AJKOER
Radically Dubious
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A Better Understanding of a Pathway to Problematic Silver Nitride
A recent review of the literature on possible paths to Silver nitride (Ag3N), I found somewhat disappointing. My criteria for a theoretically improved
path to account for Silver nitrite (Ag3N) formation should also provide paths for Silver amide (AgNH2) and also the imide (Ag2NH), as known reported
secondary products. Also, the role of Ag2O, water concentration and the presence of OH- ions in promoting Ag3N should be evident.
To begin, I cite the more common background description, per a source (see https://hazmap.nlm.nih.gov/category-details?table=copytblage... ):
"Silver nitride is an explosive chemical compound with symbol Ag3N. It is a black, metallic-looking solid which is formed when silver oxide or silver
nitrate is dissolved in concentrated solutions of ammonia, causing formation of a silver-amide or imide complex which subsequently breaks down to
Ag3N. .”
Of interest is the cited reaction:
Ag2O(s) + 4 NH3 + H2O ⇌ 2 [Ag(NH3)2]OH
where any Silver oxide formed from the previously dissolved Ag2O by ammonia will likely now have more surface area and enhanced reactivity. Further,
it should be noted importantly that the reaction is reversible (see as a source, "Second year college chemistry" by William Henry Chapin, page 255):
"As might be expected, the silver-ammonium complex dissociates slightly into its constituents as indicated by the equation
[Ag(NH3)2]+ ⇌ Ag+ + 2 NH3
This is a reversible reaction, very much like the ionization of a very weak acid or base."
A very important new point is that once any Ag2O is formed (by say moving the equilibrium to the left from a possible loss of water on standing in an
open vessel or upon addition of dry alcohol), light can further accelerate the process. See, as a source, "Ag2O as a New Visible-Light Photocatalyst:
Self-Stability and High Photocatalytic Activity", by Xuefei Wang, link: http://onlinelibrary.wiley.com/doi/10.1002/chem.201101032/ab... . To quote from the abstract:
"Ag2O is unstable under visible-light irradiation and decomposes into metallic Ag during the photocatalytic decomposition of organic substances.
However, after partial in situ formation of Ag on the surface of Ag2O, the Ag2O-Ag composite can work as a stable and efficient visible-light
photocatalyst"
The use of photolysis and also other irradiation paths (like X-rays) are commonly associated with radical formation. Here is an interesting radiation
study, ‘The X-ray activated reduction of silver (I) solutions as a method for nanoparticles manufacturing' by M. Staszewski, et al., published in
Journal of Achievements in Materials and Manufacturing Engineering, Volume 28, Issue 1, May 2008, link: https://pdfs.semanticscholar.org/3877/7e9bee7cee0fba5e9bf41b... . To quote a passage discussing radiation treatment:
“The next step was aimed at eliminating glucose-containing ammonia solutions. The reactions taking place in these solutions resulted sometimes in
the formation of precipitates with unstable composition. These precipitates contained considerable amounts of silver nitride Ag3N, which formed in
some ammonia solutions of the silver salts and exhibited explosive properties in a dry state [15]. The results from analysis of the deposits obtained
for three selected types of solutions are presented in Table 1. “
As the irradiation of glucose is a path to radicals, my proposed mechanics incorporates an ostensible radical pathway (but not claiming any
exclusivity for the necessity of radical formation) for the creation of Ag3N, Ag2NH and AgNH2, which also appears to be accelerated in the presence of
OH- and Ag2O in the presence of NH3.
The proposed reaction path is:
NH3 + H2O ⇌ NH4+ + OH-
NH4+ ⇌ NH3 + H+
[Ag(NH3)2]+ ⇌ Ag+ + 2 NH3 (per above)
In the presence of light:
Ag+ + hv --> •Ag + h+ (see 'Photodecomposition and Luminescence of Silver Halides', by Vitaliy Belous, et al., 1999, at http://www.imaging.org/site/PDFS/Papers/1999/PICS-0-42/1080.... )
OH- + h+ --> •OH + hv (same source as above)
OH- (aq) + hv --> •OH + e- (aq) (see ‘Flash photolysis in the vacuum ultraviolet region of sulfate, carbonate, and hydroxyl ions in aqueous
solutions’ by Elie Hayon, and John J. McGarvey, in J. Phys. Chem., 1967, 71 (5), pp 1472–1477, DOI: 10.1021/j100864a044 , link: https://pubs.acs.org/doi/abs/10.1021/j100864a044?journalCode... )
Ag+(aq) + e-(aq) --> •Ag (or is it just Ag, not radicalized?, see discussion at http://images.biomedsearch.com/7621791/envhper00361-0024.pdf... )
NH3 + •OH --> •NH2 + H2O (See "Kinetics and Mechanism of the Reaction of •NH2 with O2 in Aqueous Solutions", by B. Laszlo , Z. B. Alfassi ,
P. Neta , and R. E. Huie, J. Phys. Chem. A, 1998, 102 (44), pp 8498–8504, DOI: 10.1021/jp981529+, July 15, 1998, at https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=831494 )
NH3 + •H = •NH2 + H2 (see https://www.degruyter.com/view/j/zpch.2000.214.issue-8/zpch.... )
NH4+ + e- ⇌ NH3 + •H (as H+ + e- = •H see https://pubs.acs.org/doi/abs/10.1021/ba-1965-0050.ch017 )
•NH2 + e- --> NH2-
•H + •OH = H2O + Photon (hv) (see p.70 at https://books.google.com/books?id=mO6Z07lHQO4C&pg=PA70&a... )
•Ag + •NH2 --> AgNH2
Ag+ + NH2- = AgNH2
2 AgNH2 --> Ag2NH + NH3
AgNH2 + Ag2NH --> Ag3N + NH3
Note, per Wikipedia on what I claim is a related chloramine disproportionation reaction (link: https://en.wikipedia.org/wiki/Chloramine ):
2 NH2Cl + H+ ⇌ NHCl2 + NH4+
3 NHCl2 + H+ ⇌ 2 NCl3 + NH4+
And, as NH4+= NH3 + H+, there is a possible parallel pH dependent disproportionation with silver amine in the manner of chloramine.
