Sciencemadness Discussion Board

Silver in high oxidation state -- experiment

woelen - 16-2-2013 at 12:11

This is an experiment I want to share with you:

http://woelen.homescience.net/science/chem/exps/Ag2O2/index....

I made the remarkable compound Ag2O2, a mixed silver(I)/silver(III) oxide, sometimes written as Ag2O.Ag2O3 or Ag4O4.

This stuff gives very dark brown solutions in strong acids, due to formation of the Ag(3+) ion, an ion which many people do not know at all.

More experiments will follow with this compound.

AndersHoveland - 16-2-2013 at 21:46

I wonder what exactly forms when the Ag2O2 is dissolved by the concentrated acid. I doubt Ag+3 ions can exist in aqueous solution. Perhaps AgO+ ions form. This would be analogous to the blue-colored vanadyl ion in aqueous solutions of vanadium +5 compounds.

I do not even think Ag+2 can exist in aqeous solution. If I remember correctly, it can only exist in concentrated perchloric acid. One investigation measured the reduction potential of Ag+2 dissolved in 6.5M perchloric acid at 1.987 v. That is just slightly less than the strength of persulfate. AgF2 dissolved in anhydrous HF can oxidize xenon to XeF2, so I am fairly certain that Ag+2 would immediately hydrolyze with any water.

Gold(III) oxide is also soluble in concentrated acidic solution. When diluted with water, it is actually the metahydroxide, AuO(OH), which precipitates out. Heating to 150° dehydrates it to Au2O3. At 220° the auric oxide decomposes to its elements.

[Edited on 17-2-2013 by AndersHoveland]

Adas - 17-2-2013 at 01:24

Quote: Originally posted by AndersHoveland  
Gold(III) oxide is also soluble in concentrated acidic solution. When diluted with water, it is actually the metahydroxide, AuO(OH), which precipitates out. Heating to 150° dehydrates it to Au2O3. At 220° the auric oxide decomposes to its elements.

[Edited on 17-2-2013 by AndersHoveland]


Wikipedia: Anhydrous Au2O3 can be prepared by heating amorphous hydrated gold(III) oxide with perchloric acid and an alkali metal perchlorate in a sealed quartz tube at a temperature of around 250 °C and a pressure of around 30 MPa.

blogfast25 - 17-2-2013 at 07:14

I'm rather suprised (and still a little sceptical) at the existence of Ag (III). Ag has basically the same electron configuration as Cu, so an unstable Ag(II) might be expected, on these grounds. But Ag(III)? I'll have to read your article in its entirety, woelen...

[Edited on 17-2-2013 by blogfast25]

AndersHoveland - 17-2-2013 at 07:42

Quote: Originally posted by blogfast25  
I'm rather suprised (and still a little sceptical) at the existence of Ag (III).

It is true, and the structure has been confirmed by neutron diffraction. However, I believe it typically only exists in the +3 oxidation state in its solid state.

The compound can be thought of as containing Ag+ ions and AgO2- ions.

It should not really be that surprising, as copper typically forms in the +1 or +2 oxidation state, while gold usually in the +3 oxidation state. The compounds CuF3, AgF2, and AgF3 do exist, but they are highly reactive and difficult to prepare. AgF3 cannot even be formed by any direct reaction of the elements. Gold can also exist in the +5 oxidation state, in the compound Au2F10. A bit of trivia, it is supposedly a stronger fluoride ion abductor than even SbF5.

BromicAcid - 17-2-2013 at 08:16

I love reading about obscure high oxidation state transition metal compounds. I think because there was a PhD thesis in my college library all on pernickelate compounds and up to that point I had never heard of such a thing. For a time research wasn't too focused in that area either however more recently with materials science applications and the semi-conductor industry it seems that these compounds are again being brought up in the literature. Great to see these types of experiments Woelen!

Mailinmypocket - 17-2-2013 at 14:08

Nice job! I'm definitely going to try some of your experiments next weekend!

woelen - 17-2-2013 at 14:18

I also tried adding solid Ag2O2 to 48% HF. When this is done, then a soft hissing noise is produced, but the solid does not dissolve. Probably the hissing noise is due to small bubbles of air escaping the loosely packed powder. When 50% HNO3 is added to the HF (appr. same volume), then the black solid dissolves and a dark brown solution is obtained.

