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woelen
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Silver in high oxidation state -- experiment
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.
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AndersHoveland
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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]
I'm not saying let's go kill all the stupid people...I'm just saying lets remove all the warning labels and let the problem sort itself out.
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Adas
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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.
Rest In Pieces!
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blogfast25
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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]
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AndersHoveland
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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.
I'm not saying let's go kill all the stupid people...I'm just saying lets remove all the warning labels and let the problem sort itself out.
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BromicAcid
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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!
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Mailinmypocket
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Nice job! I'm definitely going to try some of your experiments next weekend!
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woelen
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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.
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woelen
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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.
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AndersHoveland
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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]
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AndersHoveland
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| 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.
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woelen
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I also have Ag2SO4 and then I'll try making Ag2O2 from that and see what happens if that is added to HClO4.
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AndersHoveland
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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]
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woelen
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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...
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AndersHoveland
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| 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]
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woelen
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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.
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AndersHoveland
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This is indeed a puzzling mystery. What do you think, Woelen? Do you have any hypothesis for what could be happening?
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woelen
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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.
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Vargouille
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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.
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woelen
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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]
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woelen
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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]
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AndersHoveland
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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.
I think Vargouille might be right.
[Edited on 3-3-2013 by AndersHoveland]
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AJKOER
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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."
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AndersHoveland
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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.
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woelen
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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.
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