fdnjj6
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Sodium nitrite from NO2 and NO?
I have been looking into sodium nitrite synthesis since many people fail to make high purity sodium nitrite. It is rarely extracted from what I have
seen and the purity is still lacking heavily.
On the sciencemadness wiki article on sodium nitrite it claims that NO2 and NO, reacting with sodium hydroxide solution would form NaNO2. How true is
this and would this be of decent purity?
I had looked into some other synthesis routes involving nitrogen oxides and I have found that dinitrogen triioxide will also react to form NaNO2 in
sodium hyroxide solution. The problem is that the equilibrium that exists between NO2 and NO to form N2O3 lies to the reactant side and so if the
sciencemadness wiki is wrong, you'd form other impurities.
Has anyone ever tried to make sodium nitrite this way? It's pretty dangerous due to the amounts of incredibly toxic NO2 gas that is needed but I see
it as a viable method. Since the nitrous acid produced should be reacted right away I don't see it being able to decompose quickly enough to make this
impossible.
I'm also aware of NO2 gas being able to form nitric acid but that requires a reaction between NO2 and the nitrous acid. So if the nitrous acid is able
to react quickly enough with sodium hydroxide, the salt produced should be NaNO2.
On that note, I think that the bubbling of NO2 gas would need to be stopped after the concentration of sodium hydroxide falls too low to discourage
the nitrous acid from having enough time to react with more NO2 to form nitric acid, which would form sodium nitrate and make our product less pure.
What does everyone think of this? I have used a sodium hydroxide bath to try and scrub off NO2 from when I needed to make silver nitrate from metallic
silver but never thought to try and recover what may have formed. I think chilling the sodium hydroxide solution as cold as possible is also a good
idea to try and encourage nitrous acid from not reacting or decomposing too quickly.
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fdnjj6
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I also wanted to add something I had forgotten to include in the original post.
The wiki also states that the reaction is air sensitive and oxygen will lead to nitrate formation. This however, shouldn't be too much of an issue as
an inert gas could be used to flush the system or one could simply let the NO2 generator run for a little and have the gas vent to a separate scrubber
(to avoid lots of gas from venting) until the whole system is purged with primarily nitrogen oxides with the least amount of oxygen as possible.
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Bedlasky
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NO2 reacts with NaOH in this way:
2NO2 + 2NaOH - -> NaNO2 + NaNO3 + H2O
If you want to create pure NaNO2, you need (in theory) 1:1 molar mixture of NO/NO2.
NO2 + NO <--> N2O3
N2O3 + 2NaOH - - > 2NaNO2 + H2O
But I think that in reality if you bubble 1:1 molar mix of NO/NO2 in to NaOH, you still have some nitrate as a byproduct, because there is still some
NO2 which react with NaOH to form nitrite and nitrate.
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fdnjj6
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Yea you are right, I have seen that dinitrogen triioxide requires NO and NO2 to be in a 1:1 molar ratio, but the temperature required for N2O3 to form
is also well below 0C.
I have been doing some more research and realized that if I were to purge a system of oxygen, and have an addition funnel drip dilute nitric acid onto
some copper metal, and were to direct the gas into a cold sodium hydroxide solution, sodium nitrite should be able to form as the major product. Some
nitrite and hydroxide contamination should be expected but a recrystallization should take care of getting rid of most contaminants.
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fdnjj6
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At what concentration of nitric acid would NO be the major product anyway? All I ever see is people saying dilute nitric acid but never saying what
concentration will actually form NO as the major gas evolved.
If I were to purge the system of oxygen and figure out what concentration of nitric acid would mainly produce NO instead of NO2, I would theoretically
be able to produce mainly sodium nitrite correct?
I think using a 3 necked flask with an addition funnel to add dilute nitric acid, a condenser to cool the nitric oxide and reflux any water of nitric
acid vapors, and then having a stopper on the other neck would be the best way to perform this reaction. Adding a gas outlet adapter on top of the
condenser and bubbling the NO gas into a cold sodium hydroxide solution would be the best way to do this in my opinion.
To make sure that the reaction is complete, the pH of the hydroxide solution should be monitored. Sodium nitrite is a basic salt so when the pH starts
approaching the pH of that salt, the reaction would be complete. Theoretical yields of the NO production could also be easily calculate and made into
a 200% excess to account for the initial purging loss and the assumption that not all of the gas reacts fast enough. A magnetic stirrer in the sodium
hydroxide solution would also be a good idea.
After the reaction is almost complete, the NO generation is stopped to prevent nitrate formation from nitrous acid decomposing in the water into NO2.
Then the remainder of the solution is boiled down and then recrystallized to get pure sodium nitrite. The hydroxide and nitrate are so soluble in
water I think that as long as nitrite is the major product then reasonably pure nitrite is feasible.
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Bedlasky
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N2O3 is present in NO/NO2 mix also at room temperature. Equilibrium strongly favors NO and NO2, but N2O3 is still present in small amounts. As you
said, cooling should increase nitrite yield, because equilibrium is more shifted to N2O3 side.
NO is air sensitive and it is oxidized in to NO2.
1+1 HNO3 should be fine for NO generating.
Btw. cadmium metal is used in analytical chemistry to reduce nitrate in to nitrite.
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Pumukli
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I'd say dilute HNO3 is around 30%, it should produce mostly NO. Around 60% it makes NO2 mostly.
There's an interesting, century old paper regarding sodium nitrite synthesis, one, that I saved ages ago to try on a (seemingly never arriving) day
when I'll have the time/equpment/mood constellation just right. :-)
Anyways, the synth is basically what you attempt to do, but (if memory serves well) it generates NO from starch/saw dust intead of copper. And
manipulates the reaction temperature such a way that only the colourless NO is allowed to evolve, when brown NO2 appears they'd regulate back the
heat. According to the article the NaNO2 is of very good quality in the end.
I'm sure someone else in the group has this article saved too and would share happily. (Because I put mine somewhere on a pendrive and put the
pendrive into a very safe place... It is so safe I don't even remember the location. But surely will find in the coming years.) :-)
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AJKOER
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Actually, there may be an easy way:
.NO2 reacts with KOH in this way:
2 .NO2 + 2 KOH - -> KNO2 + KNO3 + H2O
[EDIT] An aqueous path:
.NO2 + .NO2 = N2O4
N2O4 + H2O = HNO2 + HNO3
Neutralize with KHCO3.
If one checks out a Solubility Table, the nitrite is amazingly 10 times more soluble! See, for example, https://en.wikipedia.org/wiki/Solubility_table . Adding KCl would further help to salt out the KNO3.
If you really need NaNO2, once you have pure KNO2 (or mixed with KCl), add NaCl and separate out the KCl.
[Edited on 5-8-2020 by AJKOER]
[Edited on 5-8-2020 by AJKOER]
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fdnjj6
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Good point, I have thought of that way but the NO3 contamination was pretty high. Although I guess the nitrate can be very easily gotten rid of
through 2-3 recrystallizations.
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AJKOER
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fdnjj6:
I have edited slighty. Note the route involving further addition of KCl to salt out the KNO3 is OK with a residual amount of KCl as you will be adding
NaCl (to prepare aqueous NaNO2) and removing all KCl.
Remember, boil solutions to remove any dissolved O2 and avoid further oxygen exposure, or some of the nitrite converts back to nitrate with time.
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