Another funny experiment is mixing some of the Ag2O2 with finely divided red phosphorus and igniting the mix. This gives an extremely violent reaction, the mix deflagrates with a very bright orange flame.

Now I am in the process of making a periodato-complex of silver(III), which has a deep orange/maroon color. More will follow in a webpage one of these days.

woelen - 19-2-2013 at 06:59

I found something very interesting. As AndersHoveland already suggested, the presence of free Ag(3+) ions is doubtful. I now also am inclined to think that the brown solutions in conc. HNO3 are not due to Ag(3+).

I did the following experiment:

Add a tiny speck of Ag2O2 to 60% HClO4: The black solid quickly dissolves, while fizzling. A colorless gas is produced and the solution becomes nearly colorless (very pale brown/red). I added a little more Ag2O2. When this is done, there is more fizzling and now the solution becomes clearly visible pale brown with a somewhat pinkish hue (pictures will follow later). The color still is quite weak, but one can clearly see that the solution is not colorless.
To this solution I added a small amount of 53% HNO3 (appr. 1/4 of the total volume of the solution). When this is done, then the solution becomes very dark brown. Apparently silver(III) forms a strongly colored complex with nitrate ions or with nitric acid.

I again made the pale brown/pink solution by adding Ag2O2 to 60% HClO4. To this solution I now added some very pure Sr(NO3)2. When this is done, part of the Sr(NO3)2 dissolves and the liquid becomes colorless. When some 53% HNO3 is added to this liquid, then it remains colorless (actually, it becomes somewhat opalescent, but this may be due to the precipitation of very small amounts of Sr(NO3)2). So, most likely the brown complex is not with nitrate ion, but with molecular HNO3.

I have tried finding info on this brown complex on internet, but no success.

AndersHoveland - 19-2-2013 at 09:13

Woelen, can I make a suggestion? First dissolve some Ag2O2 into nitric acid to form the brown solution again. Then try adding perchloric acid to see if the color goes away.

I am not really sure what is happening. Another possibility may be that HOAg+ ions are forming in solution. You said that you observed gas being given off. Here is just a guess at what the reaction might be:

4 Ag2O2 + 8 H+(aq) --> 4 Ag+ + 4 HOAg+ + 2 H2O + O2


Quote: Originally posted by BromicAcid  
For a time research wasn't too focused in that area either however more recently it seems that these compounds are again being brought up in the literature. Great to see these types of experiments Woelen!

Actually, "silver peroxide" (Ag2O2) has been known for quite some time. It had been early established that the compound was probably not a true peroxide, but it was not until much later that the actual structure was determined. Before modern-technology methods of chemical analysis, many just assumed it was just silver(II) oxide.

Another one of these so-called "peroxides" which have not been well investigated is NiO2.

[Edited on 19-2-2013 by AndersHoveland]

AndersHoveland - 20-2-2013 at 05:54

Quote:
"The peroxide of silver is converted by sulphuric acid into the usual silver salt, oxygen being evolved. With colourless concentrated nitric acid it forms a red solution, which, however, on being heated, evolves oxygen gas, and becomes changed into the common salt of silver. It undergoes the same change without evolution of oxygen, by the action of nitric acid containing nitrous acid... With muriatic [hydrochloric] acid it forms white chloride of silver, with evolution of chlorine. It is dissolvedby ammonia with a brisk evolution of nitrogen gas. The decomposition is so violent, that the crystals are driven about the solution like rockets. Hydrogen gas at ordinary temperatures has no influence upon it. ...In peroxide of hydrogen [H2O2] it is changed into metallic silver, with a violent foaming of oxygen gas."

