# How to prepare potassium cyanide from potassium ferrocyanide

How do we prepare potassium cyanide from potassium ferrocyanide?

It is something which I am curious about, but cannot find the answer. I am studying inorganic chemistry but this reaction cannot be found.

Until about 1900, potassium cyanide was produced by melting dry potassium carbonate with dry potassium hexacyanidoferrate(II): $$\ce{K4[Fe(CN)6] + K2CO3 -> 6KCN + FeO + CO2}$$ However, the product contains potassium cyanate, which is approximately formed as follows: $$\ce{K4[Fe(CN)6] + K2CO3 -> 5KCN + KOCN + Fe + CO2}$$

In a small scale, dry potassium hexacyanidoferrate(II) is melted in a covered porcelain crucible. It is heated as long as nitrogen escapes. $$\ce{K4[Fe(CN)6] -> 4KCN + N2 +FeC2}$$ The iron carbide sinks to the bottom and the molten potassium cyanide can be poured off.

In addition to Loong's excellent answer, I'm going to shed some light on the last process i.e. the decomposition reaction of potassium ferrocyanide.

Actual reaction is this:

$$\ce{3K4[Fe(CN)6] ->[\Delta]12KCN + Fe3C + 5C + 3N2}$$

The thermal decomposition of potassium hexacyanoferrate(II) to produce potassium cyanide, iron carbide, carbon and nitrogen. This reaction takes place at a temperature near 650°C.(source)

As always nitrogen gas escapes, iron carbide and carbon sinks to the bottom and the molten potassium cyanide can be poured off.

There is a paper which discuss the decomposition of potassium ferrocyanide trihydrate($\ce{K4[Fe(CN)6].3H2O}$) to give anhydrous potassium ferrocyanide. Further increase in heat gives various decomposition products like $\ce{α-Fe2O3, Fe3O4, Fe3C, Fe, FeO, KFeO2,\beta-FeOOH, KOCN, K2CO3}$ and $\ce{KCN}$.