Sciencemadness Discussion Board

Purple acid, N2O2HSO4

Diachrynic - 5-1-2021 at 09:08

woelen will probably like this one ^^

I think I found this one on atomistry, which seems to be down at the moment. The relevant page, thankfully, is saved here in the Wayback Machine.

For completeness sake, I will include the full quote right here:

Sulphonitronic Acid

Sulphonitronic Acid or "Purple Acid"-For a considerable time the existence of an unstable compound which yields a purple solution in sulphuric acid has been known. Of various names which have been suggested for this compound, the designation "sulphonitronic acid" possesses the advantage of indicating the nature of the chief constituent elements without committing itself to any definite conception of the structure or constitution. The acid has been regarded as a derivative of quadrivalent nitrogen, viz. " nitrosisulphonic acid," O:N(OH).SO2.OH, or peroxylaminic acid, NO(SO2.OH)2. Unfortunately the question of the actual constitution is still undecided, and even the composition is uncertain. According to more recent investigations it appears probable that the " acid" is either an oxide of nitrogen intermediate between NO and N2O3, or a compound of sulphuric acid with such an oxide. On account of this uncertainty the compound is frequently referred to merely as "purple acid" (also "blue acid " and "violet acid"). It has possibly acquired undue importance on account of its occurrence as an intermediate product in the "lead chamber" process for the manufacture of sulphuric acid.

The coloured solution in sulphuric acid is obtainable by passing nitrogen dioxide with air into a saturated solution of sulphur dioxide in diluted sulphuric acid (1:1 by volume) at 0° C. It can also be formed by the addition of sodium hydrogen sulphite to a solution of nitrosulphonic (nitrosylsulphuric) acid, produced by dissolving sodium nitrite in slightly diluted sulphuric acid. These methods depend on the reduction of the nitrosulphonic acid by sulphurous acid or sulphur dioxide; the reduction can also be effected by metals, e.g. mercury.

The blue solution is unstable and decomposes slowly, with formation of sulphuric acid, sulphur dioxide and nitrogen dioxide. When shaken with air or submitted to oxidation by chlorine, nitric acid or hydrogen peroxide, conversion into nitrosulphonic acid is effected, brown fumes being liberated. Dilution with water also destroys the coloured substance. If strongly cooled, the solution changes to an intense red, so that if a solution is too weak to possess a marked colour at the ordinary temperature, the presence of the "purple acid" can easily be detected by cooling in a mixture of acetone and solid carbon dioxide.
Certain of the salts in solution have a stronger colour than the acid, and in some cases are more stable; thus, a deep blue solution of the copper salt may be obtained by the reduction of nitrosulphonic acid (in sulphuric acid) by mercury in the presence of copper. A suggestion has been made that the colour in the "brown ring" test for a nitrate is due to the formation of the ferrous salt of "purple acid," but this is improbable.

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.

IMG_20200207_144904.jpg - 1.4MB

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.

N2O2HSO4.png - 5.1MB

IMG_20200209_143152.jpg - 2.1MB

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.

IMG_20201012_112141.jpg - 2.2MB

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.

IMG_7089_cc.jpg - 1.9MB

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]

RustyShackleford - 5-1-2021 at 09:20

Good writeup! i recall trying it on your behalf some months ago but never adding the copper wire, so i was dissapointed.

Bedlasky - 5-1-2021 at 09:44

Very nice! I will definetely try it! I plan to make some highly electrophilic species in conc. H2SO4 and oleum, so this will be nice addition to these experiments.

woelen - 5-1-2021 at 13:47

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 :)

EthidiumBromide - 5-1-2021 at 23:55

This is very interesting, I have to try this when I have some spare time. Thanks for sharing!

Diachrynic - 6-1-2021 at 01:49

I'm glad that I am not the only one that finds this fascinating ^^

RustyShackleford, I remember that. It seems that the original claim that formic acid reduces it by itself isn't true or as easy to reproduce. Either way, I think with the CO involved it is an impractical method when better alternatives exist.

Bedlasky, that sounds really nice, I hope the experiments work!

woelen, please do! That would be amazing to see since I'm a big fan of your website.

