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Author: Subject: Ammoniacal Silversulfamate Solution, Dangerous?
rolynd
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[*] posted on 18-10-2016 at 02:09
Ammoniacal Silversulfamate Solution, Dangerous?


Hi,
I was looking for a non-cyanide Silverplating solution and came upon Patent US 4279708 A which proposes a Silver sulfamate based solution. Main Ingredients are Silversulfamate, Amminiumsulfamate and regulated with ammonia to ph 9-10

I know about the possible formation of dangerous azides when adding Ammonia to Silvernitrate solution - thats not a good idea.
I tried to do some research on silver sulfamate in this regard but turned up nothing.
I wanted to ask the more experienced members here if they forsee any danger of possible azide formation?

I also investigated other possible non-cyanide silverplating solutions but those based on KI or Thiosulfate seem to be not ideal since they suffer from poor adhesion(both), off-white color(KI) or increased tarnishing of the deposit in case of the thiosulfate based due to incorporation of sulfur.

The Sulfamate based one sounds nice at first glance, uses fairly easy to obtain reagents and (at least acording to the patent) doesnt suffer from the aforementioned problems.The plating solution might be a fail, but I am inclined to give it a try , so thats why I am doing my research first and asking about the things I am unsure about...

The only thing that raised some concern was the combination of silver salts with ammonia which I know in general is probably not a good idea but as said -I dont know if this is also the case here.
.



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AJKOER
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[*] posted on 22-10-2016 at 12:53


Good news, dangerous azides are not formed when adding Ammonia to a Silver nitrate solution. However, you can form equally or more dangerous AgNH2, Ag2NH and Ag3N. As to exactly how, I will attempt an explanation of the underlying chemistry. Apply it to answer your question as to the likelihood of the formation of unwanted compounds in any new process.

First, we note that the amide anion is the conjugate base of ammonia:

H2O + H2N- → OH- + NH3 (see https://books.google.com/books?id=y-O1mo6Lg2wC&pg=PA156&lpg=PA156&dq=conjugate+base+of+ammonia+(the+anion+H2N−)&source=bl&ots=Rx 0kd-Vf0t&sig=2aiUenC 5rKejLMm8EYXyxRs6uSo&hl=en&sa=X&ved=0ahUKEwiotqvvqO7PAhVCbiYKHcYFBWQQ6AEIIDAC#v=onepage&q=conjugate%20base%20of%20ammonia%20(the%20ani on%20H2N%E2%88%9 2)&f=false )

So, while this reaction is perceived to greatly favor moving to the right, but perhaps in the presence of a silver ion (Ag+) together with concentrated aqueous ammonia (or just NH3) and/or an elevated pH (like AgOH in presence of NaOH) and/or a dehydrating agent (dry ethanol), the above reaction could be moved back to the left with the creation of an insoluble silver amide, AgNH2. That is, in the case of Silver salts (where 2 AgOH = Ag2O + H2O), we could have the formation of Silver amide from:

AgOH + NH3 =?= H2O + AgNH2

Note, the cited compounds, NH3, NaOH and ethanol, apparently accelerate the known formation of the mixed product referred to as fulminating silver along with high pH. Reference: See, for example: https://books.google.com/books?id=GuxPDAAAQBAJ&pg=PA101&...

As exactly how to explain the the reported formation of Silver imide (Ag2HN) and Silver nitride (Ag3N), I would guess perhaps a reaction in alkaline conditions, given its apparent important, with the amide anion, and then possibly with imide:

H2N- + OH- ⎯?→ H2O + HN(2-)
HN(2-) + OH- ⎯?→ H2O + N(3-)

Interesting, by the source cited above, the formation of Silver nitride, Ag3N, occurs on the decomposition of ammoniacal solution of argentous oxide in acetone or alcohol. Wikipedia ( https://en.m.wikipedia.org/wiki/Silver_nitride ) also clearly details a pH dependence, to quote:

"Silver oxide in 1.52 M ammonia solution readily converts to the nitride, while silver oxide in 0.76 M solution does not form nitride.[3] Silver oxide can also react with dry ammonia to form Ag3N. "

Barring the addition of dry alcohol, the loss of water on standing, may present an issue.

