Sciencemadness Discussion Board - when is HCN gas evolved from ferrocyanide?

when is HCN gas evolved from ferrocyanide?

bob800 - 9-11-2010 at 16:39

I was following instructions for synthesizing Prussian Blue ink from Burton Hawk's Experimenting with Chemistry: Experiments for the Home Lab. Here's snippet of the procedure:

Quote:

Prussian blue (ferric ferrocyanide) provides the coloring pigment for this
ink. Dissolve 1 gram of ferric ammonium sulfate in 10 cc. of water. Dissolve 2 grams of potassium or sodium ferrocyanide in 15 cc. of water. Mix the two solutions, filter and thoroughly wash the precipitate. Then add just enough oxalic acid solution to dissolve the precipitate forming a syrupy liquid. Finally, thin with water and your ink is ready for writing.

I was about to add the oxalic acid when I rememberd that acids + ferrocyanide = HCN! However, I don't know why this would be used without warning if that was the case for this. Is oxalic acid an exception for this? If so, then why do some acids liberate HCN while others don't?

12AX7 - 9-11-2010 at 18:02

Oxalic acid forms a fairly stable complex with iron, so it could in principle displace the cyanide if in sufficient excess. Ditto for strong acids, which displace it by brute force protonation. The Kf of cyanide is pretty serious though, and I doubt the pKa * Kf of oxalate is enough to do much. Someone with the numbers can calculate if it's possible at all. In the mean time, just don't add an excess, use a minimum as stated.

Tim

The WiZard is In - 10-11-2010 at 16:46

Quote: Originally posted by bob800

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Lab prep uses K ferro and H2SO4.

entropy51 - 10-11-2010 at 17:39

A Google using "prussian blue" and "cyanide release" would be educational. I'm surprised you didn't try it.

Fear not unless you are working on a kilogram scale. Common sense says that all chemistry should include a little ventilation.