The Chemical Gazette, William Francis, Volume 10, (year 1852), p263

The article also mentions that silver peroxide formed from silver nitrate invariably seems to contain nitrate as well, and that it appears impossible to separate them by washing, slight evolution of oxygen always resulting. The existence of an addition compound is suspected. Quantitative analysis appeared to indicate the solid salt contained a ratio which corresponds to the formula 3 Ag2O2·HNO3

So it would indeed seem that Woelen's observation may be correct, that there is some sort of complex being formed with the nitric acid.

woelen - 20-2-2013 at 23:13

I also have Ag2SO4 and then I'll try making Ag2O2 from that and see what happens if that is added to HClO4.

AndersHoveland - 21-2-2013 at 14:33

Just one more quick thought,

It might be HOAgONO3, or possibly O2NOAgO- ions that are forming. If you look at the resonance structure of a nitrate ion, it has two extra electrons distributed around the oxygen atoms (the central nitrogen effectively having a positive charge). Thus the complex could be between the hypothetical " HOAg+ " ion and common nitrate ions. Perhaps HOAg+ can just not exist in aqueous solution (or at least remain dissolved) since it would just tend to protonate water.

HOAg+ + H2O --> H3O+ + AgO
— or —
HOAg+ + H2O --> Ag(OH)2 + H+(aq)


Perchlorate would not be able to bind with this species, since the oxygens in the perchlorate have only a negative charge of -1. And sulfate might not work either because the central sulfur atom has no positive charge to counterbalance the second negative charge, so as a result, such a complex would tend to be readily protonated and not be stable.

HOAgOSO3- + H+(aq) --> HOAg+ + HOSO3-

If we remember that B(NO3)4- ions seem to be stable against hydrolysis in aqueous solution, O2NOAgO- seems plausible.

This is all just speculation, of course. I am just trying to think of a justification why nitric acid seems to be able to form a complex with silver in its higher oxidation state.

[Edited on 21-2-2013 by AndersHoveland]

woelen - 21-2-2013 at 23:32

How do you come to HOAg(+)? This would have to contain silver in oxidation state +2. Ag2O2 however contains silver in oxidation states +3 and +1. You think that HOAg(+) is formed from Ag(3+) and Ag(+)?

The experiment with the Ag2SO4 is not sufficiently conclusive. Ag2SO4 is only very sparingly soluble, so I need to work with great dilutions and this leads to excessive losses. I also have Ag2O, I'll try dissolving some of this in dilute HClO4 and then adding NaOH + Na2S2O8 to make the Ag2O2. This route also leads to Ag2O2 free of nitrate. To be continued...

AndersHoveland - 22-2-2013 at 02:18

Quote: Originally posted by woelen  
How do you come to HOAg(+)? This would have to contain silver in oxidation state +2. Ag2O2 however contains silver in oxidation states +3 and +1. You think that HOAg(+) is formed from Ag(3+) and Ag(+)?

No, but I would think the Ag+3 is being reduced to Ag+2, while the Ag+1 just goes off into solution.

(I am just throwing ideas out there)

[Edited on 22-2-2013 by AndersHoveland]

woelen - 22-2-2013 at 03:20

By what should it be reduced? If I add the Ag2O2 to nitric acid, then no gas is formed at all, it simply dissolves. The only reduction which I can imagine is reduction by water and this would produce oxygen gas. This indeed happens when I add Ag2O2 to perchloric acid, but not when I add this to nitric acid.

AndersHoveland - 22-2-2013 at 06:18

This is indeed a puzzling mystery. What do you think, Woelen? Do you have any hypothesis for what could be happening?

woelen - 22-2-2013 at 14:18

Actually, I'm quite lost on this. The brown compound in the solution in nitric acid is a mystery to me as well. I'll have to change my web page about this subject. First I'll do more experiments (as described in the previous post, I have a weekend before me and will have time to do some experiments) and then I'll come back on this subject.

