woelen
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Reaction between nitrite and cyanide?
I noticed a peculiar reaction between sodium cnitrite and sodium cyanide in acidic environments.
When the two salts are mixed and dissolved in water, then a colorless solution is obtained. When this solution is acidified, then the solution becomes
light blue and some colorless gas is formed, which becomes brown in contact with air. So, the gas must be NO and that can be expected from an acidic
nitrite solution.
If I add solid NaNO2 to a solution of NaCN in 10% HCl, then a colorless gas is produced and the liquid becomes very nice blue. I did a counter
experiment with the same amount of NaNO2 added to 7% HCl, but then the liquid only becomes very pale blue, hardly visible.
I also did an experiment, the other way around. I dissolved NaNO2 in dilute HCl. This gives an almost colorless solution and formation of NO. If I add
solid NaCN to this, then the liquid becomes brown/yellow. However, after some time, the color shifts towards light green.
Solid pieces of NaCN, sticking to the glass become dark brown quickly, when exposed to the NO-gas (or NO2 formed from oxygen and NO).
The green color probably is due to mixing of the blue color, mentioned above and the brown material, produced here from the solid NaCN.
So, there are quite some peculiar reactions:
What is the brown stuff, formed from solid NaCN and NO/NO2?
What is the blue compound, formed from solid NaNO2 and NaCN, dissolved in excess HCl? This is not simply the well-known blue nitrous acidic solution,
the blue color is much more intense and it is much more stable.
I have read about nitrosyl cyanide, being a blue gas. But I did not see any blue gas above the liquid, only pale brown, due to NO2. Does anyone know
something about the properties of nitrosyl cyanide?
If anyone wants to repeat the experiment, please be extremely careful. NaCN is exceedingly toxic and with these experiments quite some HCN may be
formed. Not all people are capable of smelling HCN (including me).
[Edited on 16-3-06 by woelen]
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guy
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Can any other acid be used, say sulfuric acid? If that works, or doesn't, then you could decide if the chloride participates. My guess is that since
NO and NO2 are oxidizers, it could have oxidized the cyanide to some form some sort of compound, like NOCN, or if possible,
NO(CN)<sub>2</sub><sup>-</sup>
[Edited on 3/17/2006 by guy]
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woelen
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I repeated the experiment with H2SO4 instead of HCl. With this, I still have the formation of the brown stuff on solid NaCN, but the blue solution is
not formed. The solution only becomes very pale blue, due to the formation of nitrous acid.
Apparently, the chloride plays an important role in the formation of the blue compound. I also was thinking of nitrosyl cyanide. This is a known
compound, and it is blue. It is a blue gas, but I did not observe any blue gas above the liquid (that would be very cool, I've never seen blue
gasses). It might be that nitrosyl cyanice is very soluble in water.
On dilution of the blue liquid (in the experiment with HCl), the blue color disappears. Apparently, the blue compound is hydrolysed.
This calls for more investigations... it is very interesting.
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guy
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This is what I would do to try and narrow down the choices.
We know that chloride is necessary for the formation of blue color.
>Try adding adding NOx to Cl-.
or try
>Adding (CN)2 + Cl- (could be kind of like a polyhalide compound).
Also, is the blue color like the blue color you got when you added NO2 to CuCl2?
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hodges
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Woelen, are you sure of the purity of your HCl? I find in other reactions that my HCl (from muriatic acid) is often contaminated with iron compounds.
Given how dark the iron cyanide compounds are it wouldn't take more than a slight trace of iron to give a blue color.
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woelen
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I'm quite sure it is not due to the ferro/ferri cyanide compounds. If I dilute the liquid by a factor of 3 or 4, then it becomes colorless, not just
lighter blue.
If I look through 2 cm of the liquid, then it is blue. If I dilute it to four times its volume with water and then I look through 8 cm of liquid, then
it is colorless. With a ferri/ferro cyanide solution, the 8 cm of diluted liquid would appear to be as blue as 2 cm of the more concentrated solution.
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Dr. Beaker
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you've forgot about good 'ol La Chatelier.
Fe+2 + 6CN- <------> [Fe(CN)6]-4
dilution favours the decomposition of the blue ferrocianate.
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Dr. Beaker
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p.s
1. try compairing the intensity of color on RT and at a lower temp. (say - near 0 oC)
the complexation should be exothermic (negative delta S) and therefore more color at lower temp is expected!
2. try bubling air to the reaction mixture. it may oxidize the blue ferrocianate to the faint yellow ferouscianate.
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