# Disposal of Silver Halides dissolved in Ammonium Hydroxide

I was reading the Lab Manual for Chemistry prepared by the NCERT . While going through the the test for Chloride ions (pg 88) I came across the following Hazard Warning :

The solution obtained after dissolving AgCl/AgBr/AgI precipitate in ammonium hydroxide should be acidified with 2M HNO3 and should be discarded quickly to avoid serious explosion.

How does it cause a serious explosion?

• Feb 4 at 4:44

In the test for Cl- the chloride ions are converted into AgCl by the following reaction,

Cl- + AgNO3 → NO3- + AgCl

which is then dissolved in ammonium hydroxide by the formation of diammine silver chloride by the following reaction:

AgCl + 2NH4OH → [Ag(NH3)2]Cl + 2H2O

On standing, the diammine silver complex subsequently breaks down to Ag3N.

The standard free energy of Ag3N is about +315 kJ/mol . Hence it is unstable and decomposes explosively to metallic silver and nitrogen gas .

2Ag3N → 6Ag + N2

So the diammine silver complex has to be destroyed after use so that it does not break down into Ag3N.

One way of destroying the complex is by adding nitric acid.

[Ag(NH3)2]Cl + 3HNO3 → AgNO3 + 2NH4NO3 + H2O

So in all reactions involving the formation of diammine silver complex, the complex must be destroyed using acids after the purpose.

The complex is formed when AgCl, AgBr(sparingly soluble)and AgI(the complex will be formed only in trace amounts) are dissolved in NH4OH . The comlex is also formed in the well-known Tollen's test. Tollen's reagent is [Ag(NH3)2]OH. Ammoniacal Silver Nitrate is also used in Fontana–Masson Stain and also in silver mirroring(such as inside insulated vacuum flask).

A reported accident

Actually, the mechanism of $$\ce{Ag3N}$$ creation (and also the associated products, Silver amide $$\ce{AgNH2}$$, and the imide $$\ce{Ag2NH}$$), have not been definitely accounted for, either by accident investigation reports or per a recent review of the literature, on possible paths to explosive Silver nitride.

I have ideas of a wider chemistry spectrum based answer which accounts for the apparent role involving $$\ce{Ag2O}$$.

To begin, I cite the more common background description, per a government source, to quote:

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.

So, of interest is the cited reaction:

$$\ce{Ag2O(s) + 4 NH3 + H2O ⇌ 2 [Ag(NH3)2]OH}$$

where I would expect any Silver oxide formed from the reverse reaction of dissolving Ag2O in ammonia would then likely create a high surface area (with enhanced reactivity) $$\ce{Ag2O}$$. It should also importantly be noted that the reaction, itself, is reversible (see as a source, "Second year college chemistry" by William Henry Chapin, page 255), to quote:

As might be expected, the silver-ammonium complex dissociates slightly into its constituents as indicated by the equation

$$\ce{[Ag(NH3)2]+ ⇌ Ag+ + 2 NH3}$$

This is a reversible reaction, very much like the ionization of a very weak acid or base.

So, 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), a radiation source could further accelerate the process (see, for example, Ag2O as a New Visible-Light Photocatalyst: Self-Stability and High Photocatalytic Activity). 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.

As such, the introduction of appropriate light, or other irradiation paths (like X-rays commonly associated with radical formation) may be instrumental. In fact, here is an interesting pertinent radiation study: The X-ray activated reduction of silver (I) solutions as a method for nanoparticles manufacturing. To quote from a passage discussing an associated 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 promotive radical pathway for the allude to creation of $$\ce{Ag3N, Ag2NH and AgNH2}$$, which also appears to be accelerated in the presence of $$\ce{OH- and Ag2O}$$ in the presence of ammonia.

The proposed reaction path is:

$$\ce{NH3 + H2O ⇌ NH4+ + OH-}$$

$$\ce{NH4+ ⇌ NH3 + H+}$$

$$\ce{[Ag(NH3)2]+ ⇌ Ag+ + 2 NH3}$$ (per above)

In the further presence of light:

$$\ce{Ag+ + hv -> •Ag + h+}$$

$$\ce{OH- + h+ -> •OH + hv}$$ (same source as above)

$$\ce{OH- (aq) + hv -> •OH + e- (aq)}$$

$$\ce{NH3 + •OH -> •NH2 + H2O}$$

From a related work: Kinetics and Mechanism of the Reaction of •NH2 with O2 in Aqueous Solutions

$$\ce{NH3 + •H <=> •NH2 + H2}$$

As noted in this work.

$$\ce{•NH2 + e- -> NH2-}$$

$$\ce{Ag+ + NH2- <=> AgNH2}$$

$$\ce{AgNH2 -> Ag2NH + NH3}$$

$$\ce{AgNH2 + Ag2NH -> Ag3N + NH3}$$

Note, my suggested pathway is largely promoted from the recognition of Ag2O as a recent (article was 2011) visible-light photocatalyst with the possible introduction of contributing radical based reactions.

In summary, I am suggesting chemically diverse mechanisms, involving both light and radical based reactions, as a more likely explanation.

In layman's terms, silver would have to be in the form of an oxide to set the creation of silver nitride, when dissolving in ammonium hydroxide. It is a simple tradeoff... the silver oxide takes on the nitrogen from the ammonia and donates it's oxygen to the hydrogen. Then you get water and silver nitride. Without silver being bonded to the oxygen, silver will not form an explosive compound when ammonium hydroxide is added to the reaction.