Also, interestingly, a source "A study of complexes Mg(NH3)n and Ag(NH3)n, where n = 1-8: Competition between direct coordination and solvation
through hydrogen bonding" by Tamer Shoeib, et al, available at https://www.google.com/url?sa=t&source=web&rct=j&... reported "that reaction of Ag+ with two NH3 molecules is exothermic by 85.6 kcal
mol", which could help fuel the formation of the endothermic Ag3N compound..
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Note, my suggested pathway is largely promoted from the recognition of Ag2O as a new (article was 2011) visible-light photocatalyst with the possible
introduction of contributing radical based reactions.
Open to comments on the new proposed pathway as well as how it may be compromised to reduce the occurrence of problematic Ag3N.
[Edited on 25-8-2018 by AJKOER]
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AJKOER
Radically Dubious
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Registered: 7-5-2011
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Based on the assumed validity of the above proposed reaction path, the first possible preventive measure is to reduce light exposure especially to
yellow and orange rays (see
https://books.google.com/books?id=JfkVEnuyBS0C&pg=RA30-P... ). This may limit radical formation leading to the amino radical, •NH2 and
subsequent creation of Ag3N.
Next, a complementing procedure, scavenge any created •NH2 radicals by continuous pumping of air into the solution and collecting the outgas (likely
rich in ammonia) in cold water, which would be then added back to the solution. Support for this approach is based on the work: ‘Direct observation
of NH2* reactions with oxygen, amino acids, and melanins’, by Clarke K, Edge R, Johnson V, Land EJ, Navaratnam S and Truscott TG, in J Phys Chem A.
2008 Feb 14;112(6):1234-7. doi: 10.1021/jp076395r, link: https://www.ncbi.nlm.nih.gov/pubmed/18215026. From the abstract, to quote:
‘We report the direct observation of the quenching of the weakly absorbing transient due to the amino radical by oxygen and, hence determine, by a
totally direct method, the corresponding rate constant (k = (1.1 +/- 0.1) x 10(9) dm3 mol(-1) s(-1)). “
Also, see "Kinetics and Mechanism of the Reaction of •NH2 with O2 in Aqueous Solutions", by B. Laszlo , Z. B. Alfassi , P. Neta , and R. E. Huie, J.
Phys. Chem. A, 1998, 102 (44), pp 8498–8504, DOI: 10.1021/jp981529+, July 15, 1998, at https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=831494 .
So, no amino radical likely implies no Ag3N.
Now, if there's some photolysis leading to solvated electrons in the presence of oxygen, this results in elemental silver via:
O2 + e-(aq) --> •O2- (aq)
Ag(+) + •O2- = Ag (s) + O2 (see, for example, https://pubs.acs.org/doi/abs/10.1021/es103757c?src=recsys&am... )
With silver metal, we could have a metal-air battery scenario, with half cell reactions similar to the action of O2 on Cu and aqueous NH3, which in
the case of silver, results in moving away from Ag2O formation to Ag(NH3)2]OH (a good thing also).
In particular for the case of copper, the reported half cell reactions are:
1/2 O2 + H2O + 2 e- ---> 2 OH- (cathodic reduction of O2 at surface of the elemental copper)
And, at the elemental copper anodic zone, the formation of the complex:
Cu + 4 NH3 + 2 H2O --> [Cu(NH3)4(H2O)2]2+ + 2 e- (anodic dissolution of Cu by a complexing agent)
With an overall reaction:
Cu + 4 NH3 + 1/2 O2 + 3 H2O --> [Cu(NH3)4(H2O)2]2+ + 2 OH-
Also, some standard chemical reactions, only for copper where copper ion oscillates between cuprous and cupric states as noted by the source below:
2 Cu + 4 NH3 + 1/2 O2 + H2O --> 2 [Cu(NH3)2]OH
2 [Cu(NH3)2]OH + 4 NH3 (aq) + 1/2 O2 + H2O --> 2 [Cu(NH3)4](OH)2
Cu + [Cu(NH3)4](OH)2 <---> 2 [Cu(NH3)2]OH
Reference: "Kinetics and Mechanism of Copper Dissolution In Aqueous Ammonia", at https://www.google.com/url?sa=t&source=web&rct=j&...
