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Okay, so I was fiddling about in the chemistry lab, and I added:
Sodium Nitroprusside (soln) $\ce{Na_2[Fe(CN)_5NO]}$ with Potassium Ferrocyanide (soln) $\ce{K_4[Fe(CN)_6]}$.

$\ce{Na_2[Fe(CN)_5NO]}$ was a yellowish-reddish colour - actually don't remember the exact colour, while $\ce{K_4[Fe(CN)_6]}$ was pale yellow.

Nothing really happened to the mixture - except the colours mixed giving a weird yellow colour. I assume the only thing that happened was a double decomposition reaction:

$\ce{K_4[Fe(CN)_6] + Na_2[Fe(CN)_5NO] \rightarrow K_2[Fe(CN)_5NO] + Na_4[Fe(CN)_6]}$
or something like that.

Next I added a bit of dil Hydrochloric Acid. I expected nothing to happen - nothing ever happens with the chemicals they keep in labs. :P However, the colour of the mixture turned a beautiful Emerald Green!

Now I've been trying to figure out what exactly occured to produce the green colour, but I don't know. Could anyone help me?

$P.S.$ My friend's gonna send me a picture of the green we took. I'll edit it to the question when I recieve it, maybe it will help?

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    $\begingroup$ It should be noted that this is a potentially very dangerous experiment. Addition of a strong acid (note that a dilute strong acid is still a strong acid) to ferrocyanide can lead to the generation of hydrogen cyanide gas, which is extremely toxic. $\endgroup$ – bobthechemist Jul 6 '13 at 17:16
  • $\begingroup$ Whoa. :O I did NOT know that. Thankfully, HCN wasn't formed. Phew. Thanks for the info! :D P.S. Any idea why the green in colour was formed? $\endgroup$ – mikhailcazi Jul 6 '13 at 19:49
  • $\begingroup$ sodium ferrocyanide is green in colour. see this:-google.co.in/… $\endgroup$ – Nilay Ghosh Apr 22 '16 at 12:55

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