EthidiumBromide, thank you and good luck! I hope it works.

Here are some papers papers on the subject (all of which are in German):

Manchot, 1910,, "On the alleged nitrosisulfonic acid by Raschig (Sabatiers nitrosodisulfonic acid) and the theory of the lead chamber process", the doi is 10.1002/ange.19100234501
Attachment: Manchot1910.pdf (222kB)
This file has been downloaded 301 times

Manchot, 1911, "About the 'blue acid' (Raschigs nitrosisulfonic acid), reply to Mr. Raschig", the doi is 10.1002/ange.19110240106
Attachment: Manchot1911.pdf (350kB)
This file has been downloaded 306 times

Manchot, 1912, "On the recognition of the 'blue acid' (the reduction product of nitrosylsulfuric acid)", the doi is 10.1002/ange.19120252104
Attachment: Manchot1912.pdf (453kB)
This file has been downloaded 301 times

Seel, Ficke, Riel, Völkl, 1953,, "Notes about nitrogen oxide - nitrosyl salts", the doi is 10.1515/znb-1953-1011
Attachment: Seel1953.pdf (2MB)
This file has been downloaded 302 times

Wolf, Hummel, 1967, "On the reduction of nitrosylsulfuric acid in conc. sulfuric acid", the doi is 10.1002/zaac.19673540103
Attachment: Wolf1967.pdf (116kB)
This file has been downloaded 305 times

[Edit: The file extensions of the pdfs were missing]

[Edited on 6-1-2021 by Diachrynic]

Diachrynic - 6-1-2021 at 02:45

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.

IMG_7092_cc.jpg - 2MB

IMG_7098_cc.jpg - 2MB

IMG_7103_cc.jpg - 1.8MB

Boffis - 6-1-2021 at 04:11

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

woelen - 6-1-2021 at 04:12

Several years ago, I discovered a very intensely colored blue complex of copper with nitrite in strongly acidic solution. I have written a webpage about that, 10 years ago or so.

Your experiment provides new light on this. In my webpage I write that chloride is necessary for formation of the complex, but maybe any reductor will do.

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.

We should try to get the purple compound with a reductor, which certainly will not contain any copper, e.g. Na2S2O5 or gaseous SO2. I have a bottle with a solution of 5 to 6% SO2 (I once made this for experiments with V2O5, trying to convert that to VOSO4 without any other cations in the system), and I could try experimenting with this (e.g. diluting 3 volumes of reagent grade H2SO4 with 1 volume of the solution of SO2) and adding NaNO2 to that. Metallic reductors like zinc, aluminium or magnesium I do not really trust. Many commercially available metals may contain traces of copper.

Diachrynic - 6-1-2021 at 05:07

Boffis, thank you very much! When I am able to include the Gmelin pages (and maybe even translate them to English) here there will be a bunch more reading material too!

Quote: Originally posted by woelen  
Several years ago, I discovered a very intensely colored blue complex of copper with nitrite in strongly acidic solution. [...]

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.

IMG_7104_cc.jpg - 2MB

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)
This file has been downloaded 317 times

nitrosyl.png - 109kB

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.

This is a valid concern, even if my zinc is labled as 99.995% pure. There could be copper as an impurity and I agree that the experiment is more impressive without any metals at all. Gaseous sulfur dioxide should be a way to make it absolutely metal free. I despise the smell of it however and adding metabisulfite is a lot more convenient, but sulfur dioxide does work as well, according to the literature.

[Edited on 6-1-2021 by Diachrynic]

[Edited on 6-1-2021 by Diachrynic]

Diachrynic - 7-1-2021 at 02:22

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

Attachment: N2O2HSO4.pdf (1.5MB)
This file has been downloaded 329 times

Bedlasky - 7-1-2021 at 05:12

Look at this:

Speaking of copper nitrite complexes, this one is quite interesting:

woelen - 7-1-2021 at 06:15

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 . . . . :)

Diachrynic - 7-1-2021 at 07:47

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 :o

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.

Bedlasky - 7-1-2021 at 12:25

Diachrynic: Maybe purple acid is similar complex like nitrite complex mentioned in that paper I posted.

