide. And this product was initially recovered by washing these ashes and evaporating the obtained solution. In German, this compound is called Blutlaugensalz, which means "salt from washing blood" This is even the way this product was originally discovered in the old times of beginning chemistry.

So potassium ferrocyanide resists high temperature, and will not produce any $\ce{HCN}$ by heating or by combustion. The only way of producing HCN out of it, is to heat it with diluted sulfuric acid, according to the equation : $$\ce{K4Fe(CN)6 + 3 H2SO4 -> 2 K2SO4 + FeSO4 + 6 HCN}$$This reaction is difficult to carry out properly, because, if the sulfuric acid is more concentrated, it will destroy $\ce{HCN}$ according to $$\ce{H2SO4 + 2 HCN + 2 H2O -> (NH4)2SO4 + 2 CO}$$

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I contacted the manufacturer of the case hardening compound. Their chemist replied:

Specific hazards:

Contact with acids liberates very toxic gas. Hydrogen cyanide (HCN).

Thermal decomposition or combustion products may include the following substances:

Hydrogen cyanide (HCN).

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Potassium ferricyanide comes with water of hydration, $\ce{K4Fe(CN)6}\cdot\ce{3H2O}$ (See here). Decomposition in the presence of this water of hydration could give hydrogen cyanide gas.

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I am guessing you mean a fused salt bath at 1400 to 1600 F to diffuse both carbon and nitrogen into the surface of the steel. The other salts in the bath likely have an affect on fumes. I never worked with molten salts but it is definitely not for amateurs. Find a commercial heat-treat shop that performs this process.

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