Purple acid, N2O2HSO4

Atomistry, as so often, is very interesting, but gets some things wrong.

A better source of information is of course the Gmelin Handbook of inorganic chemistry series, which sadly is non digitalized. The information is from Sulfur part B pages 1630ff.



They call this compound Nitrogen Oxide Nitrosyl Hydrogen Sulfate with the formula N₂O₂HSO₄, or better (ON.NO)HSO₄, the most common name is "blue acid" but "purple acid" describes the color much better.

This compound can be formed in a variety of ways, but it is unstable, can't be isolated (probably) and even in conc. sulfuric acid only stable for a few days. Nonetheless, it is very impressive.

It can either form from the reduction of NOHSO₄ in sulfuric acid by metals, sulfur dioxide, some organic compounds, and nitrogen monooxide under pressure. There are some more but more exotic methods.

Some more information can be found here: Seel, Ficke, Riel, Völkl, 1953. The doi is 10.1515/znb-1953-1011

So far, I have made the compound in three different ways, and I tend to explore at a later time what the optimal conditions for this are.

The first method I did was this:
Some 95-97% sulfuric acid was taken and some small amount of NaNO₂ was added to it. This was shaken, but not everything dissolves. Then some 85% formic acid is dripped into it. THIS PRODUCES CARBON MONOXIDE! The liquid foams a bit from the evolved CO. At this point I was already expecting a purple color, but nothing happend. So I put everything into a hot water bath for a bit to get rid of at least some CO. I also read that copper is a catalyst for this, so I added a blank piece of copper wire. And within a few seconds, vibrantly purple streaks started to form. After shaking the entire solution was dark purple.





I left the solution standing like this for three or four days, and the color slowly faded and finally was colorless again.
In retrospect, the formic acid probably didn't do much apart from diluting the sulfuric acid, but I read it should have been able to reduce the NOHSO₄ by itself. It seems it doesn't. I assume the formic acid can be left out, but I haven't tried that yet.

Someone might say this is a copper complex, but there is good reason to believe that this is not true (although atomistry mentions something of a copper salt(?) of this which has an even stronger color, I guess it could be it?), because the same color appears with zinc:
To some 95-97% sulfuric acid some NaNO₂ is added, shaken until mostly dissolved, then some pure zinc powder is added. This takes a much longer time than before. Eventually however, the solution turns purple.



I added more zinc and the solution eventually had a quite dark color, compareable to the one in the first experiment.

Yet another experiment can be performed, and this time I made sure to record masses:
Everything was around 0 °C cold. I added 1.88 g (about 1 mL) of the 95-97% H₂SO₄ to a test tube, then I added 70 mg of NaNO₂ to it, stirred it around with a glass rod and left it there for ten minutes, didn't all dissolve, very slow gas evolution. Then I added 40 mg of Na₂S₂O₅ to it and stirred again, not much gas evolution, and no full dissolution either, left again for about 10 minutes. Then I put it into a 30 °C water bath where some gas evolved, but not much, the solution started clearing up and after ten minutes of standing outside the water bath it was mostly clear. Then I started adding distilled water dropwise to the test tube so it ran down the sides. On hitting the acid some gas evolved, and the acid started turning purple. I think I added about 5-6 drops, and shook it a bit, then left to stand for a few minutes for the bubbles that formed to rise.



The layering occurs because mixing in the test tube is fairly bad and the water only entered the top portion so far. The color continues to darken. Too much water will likely (according to what I read) destroy the compound again. Indeed: When the solution was diluted with an equal volume of water, all purple color faded away and some brown gas was evolved and it smelled of NO₂.

I'm fairly confident in saying I have successfully produced a solution of N₂O₂HSO₄ in sulfuric acid.
I hope that I can add the relevant Gmelin pages to this thread at some point. Information seems to be sparse about it.

[Edited on 5-1-2021 by Diachrynic]

This really is interesting! Thanks for sharing this. I certainly will try this myself and maybe I'll add a webpage to my website on this subject if I succeed. I never heard about this compound. Chemistry keeps surprising me

I conducted abother experiment to see if there was indeed a "more strongly colored copper salt" as atomistry claimed.

I mixed 1.90 g (about 1 mL) of 95-97% sulfuric acid and 70 mg of NaNO₂, put it into a water bath of 40-50 °C until it all dissolved (which it did quickly with just minor gas evolution), took it out and let it cool back down somewhat, then I added 40 mg of Na₂S₂O₅ into it (this time, as the solution wasn't as strongly cooled as in the last experiment, some foaming occured but it wasn't a problem). The solution was put for a minute or so back in the water bath and stirred, again, an almost colorless solution was obtained.
Now a solution of 50 mg CuSO₄ · 5 H₂O in 1 g of water was prepared which amounts to about 0.2 mol/L. I added 5 drops to the acid which was accompanied with some gas evolution. Even with the first drop a darker purple color formed than without copper(II). Again, bad mixing in the test tube caused a layering of a colored upper phase, despite everything being fully miscible.