About that energetic complex: It isn't certain, that this compound is actually copper(I) hyponitrite. There are still many unanswered questions, it may be some sort of nitrosyl/nitrite+organic complex. Precise nature of this compound is really mystery, I myself try to find some info about it year ago, but I didn't find anything.

Quote: Originally posted by Diachrynic  
What about just adding copper sulfate to sulfuric acid/sodium nitrite? Eventually I will try those out as well.

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

Diachrynic - 7-1-2021 at 12:50

Yeah, purple acid could be such a complex, but I imagine since N2O2+ has a free electron it could be colored just like that. Radicals tend to have colors, see NO2 or Frémy's salt (which coincidentally is violet/purple in solution). This compound isn't that tho, even tho it is also made from nitrite and sulfite. Frémy's salt can be isolated and is stable in water.

Quote: Originally posted by Bedlasky  
Sulfuric acid + NaNO2 + metallic copper maybe work - copper would be a reducing agent.

It probably does, I expect that to be the reason for the color in the experiment with formic acid (I assume the formic acid didn't do anything apart from diluting). I don't expect just copper sulfate to work without a reducing agent, frankly, but it would be very surprising if it did.

woelen - 7-1-2021 at 12:52

I did some experimenting, and I succeeded in making some of the purple acid! It's a somewhat elusive compound, making it requires some trial and error. The concentration of water, needed in the acid must be within fairly tight bounds. Too little or too much water, and it does not work.

For a start, you can do the following:

Take a SMALL pinch of NaNO2. A piece of 2x2x2 mm is enough. Too much nitrite and it does not work!
Do the NaNO2 in a dry test tube.
Add appr. 2 ml of conc. H2SO4.
Swirl around, until the NaNO2 has dissolved. If the acid is really concentrated, then there hardly will be any production of gas and no brown color can be observed above the liquid.
Add a similar amount (at most 10 mm3 of solid) of solid K2S2O5 to the liquid. Na2S2O5 also can be used.
Swirl till the solid has dissolved. Some of it will be sticking on the glass. That's not a problem.
Carefully, while keeping the test tube tilted, let a single big drop of water run along the glass, through some K2S2O5, sticking to the glass until it hits the acid.
If this is done carefully, then 2 or 3 mm of water will be floating on top of the acid, with a clearly visible purple layer between the colorless acid and the colorless aqueous layer.
Carefully swirl the test tube so that the water mixes with more acid. If you do this, then you clearly see a purple color, and production of some colorless gas. The liquid becomes warm, but not overly hot. This is good. If it remains cold, then the reaction does not occur (or is very slow).
If you want the purple color to last somewhat longer, then you should cool the test tube under some running tap water.
Carefully swirling more can make all liquid purple, but it might be that the color fades again and that an additional small drop of water must be added.

It is very easy to add too much water. Just one drop too much and the color disappears and you just get bubbles of NO and a brown color above the liquid.

This weekend I'll do some more extensive tests. For now, I have very limited time, but I could not resist taking out the test tubes. I just wanted to see this with my own eyes. I'll try to find a procedure, which is more reliable and gives more certain results.

But right now, I am quite convinced that the purple color is not due to trace amounts of copper.

EthidiumBromide - 8-1-2021 at 02:53

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:

purple_acid1.jpg - 113kB

purple_acid2.jpg - 144kB

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]

Bedlasky - 8-1-2021 at 09:50

I tried this experiment today. And I am unable to do this compound. I added small amount of NaNO2 in to the dry test tube and "dissolve it" (actually it didn't dissolve completely and I used very small amount of NaNO2) in cca 2ml of 96% sulfuric acid and added small amount of Na2S2O5 (which also refuse to dissolve completely). Then I added drop of distilled water. Nothing. I added second drop. Nothing. Just a gas evolution. When I added drop of copper sulfate solution, it turn purple but for a very short time. Nickel and cobalt didn't have influence on this.

