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AndersHoveland
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I found a reference which I would strongly recommend reading. Here is just part of it:
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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
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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:
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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
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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]
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Boffis
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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.
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woelen
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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.
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AndersHoveland
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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."
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I found one source that claims Ag2O3 exists and can be isolated:
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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
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I am not entirely sure whether I believe them, but it seems plausible.
[Edited on 5-3-2013 by AndersHoveland]
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AndersHoveland
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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.
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...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
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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 -
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(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)
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[Edited on 5-3-2013 by AndersHoveland]
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