Vargouille - 22-2-2013 at 14:18

I found a reference in volume 1 of Inorganic Reaction Mechanisms (a Specialist Periodical Report from 'The Chemistry Society') concerning the reaction between Co(III) and Ag(I) and part of the reaction it puts forth is "Ag(III) + H2O <=> AgO+ + 2H+" and "AgO+ --> Ag+ + 1/2O2". It could be that the oxidizing nature of nitric acid is significant so that the second reaction is hampered. It could be that more concentrated perchloric acid solutions would prevent the decomposition, exploiting the oxidative properties of more concentrated forms of perchloric acid.

woelen - 24-2-2013 at 04:45

In the meantime I have done experiments with Ag2O2, free of nitrate. I did the following:

- Dissolve some Ag2O in reagent grade 50% HClO4 and dilute with 5 times its volume of distilled water. This gives a clear colorless solution.
- Prepare a solution of NaOH and Na2S2O8. Use excess amounts, the Na2S2O8 relative to the Ag2O and the NaOH relative to HClO4.
- Pour the solution of AgClO4 in dilute HClO4 in the Na2S2O8/NaOH solution and swirl the test tube while keeping the solution hot in the meantime. This results in formation of a dark grey precipitate, which after some continued heating becomes very compact and quickly settles at the bottom.
- Decant the liquid above the precipitate and rinse two times with distilled water. Decant the final rinsing water and some compact wet precipitate remains.
- Take a small amount of this black (still wet) precipitate and add this to 65% HNO3. A dark brown solution is obtained. This test was done to be sure that the precipitate indeed is Ag2O2 and not simply Ag2O.
- Take the remaining part of the precipitate and add this to 70% HClO4. The precipitate quickly dissolves, there is fizzling and after a minute or so, the liquid is clear and very pale pink/brown.
- Add some nitric acid (65%) to the pink brown solution: The liquid becomes dark brown.

So, the theory that Ag2O2 from AgNO3 is not really Ag2O2, but some complex of Ag2O2 and HNO3 is less certain now. In a system, totally free of nitrate, there still is the pale pink/brown color and that liquid still contains silver in high oxidation state, because it reacts with HNO3 to give dark brown solutions. It is unclear however, what species are formed in the diverse acids. Maybe in HClO4 there indeed is some free Ag(3+), which has a pale color, while in HNO3 there is formation of some strongly colored complex, or maybe there is some mixed oxidation state ion, which contains silver(III) and silver(I) and due to this mixed oxidation state shows such intense colors? (A similar thing is known for [FeFe(CN)6](-) and copper(I)/copper(II)-chloro complexes).

I also made pictures of the HClO4/Ag2O experiment and now I have the material to make a write-up on the experiment. Unfortunately I cannot give any answers, and hence, I'll add this to the riddles-section of my website.

[Edited on 24-2-13 by woelen]

woelen - 3-3-2013 at 10:57

Here follows the modified web page:

http://woelen.homescience.net/science/chem/exps/Ag2O2/index....

It now contains pictures of the experiments with perchloric acid and this new web page clearly shows that nitric acid indeed forms a deep brown complex. Without nitric acid such a complex does not form.

The exact nature of the brown solution remains a riddle to me!

[Edited on 3-3-13 by woelen]

AndersHoveland - 3-3-2013 at 15:19

I have another possible explanation. In the decomposition reaction of hydrogen peroxide by silver, it is thought that the hydrogen peroxide both oxidizes, then reduces the silver oxide.

That could be one potential explanation for the existence of Ag+3 in nitric acid. The Ag+3 could be alternatively being reduced and oxidized in some equilibrium. When the nitric acid is reduced, it produces more reactive nitrous acid, which could then act to transiently reoxidize the Ag+1.

Another likely possibility is that Ag+3 does not actually exist in the solution, but rather the Ag+1 and Ag+3 in the Ag2O2 are converted to Ag+2.

Just because nitric acid or nitrous acid does not appear to oxidize silver in a test reaction does not mean that it does not. It may just be that the oxidized silver is being reduced as fast as it forms. The equilibrium constant may not be very big, but this could still allow the silver to become soluble.