EthidiumBromide - 8-1-2021 at 11:45

Bedlasky, how much water did you add? I just kept slowly adding drops of water and mixing until the color started showing up.
Also, have you tried heating up the H2SO4 after adding it to the test tube? It helps in dissolving NaNO2. Maybe try using more NaNO2 as well.

woelen - 9-1-2021 at 13:00

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:

purple_acid.jpg - 254kB purple_acid_dark.jpg - 231kB purple_acid_dark_backlight.jpg - 280kB

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.

Bedlasky - 9-1-2021 at 17:52

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.

IMG_20210110_002215.jpg - 1.5MB IMG_20210110_002253.jpg - 1.3MB IMG_20210110_002451.jpg - 1.4MB

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)
This file has been downloaded 296 times

[Edited on 10-1-2021 by Bedlasky]

Diachrynic - 10-1-2021 at 01:26

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:

N2O2+.png - 37kB

Bedlasky - 10-1-2021 at 08:11

Diachrynic: Actually, bonding between N and O in NO is little bit more complicated than you proposed:

Diachrynic - 10-1-2021 at 09:44

I have never heard of that before, thank you for pointing it out. We were taught the normal bonding with a single radical... Every concept you get taught later turns out to be horribly simplified!

Bedlasky - 10-1-2021 at 10:44

There are many "strange" bonds like three electron bonds, three-centered-two electron bond, four-centered-two electron bond, bonds in aromatic and homoaromatic compounds etc.

In the case of these radicals, boranes, strange cations (like H2+, He2+, CH5+...) etc. is good to draw MO diagram - you can see number of unpaired electrons, you can calculate bond order etc.

Btw. NO have also dimer N2O2 - both are in equilibrium in liquid state.

[Edited on 10-1-2021 by Bedlasky]

Plinius - 13-1-2021 at 02:41

I have been an occasional visitor for some time but I found this thread so interesting that I decided to register an account.

About 30 years ago I worked at a company which built a plant for production of nitric oxide from sulfur dioxide and nitric acid. The plant was made of glass, in principle large scale laboratory equipment. The raw nitric oxide was dried by washing it with concentrated sulfuric acid in a packed column.

The plant started up without problems. After a few days a surprised operator discovered that the sulfuric acid in the storage flask under the drying column had developed a beautiful violet colour. I was assigned to the task finding out why the acid had turned violet. The natural starting point was looking in Gmelin where the answer was easily found.

The plant was very colorful during production. The reaction column was brown due to the nitrogen dioxide and the sulfuric acid purple. On top of that the cooling water contained an anti-freeze which had some flourescein added to it. Visitors were impressed.

As a by product dilute sulfuric acid containing nitric acid was obtained. During the design phase I wondered how that acid solution would be disposed of in en environmentally acceptable way. The local management found a creative solution to the waste disposal problem. They donated the acid solution to a local pig farmer who added it to the manure produced by the pigs instead of purchased acid. The addition of acid decreased the smell (stench is probably a better word) from the manure. A win-win situation.

Bedlasky - 13-1-2021 at 12:25

Plinius: Welcome to the forum! I never thought that such a rare compound can be produce during industrial operation.

Diachrynic - 19-1-2021 at 07:05

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:

CuSO4 x NO.png - 554kB

[Edited on 19-1-2021 by Diachrynic]

Bedlasky - 20-1-2021 at 12:03

Diachrynic: I found paper about reaction of Cu(0), Cu+ and Cu2+ with NO in strong acids. End product of these reactions is [Cu(NO)]2+ complex, which you mentioned in your post.

Cu + 3NO + 2H+ --> [Cu(NO)]2+ + N2O + H2O

2Cu + 4NO + 4H+ --> 2[Cu(NO)]2+ + N2 + 2H2O

2Cu+ + 4NO + 2H+ --> 2[Cu(NO)]2+ + N2O + H2O

Cu2+ + NO <--> [Cu(NO)]2+

Lion850 - 20-1-2021 at 23:29

Gents the attached extracted from an old book discusses the "violet acid".

The book is "Lecture Experiments in Chemistry by Fowles G" which can be downloaded here

which in my opinion is a very interesting and informative book especially for chemistry amateurs like me.

Attachment: Violet acid.pdf (2.7MB)
This file has been downloaded 294 times

Diachrynic - 21-1-2021 at 04:05

Bedlasky, that is a great find! That is exactly what I was looking for and confirms the text from Gmelin but in more detail. Thank you!