To repeat, the above reactions suggest Ag(NH3)2]OH formation which is a favorable move of the equilibrium against Ag2O (a visible-light photocatalyst)
formation.
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I suspect more possible preventive procedures are likely as well, suggestions welcome.
[Edited on 25-8-2018 by AJKOER]
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AJKOER
Radically Dubious
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I also recommend per the postulated path analysis presented above, the employment of a hydroxyl radical scavenger, namely ammonium carbonate (and not
ammonium bicarbonate, also a scavenger, as the amphoteric HCO3- could react with the basic [Ag(NH3)2]OH ) to be applied in a concentration
approaching that of ammonia (see comments in at https://pubs.acs.org/doi/abs/10.1021/es60137a005?journalCode... and also an article ‘Hydroxyl radical scavenging role of chloride and
bicarbonate ions in the H2O2/UV process’, at https://www.ncbi.nlm.nih.gov/pubmed/11513408 ) to limit the undesirable formation of Ag3N. Reactions of interest:
•OH + CO3(2-) = OH- + •CO3- k =3.0 x 10^8 /M/s (see Eq (5) at https://s3.amazonaws.com/academia.edu.documents/2719367/nitr... )
•OH + HCO3- = H2O + •CO3- k =8.5 x 10^6 /M/s (see Eq(6), same source)
•CO3- + •CO3- = CO2 + CO4(2-) k =1.5 x 10^7 /M/s (see Eq(11), same source)
Interestingly, photolysis applied to a boiled carbonate solution (which is free of dissolved O2) is a claimed source of the carbonate radical and also
solvated electrons (see this 1969 reference: https://pubs.acs.org/doi/abs/10.1021/j100909a029?journalCode... _ ). The reaction is described as:
CO3(2-) + hv→ •CO3- + e-(aq)
The logic of using Ammonium carbonate (although I am not sure that it is necessarily completely inert with respect to [Ag(NH3)2]OH and its
applications) is that it removes hydroxyl radicals which potentially interact with ammonia as source of the amino radical (detailed previously), a
presumed precursor to Ag3N from Ag2NH and AgNH2.
A review of the literature apparently does not suggest a simple interaction of the carbonate radical anion with ammonia resulting in the amino
radical as a final major product.
Here is a sample article, ‘The carbonate radical: its reactivity with oxygen, ammonia, amino acids, and melanins.’, by Clarke K, et al., in Phys
Chem A. 2008 Oct 16;112(41):10147-51. doi: 10.1021/jp801505b , at https://www.ncbi.nlm.nih.gov/pubmed/18800821 . From the abstract:
“The CO3 (*-) radical shows complex decay patterns in the presence of ammonia, which can be understood as a balance between the radical-radical
reaction CO3 (*-) + CO3 (*-) and CO3(*-) + NH2 (*) (the amino radical).”
Another article of interest ‘Carbonate radical formation in radiolysis of sodium carbonate and bicarbonate solutions up to 250 degrees C and the
mechanism of its second order decay’ by Haygarth KS, Marin TW, Janik I, Kanjana K, Stanisky CM and Bartels DM, in J Phys Chem A. 2010 Feb
11;114(5):2142-50. DOI: 10.1021/jp9105162 .To quote from the abstract:
“Pulse radiolysis experiments published several years ago (J. Phys. Chem. A, 2002, 106, 2430) raised the possibility that the carbonate radical
formed from reaction of *OH radicals with either HCO(3)(-) or CO(3)(2-) might actually exist predominantly as a dimer form, for example,
*(CO(3))(2)(3-). In this work we re-examine the data upon which this suggestion was based and find that the original data analysis is flawed. A major
omission of the original analysis is the recombination reaction *OH + *CO(3)(-) --> HOOCO(2)(-). Upon reanalysis of the published data for sodium
bicarbonate solutions and analysis of new transient absorption data we are able to establish the rate constant for this reaction up to 250 degrees C.
The mechanism for the second-order self-recombination of the carbonate radical has never been convincingly demonstrated. From a combination of
literature data and new transient absorption experiments in the 1-400 ms regime, we are able to show that the mechanism involves pre-equilibrium
formation of a C(2)O(6)(2-) dimer, which dissociates to CO(2) and peroxymonocarbonate anion: *CO3(-)+*CO3(-)<-->C2O6(2-)-->CO2+O2COO(2-)
*CO3(-) reacts with the product peroxymonocarbonate anion, producing a peroxymonocarbonate radical *O2COO(-), which can also recombine with the
carbonate radical: *CO3(-)+CO4(2-)-->*CO4(-)+CO3(2-) *CO3(-)+CO4(-)-->C2O7(2-)."
In view of the above, I suspect that even if amino radical is created by some path, it is likely consumed in subsequent reactions (with possibly
•CO3-, •CO4l(-), O2COO(2-), CO4(2-), C2O7(2-),..) limiting •NH2 presence for the formation of the precursor AgNH2.
[Edited on 26-8-2018 by AJKOER]
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