So it seems the dark purple color in the experiment with the formic acid could be the copper salt atomistry mentions. Since a similar, although less intense, color is obtained without copper as well the existence of N₂O₂HSO₄ is not contradicted by this.

What an interesting find! Thank for the attached papers too, some light bedtime reading

Quote: Originally posted by woelen

Several years ago, I discovered a very intensely colored blue complex of copper with nitrite in strongly acidic solution. [...] https://woelen.homescience.net/science/chem/riddles/copper+n...

This is very interesting! There is one very remarkable difference between the experiments however. In your experiment, you used 20% sulfuric acid. This complex however is unstable at such concentrations. To demonstrate this, the following experiment can be performed:

To half of the blue solution prepared earlier, about 0.5 mL, was added water dropwise. About 7 drops of water were added, which the scale indicated as a gain in weight of 160 mg. The solution evolved brown NO₂ gas and warmed, as expected from sulfuric acid. The purple color disappeared within seconds completely.



An estimation of the concentration: 7 drops weighed about 160 mg here, plus 5 drops earlier divided by two means about 200 mg of water were added to about 0.95 g of 95-97% sulfuric acid, which means a concentration of about 75% is remaining in the colorless test tube.

It is however likely that the deep blue coloration is due to a complex between the N₂O₂⁺ and the Cu²⁺ (what atomistry calls a "copper salt" of the purple acid), since according to your experiments it can complex with NO/NO₂ and related species. Given that chloride seems to be required, it could also involve nitrosyl chloride.

EDIT: In fact, I have found mention of this in Sharp, Thorley, 1963, "670. The infrared spectrum of the nitrosonium ion", the doi is 10.1039/JR9630003557
Weirdly enough they seem to be talking about blue Cu⁺ complexes. Strange.
Attachment: Sharp1963.pdf (367kB)
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Quote: Originally posted by woelen

This splits the situation into two different directions. One direction is the situation with the presence of copper, the other is the situation without any copper. The latter one is most striking and I really want to see this myself. With zinc as reductor, you also get a faint purple color, but how sure are you that the zinc is absolutely free of copper? Given the intense color of the copper complex, even a tiny amount like 0.01% of copper in the zinc could give rise to the color.

Quote: Originally posted by Diachrynic

Here are the relevant Gmelin pages. Might translate them to english later if I have the time.

Very interesting read. Apparently it is important to have some water in the mix (e.g. 20%) to obtain the purple compound. I have a little doubt about the NO + NO(+) situation, described in the german paper. In that paper it is written that passing NO over a solution of NO(+)/HSO4(-) in 80% H2SO4 forms the purple ion [ON.NO](+). But why have I never observed this when dissolving NaNO2 in sulfuric acid of varying concentrations? Simply adding NaNO2 to e.g. 80% H2SO4 should give at least some hints of blue/purple. There always is some decomposition of nitrite with formation of NO and HNO3, no matter how caerfully you add the nitrite to the acid.

Time to take out the test tubes and experiment with this . . . .

Bedlasky, coordinating peroxynitrate, how cool is that! Too bad it requires inert gas, vacuum and dry ice temperatures... I would have loved to try that. The thread you linked is also quite interesting, since hyponitrites are fascinating (always wanted to make them!), and I would have never guessed something energetic could form from ascorbic acid, copper and nitrite

woelen, it's a good question why no purple acid forms just from dissolving nitrite in sulfuric acid. My guess would be for now: Since the purple acid seems to be quite unstable already by itself and even more so with oxidizing agents like O₂ or worse NO₂ which according to Gmelin oxidize it quickly, my guess is that if some NO and HNO₃ forms in the mixture, it is more likely that the NO leaves the solution - it's solubility isn't that good, which is why most procedures making purple acid from NO seem to require pressure - you will have more nitrogen in a higher oxidation state than +3 overall, and if some purple acid would form, the nitric acid will oxidize it immediately. This is all pure speculation of course.

Good luck with the experiments!

There are many unanswered questions for me still. Does the order of addition matter, i.e. first nitrite then sulfite and reverse? What happens when I dilute the acid before adding the salts? What happens when I separately dissolve nitrite and sulfite in sulfuric acid, then mix them? What about solid NOHSO₄ + SO₂ gas, Gmelin says this should form the purple acid too? What about just adding copper sulfate to sulfuric acid/sodium nitrite? Eventually I will try those out as well.

Quote: Originally posted by Diachrynic

What about just adding copper sulfate to sulfuric acid/sodium nitrite? Eventually I will try those out as well.

Quote: Originally posted by Bedlasky

Sulfuric acid + NaNO2 + metallic copper maybe work - copper would be a reducing agent.

I have also tried this now and can confirm that this color has nothing to do with the presence of copper or any other transition metal contamination. I loosely followed the metabisulfite method described in the OP.

