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For example, will squeezing lemon juice on top of table salt with sodium ferrocyanide ($\ce{Na4[Fe(CN)6]}$) additive (E 535) release hydrogen cyanide gas?

Examples of ferrocyanide additives:

  1. sodium ferrocyanide (E 535)
  2. potassium ferrocyanide (E 536)
  3. calcium ferrocyanide (E 538)
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  • $\begingroup$ You can see this video at 2:02 for reference but seriously don't try this. $\endgroup$ – Nilay Ghosh Nov 6 '17 at 8:19
  • $\begingroup$ @NilayGhosh your video is irrelevant. This question is about hexacyanoferrate not sodium cyanide. $\endgroup$ – matt_black Nov 6 '17 at 9:31
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No. The hexacyanidoferrate(II) complex is a very inert complex that does not undergo ligand exchange easily. This is somewhat exemplified by the harsh conditions alchemists used to synthesise hexacyanidoferrate(II), which included high temperatures and strong bases.

Furthermore, whatever amount of lemon juice you add the resulting acidity will be much less than in your stomach. If you swallow a cyanide source such as potassium cyanide, this will get protonated by your stomach acid to liberate hydrogen cyanide and potentially kill you. Food additives — especially approved ones — undergo various testing procedures to make sure that does not happen.

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  • $\begingroup$ Yes, it has been demonstrated in this video. $\endgroup$ – Nilay Ghosh Nov 6 '17 at 8:18
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    $\begingroup$ @NilayGhosh what you reference is totally misleading and irrelevant. Cody is playing with sodium cyanide. This question is about hexacyanoferrate salts which are totally different, behave totally differently and are far safer. It is easy to get HCN from sodium cyanide and almost impossible from hexacyanoferrate, which is why it is safe as a food additive. $\endgroup$ – matt_black Nov 6 '17 at 9:30
  • $\begingroup$ Hexacyanoferrate(II) maybe? $\endgroup$ – Oscar Lanzi Nov 6 '17 at 22:06
  • $\begingroup$ Is the complex as stable with Fe(III) as with Fe(II)? $\endgroup$ – Oscar Lanzi Nov 7 '17 at 1:18
  • $\begingroup$ @OscarLanzi Hexacyanidoferrate(II) is actually more stable than hexacyanidoferrate(III) (which is an oxidising agent) due to the $\mathrm{(t_{2g})^6}$ state. $\endgroup$ – Jan Nov 7 '17 at 1:20

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