Wikipedia also has some good tips on silver mirror plating and mentions some accelerates for the process (see https://en.m.wikipedia.org/wiki/Tollens%27_reagent ). These include employing clean glassware for a high quality mirror, and to increase the speed of deposition, pre-treated with tin(II) chloride stabilised in hydrochloric acid solution.

[Edited on 23-10-2016 by AJKOER]
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clearly_not_atara
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[*] posted on 22-10-2016 at 15:38


Quote:
H2N- + OH- ⎯?→ H2O + HN(2-)
HN(2-) + OH- ⎯?→ H2O + N(3-)


The pKa of NH3 -> NH2- in H2O is roughly 30; the pKa of NH2- -> NH(2-) is probably larger than 40, which means that basically no such ions will be present at all (afaik, OH- -> O2- is still more favorable than H2N- >> HN2-) in even the most basic solutions (40 - 15 = 25 > 23). I believe the mechanism involves the decomposition of the diamminesilver(I) ion possibly beginning by deprotonating one of the amine groups (which are much more acidic when part of this complex then when existing as free ammonia).

http://pubs.acs.org/doi/abs/10.1021/ie00059a023

Quote:
If the solution is relatively concentrated, the solids will contain much Ag3N. If the solution contains less than 5% ammonia, the solids will consist mostly of Ag2O and be nonexplosive.


I haven't got the paper, but if I had to guess:

(NH3)2Ag+ + L- >> LH + (NH3)-(Ag+)-(NH2-)

(NH3)-(Ag+)-(NH2-) + (NH3)2Ag+ >>

(NH2-)–(Ag+)–(NH3)
|
(Ag+)–(NH3)
|
(NH3)

^the above complex could lose NH3 to form the linear (NH3AgNH2AgNH3)+, which ought to deprotonate in the middle, and by repetition of this process forming a crystal of Ag3N.

Succinimide can also be used to form a silver complex. Succinimide can be made from succinic acid and urea, IIRC.


[Edited on 22-10-2016 by clearly_not_atara]

[Edited on 22-10-2016 by clearly_not_atara]
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AJKOER
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[*] posted on 23-10-2016 at 03:31


Perhaps, making this too complex, since "Silver oxide can also react with dry ammonia to form Ag3N", from which I would infer a possible path as:

3 AgOH + NH3 = 3 H2O + Ag3N

Or, rescaling:

18 AgOH +6 NH3 = 18 H2O + 6 Ag3N

Since: 2 AgOH = Ag2O + H2O, we have also:

9 Ag2O + 9 H2O + 6 NH3 = 18 H2O + 6 Ag3N

Or, rescaling:

3 Ag2O + 2 NH3 = 3 H2O + 2 Ag3N

This reaction (which may involve surface chemistry) requires heat or light energy as Ag3N is endothermic. As soon as water is created, I suspect it is removed by the formation of [Ag(NH3)2]OH (which may be here just a side reaction consuming more Ag2O and ammonia also), possibly driving the Ag3N creation forward. A sample reaction sequence:

Ag2O + 2/3 NH3 = H2O + 2/3 Ag3N

Ag2O + 4 NH3 + H2O → 2 [Ag(NH3)2]OH

Net: 2 Ag2O + 14/3 NH3 → 2/3 Ag3N + 2 [Ag(NH3)2]OH

Rescaling:

3 Ag2O + 7 NH3 → Ag3N + 3 [Ag(NH3)2]OH

which suggests a large excess of ammonia relative to Ag2O over the starting reaction. Further, this sample net reaction is not quite realistic as the diammine silver hydroxide exists only in solution.

Interestingly, once 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 Ag3N formation.
-----------------------

Found my prior comments on this topic, which are generally in line with Clearly_Not_Altra above. Link: http://www.sciencemadness.org/talk/viewthread.php?tid=28147 . Of interest is the cited reaction:

Ag2O(s) + 4NH3 + 4H2O <--> 2 [Ag(NH3)2]OH + 3 H2O [1]

which 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."

Further comments about Ag3N formation on standing in an open vessel is likely due to loss of water, moving [1] to the left. Once any Ag2O is formed, light could accelerate the process. See "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"

[Edited on 24-10-2016 by AJKOER]
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