Lion850, thanks for this contribution as well! The more sources mention the compound the more weird it seems I (and many others that I asked) had literally never heard of it before I randomly stumbled over it. The formula given is slightly different but not too far off from the modern interpretations.

Here are three more relevant papers:

Wright, Wu, Hayton, 2010, "Structural Characterization of a Copper Nitrosyl Complex with a {CuNO}10 Configuration", doi: 10.1021/ja105930b

Fraser, 1961, "The dissociation of nitric oxide-copper(II) halide complexes in alcohols. The preparation of the copper(II) fluoride nitrosyl", doi: 10.1016/0022-1902(61)80150-4

Fraser, Dasent, 1960, "The Composition of the Nitric Oxide Complexes of Cupric Halides", doi: 10.1021/ja01487a022

The last one seems very relevant for the experiment woelen performed. I assume halides can stabilize the blue complex so it doesn't decompose as fast in water and without it strong sulfuric acid is needed as the solvent.

This is a very interesting tangent from the purple acid. The paper Bedlasky found by Tsumori and Xu plus the Gmelin entry demonstrate that copper actually decomposes the purple acid and just using its NO to form a much more strongly colored complex itself. This means that no copper is the way to go if actual purple acid is desired. Still, a very interesting experiment in itself.

By the way, purple acid is supposed to turn cherry red when strongly cooled, most often dry ice bath temperatures are mentioned, but I am unsure whether or not is is a sharp or a gradual transition, although I assume the latter. This might be a neat addition to the preparation of purple acid if dry ice is available.

mayko - 10-1-2023 at 20:28

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:
purpleAcid_optics.jpg - 403kB

Low-temperature Red Chromomorph:
purpleAcid_coldRed.jpg - 104kB

purpleAcid_redSlush.jpg - 119kB

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
lead_process_schematic_smol.jpg - 268kB

Bedlasky - 11-1-2023 at 09:32

Nice red colour! Does Atomistry say at what temperature change in colour occur?

Diachrynic - 12-1-2023 at 10:47

Wow mayko, that's awesome! I never had dry ice to test this. Thank you for documenting it!

Translation of Gmelin pages complete

Diachrynic - 21-1-2023 at 08:03

Well, I forgot about it somewhat, but since I was reminded of this compound again, I decided to finish where I left off two years ago and translated the Gmelin pages.

It's by far the most complete account on purple acid/N2O2HSO4 I have found so far.

Attachment: purple_acid_translated.pdf (359kB)
This file has been downloaded 119 times

[EDIT] I am reworking the wording somewhat as I noticed some clunky phrases in the beginning especially, an updated version should be done soon.

[EDIT2] New version added.

[Edited on 21-1-2023 by Diachrynic]

Bedlasky - 21-1-2023 at 09:28

That's great! Thank you for translation, your work is really appreciated!

Neal - 22-1-2023 at 04:11

Thermochromism: where chemicals change color on temperature. My inorganic textbook lists very few examples.

j_sum1 - 22-1-2023 at 05:15

Quote: Originally posted by Lion850  
The book is "Lecture Experiments in Chemistry by Fowles G" which can be downloaded here

Link no longer works. But this sounds like a fantastic book.
I will have to dig up a copy.

j_sum1 - 22-1-2023 at 20:23

And here it is.

Lecture Experiments in Chemistry by Fowles G

Neal - 23-1-2023 at 06:44

So I'm looking at

And it's mp and bp are both decomposes? So what does it do at room temperature, decompose to both a solid and a gas at the same time?

Diachrynic - 24-1-2023 at 00:02

Thanks for the link Neal, I will have to update the page, since it mainly quotes literature that is out of date.
Purple acid has not been made in the pure solid state. Unless its sulfuric acid solution is stored under high pressure NO gas, it decomposes in solution to NO and NOHSO4, as it is in equilibrium with them. Read the translated Gmelin pages on the details, the constant of this equilibrium has been measured (although I haven't seeked out the values yet).

[Edited on 24-1-2023 by Diachrynic]