# Sodium nitrite, dissolving in concentrated H2SO4

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When you add solid NaNO2 to an acid at room temperature like dilute H2SO4, HCl or HNO3, then the stuff heavily bubbles and you get a big brown plume of NO2. This is well known:

NO2(-) + H(+) ---> HNO2

2HNO2 <---> H2O + NO + NO2

If I, however, add NaNO2 to concentrated H2SO4 (98%), then no gas is produced. Even if the stuff is heated gently, hardly any gas is produced and the solid completely dissolves. The liquid becomes colorless.

When I add this liquid to water, then it starts bubbling heavily and I get the well-known brown plume of NO and NO2.

My question is: What happens with the nitrite ion in sulphuric acid? Why don't I get NO2 + NO? Is some interesting sulphur/nitrogen/oxygen compound formed?

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Im only Speculating here, but could it be forming something like Nitrosyl sulphate?

EDIT: after thought here, how about: H2SO4 +NaNO2 --> Na2SO4 + HNO2 although Im fairly sure it can only exist in soln though???

again, purely speculative.

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HNO2 is not very soluble in water. That was what woelen was getting with the dilute sulfuric acid (as NOx gases).

I suppose that concentrated sulfuric acid may be powerful enough to oxidize NO2- to NO3-. Upon heating, however, I would still suspect NOx to be formed so obviously that doesn't explain anything.

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I also posted this question on a Dutch forum. There I also got the answer of formation of nitrosyl sulfate. They told me, this is a salt, and not a covalent compound. It has NO(+) cations and SO4(2-) anions, or HSO4(-) anions, in the excess H2SO4. Nitrosyl ions are interesting and as I understood, it seems to be possible to make pure nitrosyl compounds from this solution, by adding the appropriate anion. I'm going to experiment with this, by adding NaCl (could make pure ONCl, an orange gas), adding NaClO4 (could make ON-ClO4, a colorless liquid) and NaCN (could make a blue gas, ON-CN!). I'll keep you updated.

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Wahey, not a bad Guess then eh

actualy it was the NO2 bit and Nitrous acid thinking that got me started, and when you said about possibly forming something with the Sulpher, I figured Nitrosyl *something* (sulphide maybe... naah, it would be sulphATE).

this is all I can find on it of any real help: http://www.chemicalland21.com/specialtychem/finechem/NITROSYLSULFURIC%20ACID.htm

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A blue gas? I don't think I ever recall hearing about a gas of this color. This would be what, nitrosyl cyanide?

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Yes, the gas nitrosyl cyanide is blue.

There are more blue gases, the better known gas CF3NO also is blue, see

And undoubtedly there are more blue gasses, but they will be less common. In my home lab I have made gasses with yellow, brown, red, orange, green and purple color, but up to now no blue gas, so I'm definitely going to try this reaction next weekend.

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I think you get nitrosyl sulfuric acid NOHSO4 . An Italian process to make caprolactam , the nylon-6 monomer , IS based on NOx absorption in conc sulfuric acid & then this is used to alpha nitrosate hexahydrobenzoic acid (HHBA) forming cyclohexanone oxime in situ ( with de carboxylation ) & Beckmann rearrangement to caprolactam ( sulfate ) .HHBA is made by oxidation of toluene & then reduction with hydrogen .

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It is safer and more usual to isolate NO+ as a BF4- or a PF6- salt as perchlorates are explosive.

I use to use NO+ salts occasionally to produce M-NO complexes. They are very interesting as the NO ligand can act as a 3e donor or a 1e donor and so have always been of interest to those studying catalytic mechanisms.

NO+ salts are both air sensitive and moisture sensitive as the NO+ cation is both oxidising and reducing

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I think for me it is safer to use perchlorate, instead of BF4(-) or PF6(-). I don't have such fluoro-compounds anyways, but even if I had them, I fear their toxicity much more than the possible explosion risk of perchlorate (I work with just small pinches of chems, for safety, but also for keeping cost low).

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I did the experiments with colored gasses. First unexpected thing that I observed is that it is important to have a small amount of water present. E.g. when NaCl is added to a notrosyl/sulphuric acid solution, then only HCl is produced. If a small amount of water is present (e.g. wet NaCl), then a nice orange/yellow gas is produced. This gas is ONCl.

I did the same experiment by adding NaBr (also wet). This produced a chocolate/brown very dark compound in solution, and a dark brown gas. This gas is very heavy and can be poured out of the test tube and see really nicely see it falling downwards. This is an indication of large molecular weight. I think that the brown compound is ONBr. It definitely is not bromine, because that does not have a chocolate/brown color, that is much more red.

With dry NaBr, I get HCl and orange/red/brown fumes of Br2. The color of these are quite different from the color of the chocolate/brown compound. A small whiff of this dark brown gas (done by moving the hand along the open end of the test tube towards my nose) has a weak smell, totally different from the smell of bromine (once you have smelt bromine, you recognize it for ever).

Most funny result with NaBr is that even on strong dilution, this dark brown compounds still exists. As a counter test, I did an experiment by adding NaNO2 to a dilute solution of H2SO4 and KBr. This results in formation of the dark brown compound, even in water! Under these conditions, the color is dark brown/green, very different from the color of bromine in water.

With NaSCN, a lot of a colorless gas is produced and the liquid starts foaming a lot, and an orange/yellow solid is produced. The liquid itself turns grey/blue and turbid, but that color is transient. In contact with air, the colorless gas turns brown, so it at least contains NO, it may even be pure NO. The orange/yellow solid is insoluble in any aqueous solvent, either acid or base. Only with strong scrubbing, it could be removed from the test tube.

Finally, I did experiments, hoping to get blue ON-CN. This gas, however, is not formed. The solid NaCN is covered by a dark brown/black layer and the solid hardly further reacts. There is some formation of gas. I did not dare to take a small whiff of this. When a small amount of water is added, then I just get copious amounts of brown gas, indicating quick decomposition of the nitrosyl to NO + NO2. This brown gas could contain some HCN as well, I have been overly cautious while doing this experiment and did not smell anything.

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"With nitric acid itself, sulfuric acid acts as both an acid and a dehydrating agent, forming the nitronium ion NO2+, which is important in nitration reactions involving electrophilic aromatic substitution. This type of reaction where protonation occurs on an oxygen atom, is important in many reactions in organic chemistry, such as Fischer esterification and dehydration of alcohols."

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