Such an equilibrium might look like:

2 Ag+2 + HNO2 <==> 2 Ag+1 + HNO3 + 2 H+(aq)

If you begin with Ag2O2 in the reaction, rather than just dissolving silver in nitric acid, the equilibrium will be in a higher oxidation state, and there will mostly NO2 in the equilibrium, even in dilute nitric acid solution. The reason NO2 is not given off like the normal reaction between concentrated nitric acid and silver could have to do with equilibrium and solubilities of this gas. Because in more dilute nitric acid with silver, nitric oxide is formed, as so the nitrogen oxides readily escape, but also in more concentrated nitric acid, the NO2 would become less soluble. But with Ag2O2 in nitric acid, it could just be all NO2 forming in dilute solution.

Yet another potential explanation for the color could be that the Ag+2 is being complexed to NO, which stabilizes the ion. For example, NO•CuCl2 is well known (although it is not actually stable in aqueous solution. The NO•Ag(NO3)2 would not have to really be stable in solution either, it would only need some slight equilibrium to impart its color to the solution.
Quote: Originally posted by Vargouille  
It could be that the oxidizing nature of nitric acid is significant so that the second reaction is hampered.

I think Vargouille might be right.

[Edited on 3-3-2013 by AndersHoveland]

AJKOER - 3-3-2013 at 15:33

Here are comments from Atomistry (link: http://silver.atomistry.com/higher_oxides_of_silver.html ) on higher oxides of Silver, which may be of interest. To quote:

"Electrolysis of Silver nitrate solution at 0° C. yields at the anode a black, crystalline substance of metallic lustre. It readily loses oxygen, Silver nitrate entering into solution, and the residual crystals of silver peroxynitrate have the formula 2Ag3O4,AgNO3. It changes slowly, with evolution of oxygen, into 3Ag2O,AgNO3. According to Weber, the presence of between 15 and 25 per cent, of nitric acid inhibits the deposition of the peroxynitrate at the anode, but produces a brown solution. Weber regards the oxide portion of the salt as having the formula Ag(AgO2)2, analogous to that of magnetic iron oxide, and considers it to be the silver salt of an unstable argentic acid, HAgO2. It is a compound of silver in which the metal has a valency greater than unity.

An analogous derivative of the oxide Ag3O4 has been prepared by the electrolysis of silver fluoride. It has the formula 2Ag3O4,AgF.

A peroxide, probably Ag2O3, is stated to be produced by anodic oxidation of silver in acid solution. When solutions of sodium or potassium persulphate react with silver or Silver nitrate, a peroxide with a higher percentage of oxygen than Ag2O2 is produced, the process being attended by catalytic decomposition of the persulphate with formation of the acid sulphate. Ammonium persulphate does not yield a peroxide, but the ammonium radical becomes oxidized to nitric acid."

AndersHoveland - 3-3-2013 at 19:11

I wonder if you could get the same dark colored brown solution by just passing NO2 into a solution of silver nitrate in 40% nitric acid.

woelen - 3-3-2013 at 23:37

No, you won't get the same dark color. Actually, I have dissolved some Ag metal in conc. HNO3 and when this is done, a lot of Ag(+) is formed and the remaining silver causes formation of more NO2 which simply bubbles through the acid with all that Ag(+) and the liquid does not become dark. It only becomes yellowish, due to some dissolved NO2, but that yellowish color appears with many metals which give colorless ions (e.g. Pb, Bi, Zn).

I do not believe that NO and/or NO2 has anything to do with the intense brown color. Keep in mind that the color is REALLY intense, something like the intensity of the color of permanganate in aqueous solution.

@AJKOER: Your link indeed gives an interesting idea. Nowadays it is known that Ag2O2 is formed and not Ag3O4 (which is the Ag(AgO2)2, mentioned in your link), but it might be that the Ag2O2 is Ag(AgO2) and that AgO2(-) ions can be coordinated to nitric acid or nitrate ion at very high acid concentration. The brown color also exists in dilute nitric acid solutions, as long as another acid takes care of acidity. E.g. a liquid containing 5% nitric acid and 50% perchloric acid also produces deep brown solutions. A solution of 5% nitric acid and the rest water does not produce such solutions though. The liquid then quickly becomes turbid and a dark precipitate is formed.