In about 10 ml of 96% H2SO4 i dissolved 0.5 g of NaNO2 (heating the mixture slightly speeds this up). Then I added 0.3 g of K2S2O5, again waiting for it to dissolve (very little SO2 formed in the process). I then waited it to cool down a bit. Once it did, I started adding water dropwise with constant stirring until an intense color developed.

Here was my result:






It surprises me how the color was a very intense purple-indigo. Again, not a result of metal contamination, as I used high purity reagents.

[Edited on 8-1-2021 by EthidiumBromide]

The acid can be obtained in quite high concentrations, such that even 2 cm of liquid is nearly black. I made some pictures of the compound and will write a webpage about this. The pictures I can share here already:





For these experiments I used fairly high concentrations of NaNO2, but also quite high concentrations of K2S2O5. You can see solid material sticking to the glass, not all of the solid material could be dissolved. The left picture shows high concentration, with only a little amount of water added. The middle picture shows high concentration, with a little more water added. The right picture shows the same test tube, with bright LED-light from behind, showing the color somewhat better than the middle picture.

I really is remarkable to see formation of this compound from only NaNO2, K2S2O5 and H2SO4.

When the dark material is diluted with water, then a nice blue liquid is obtained, like a dilute solution of CuSO4, but slightly more cyan. This most likely is N2O3, dissolved in water. More on that follows in the webpage.

So I tried this reaction again. I folowed EthidiumBromide instructions, instead of K2S2O5 I used anhydrous Na2SO3. And I obtained this beautiful solution. I tried to add some copper nitrate which I made from piece of copper wire and nitric acid (I didn't have copper sulfate on hand), but this caused decolorization of solution due to presence of nitrate (see the third picture).

I tried to add copper wire in to purple solution. Some purple colour appeared around copper but after a while I didn't saw further formation of this colour.

I tried to use copper as a reductor instead of sulfite. At the surface of wire a saw some purple colour, but solution remained yellowish. When I added drop of water, purple colour disappeared. It seems that formation of small amount of NO2 caused oxidation of this acid. This compound is really sensitive to any oxidizing agent.



Btw: In this document is mentioned that N2O2+ is an intermediate step in reduction of nitrosyl in to NO. It is a radical cation.

Attachment: Trace Addition of Nitric Oxide and Nitrogen Dioxide to Air by Electrolysis.pdf (930kB)
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[Edited on 10-1-2021 by Bedlasky]

woelen, EthidiumBromide, Bedlasky, the pictures look amazing! Awesome job! It seems you obtained a far more concentrated solution than I did, judging by how deep the color is.

The experiment that Bedlasky did is fascinating, since copper sulfate seemed to deepen the color for me, while copper nitrate caused decomposition. Seeing how there are multiple mentions of how oxygen and nitrogen dioxide decompose the purple acid by oxidation, I'm assuming that this happend here too, with nitric acid being the oxidizing agent. It would fit with the literature.

I'm still making slow progress on the translation of the Gmelin pages, once I am done I'll append them here, to make it accessable to non-german speakers as well. It should be close to comprehensive for all literature on the compound up until the 1960s.

While I have not found an exact structure, I am assuming from all the facts so far the formation and structure will look something like this:

Plinius, that is very cool! I would love to see that plant in operation.

I have in the meanwhile found the answer to the questions on what the copper now actually does. It turns out it is something quite different, not a salt of the purple acid. Rather, the purple acid just serves as a donor of NO. In strong sulfuric acid, copper will form an addition compound with NO, denoted by Gmelin with CuSO₄ · NO. This is indicated by the fact that it can be made from pure NO and copper sulfate in strong sulfuric acid, and its absorption spectrum has nothing in common with the one from purple acid. It is water sensitive and hydrolyses below 70% it seems. Another nail in the coffin is that copper chloride actually forms the blue color too, giving CuCl₂ · NO, and its absorption spectrum is the same. It would be interesting to try this, dissolving anhydrous CuCl₂ in alcohol or acetone, then passing NO into it. I assume this is or at least is related to the blue complex that woelen obtained in the experiment linked earlier.

The source is Gmelins handbook of inorganic chemistry copper part B-1 page 579:



[Edited on 19-1-2021 by Diachrynic]

This is a fun and relatively easy experiment! I can confirm the section of the Atomistry article which says the substance becomes red on cooling. After chilling in a dry ice/acetone bath for a few minutes, the blue-purple color changed to a deep cherry red. When cooled further, the mixture mostly solidified into a red slush. The purple color returned upon warming.

Purple Acid:


Low-temperature Red Chromomorph:


Solidification:


The section from the Fowles book mentioned earlier is really interesting, especially the part describing the catalytic cycle in the lead chamber process. If I had the capacity I'd try out the whole range of demonstrations.

Lead Chamber Cycle

Translation of Gmelin pages complete

Quote: Originally posted by Lion850

The book is "Lecture Experiments in Chemistry by Fowles G" which can be downloaded here https://b-ok.global/book/3164501/d69c55