AndersHoveland - 4-3-2013 at 00:32

I found a reference which I would strongly recommend reading. Here is just part of it:
Quote:

The black oxide formed is quite soluble in nitric acid and fairly stable in such solutions, the color being discharged only in 24 to 36 hours if the solution is kept cold. Warming or dilution with water at once decomposes the compound. On freezing the strong nitric acid solution, which may be almost black, a yellowish crystal-mass is obtained at liquid air temperatures. This upon warming melts to a black liquid without any evidence of separation into different constituents.
Solution in nitric acid was always proceded by evoltion of oxygen which soon ceased, the bulk of the material dissolving without further decomposition.

"Silver Peroxide and the Valence of Silver", H. C. P. WEBER;
Transactions of the American Electrochemical Society, Volume 32, starting page 391

The reference also mentions that a tiny ammount of chlorine can catalyze the decomposition of these Ag2O2 solutions in nitric acid, while manganese sulfate is merely oxidized to permanganate, without any catalytic effect.

The article seemed to suggest the some oxygen is given off when the silver peroxide reacts with nitric acid. Woelen, are you sure you did not notice any oxygen bubbles?

What I find particularly interesting is that this black substance loses most of its color when frozen into a solid. That is consistent with my hypothesis that there may be a nitrosyl complex. Obviously since the nitric oxide would only be in equilibrium, it would go away when the solution was frozen.



Also found this, which may be of less value:
Quote:

L. J. Thénard found that in the presence of nitric acid silver oxide is partly reduced and partly dissolved. According to M. Berthelot, there is a cyclic series of reactions in which the metal is alternatively peroxidized and reduced.

M. Berthelot, Ann. Chim, Phys,., (5), 21. 164, 1880 ; (7), 11. 217, 1897 ; (7), 23, 52, 1901 ; L. J. Thénard, Ann. chim. Phys. (2) 9. 316, 441, 1818


I am not entirely sure what it means. It was mentioned in the context of hydrogen peroxide reactions, so perhaps the author meant that hydrogen peroxide is present also.

Edit(woelen) on request of AndersHoveland, fixed layout

[Edited on 5-3-13 by woelen]

Boffis - 4-3-2013 at 11:44

Better references exist, particulary about Silver III periodate complexes and there are many published examples of the use of higher valency silver compounds in organic chemistry.

Try: The Chemistry of Argentic Oxide. The formation of a Silver(III) complex with Periodate in Basic Solution; G. L. Cohen & G. Atkinson; Inorganic Chemistry, v3, p1741 (1964).

This paper not only contains a discussion on the Ag+ + Ag3+ <-> 2 Ag2+ equilibrium but also detailed preparation of some of the periodate complexes.

woelen - 4-3-2013 at 13:50

I know about the orange/maroon periodato-complexes of silver(III). I actually made some of them from part of my Ag2O2, but these are not part of the riddle at this place. Here we are discussing the brown stuff in HNO3.

@AndersHoveland: I'll try the experiments again, trying to see whether any oxygen is formed or not from Ag2O2. I'll also do the experiment by adding a tiny amount of HCl to the dark brown nitric acid solution. I'll come back on this...
I already tried adding some Ag2O2 to conc. HCl. When this is done, then the solid immediately turns white and the smell of chlorine appears. It oxidizes HCl and insoluble AgCl is formed.
The brown solution is much more stable than what you mention. I have it around for more than 2 weeks already (in a room with a temperature around 15 C in this time of year) and still it is quite dark brown. It has become lighter, but still it is very dark.

AndersHoveland - 5-3-2013 at 01:47

Quote: Originally posted by woelen  
I know about the orange/maroon periodato-complexes of silver(III). I actually made some of them from part of my Ag2O2, but these are not part of the riddle at this place. Here we are discussing the brown stuff in HNO3.

I wonder if there could be a relation. Periodic acid acts as an oxidizer also.

I still suspect there may be some complex equilibrium effect which we do not understand, which would account for this strange phenomena.
Quote: Originally posted by woelen  
The brown color also exists in dilute nitric acid solutions, as long as another acid takes care of acidity. E.g. a liquid containing 5% nitric acid and 50% perchloric acid also produces deep brown solutions. A solution of 5% nitric acid and the rest water does not produce such solutions though. The liquid then quickly becomes turbid and a dark precipitate is formed.

The color may go away, but the interesting thing is that oxygen is still not evolved when the solution is diluted. So the oxidation state is independant of acidity.


There is one more possibility which we have not considered. It may be doubtful, but it is a potential possibility to consider nonetheless. It may be possible that there is some sort of equilibrium between different oxidation states of the silver in the solution, and that this equilibrium shifts depending on the pH, would presumably have a relation to the color change.
Quote: Originally posted by Boffis  
Better references exist, try The Chemistry of Argentic Oxide. (1964).

from the paper:

"Although chemical studies have shown that, when dissolved in acid, Ag2O2 does indeed yield Ag(II), the seeming contradiction could be accounted for by the equilibrium:

AgIIIO+ + Ag+ + 2H+ <==> 2Ag+2 + H2O

It has been known for some time that Ag(III) compounds are formed when Ag(I) is oxidized in the presence of a stabilizing ligand."




I found one source that claims Ag2O3 exists and can be isolated:
Quote:

Silver (III) oxide Ag2O3 is obtained as a black oxide by anodic oxidation of Ag+ in alkaline solution. It is stable at 100 °C and is used as a semiconductor material.

"Inorganic Chemistry", Balaram Sahoo, Nimain C. Nayak, Asutosh Samantaray, Prafulla K. Pujapanda, Sahoo Balaram, nayak Nimai Charan, samantaray Asutosh, pujapanda Prafulla Kumar, (2012) p796, India

I am not entirely sure whether I believe them, but it seems plausible.

[Edited on 5-3-2013 by AndersHoveland]

AndersHoveland - 5-3-2013 at 11:19

I found something else mentioned in the literature which may be another lead (or it may have nothing at all to do with the reaction)...

Woelen's strange reaction may have a relation to another poorly understood reaction, the reaction between silver nitrate and iodine.

In the presence of silver nitrate reacting with iodine, otherwise resistant organic compounds (including alkanes) can be oxidized. In the absence of water, the alkanes can even be partially nitrated. It is thought that the reaction procedes through the formation of an intermediate IONO2.
Quote:

...It undergoes in part hydrolysis into HOI and nitric acid, producing a reddish-brown solution precisely similar in appearance and properties to that obtained by dissolving "silver peroxide" in nitric acid, and which presumably contains silver pernitrate.

"Chemical News and Journal of Industrial Science, Volumes 81-82, Sir William Crookes, (1900) p286


Obviously the production of actual "silver pernitrate" is doubtful, but this is the compound name referred to in other early literature used to describe the silver peroxide with some HNO3 apparently part of the composition (the 3 Ag2O2·HNO3 previously described in an earlier post).

If this is true, it would be remarkable for an instance where the black silver solution could be created without starting from persulfate, and without electrolysis. If it was actually the same compound forming, one would think the iodine would also have slowly catalyzed its decomposition, but unfortunately the article made no mention to whether such black solutions are actually stable, as the dark color might have just resolved away on its own after a short time.

Again, could the formation of a dark colored oxidizing solution of silver be just a coincidence? Is silver actually transiently forming in its +2 oxidation state in the mixture of AgNO3 and I2 ? I do not know.

I do not want to go off on a tangent topic, but here is a quick note about the reaction between silver nitrate and iodine -
Quote:

(10)AgNO3 + (7)I2 --> (10)AgI + (2)I2O5 + (10)NO2

in the absence of water, silver nitrate reacts with elemental iodine to give off brown nitrogen dioxide gas. If anhydrous silver nitrate is used dissolved in benzene (one of the few non-polar solvnts that can dissolve silver ions), then iodine pentoxide will form I2O5.
If water is present, iodic acid will form instead, and some nitric acid is more likely to form. Silver iodide will also be precipitated.

The reaction of silver perchlorate with iodine is similar, and can produce I(ClO4)3 (obviously not in the presence of water)


[Edited on 5-3-2013 by AndersHoveland]