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Author: Subject: Silver nitrate giving off HCl ??
nimgoldman
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[*] posted on 10-12-2018 at 10:20
Silver nitrate giving off HCl ??


I've made silver nitrate the common way (dissolving silver metal in conc. nitric acid).

Once most of the silver nitrate crystallized, I poured off the water and left the crystals to dry.

I kept the little water and slowly evaporated it, yielding nice square crystals.

However, the crystals slightly yellowed and smell exacly like hydrogen chloride. The putrid smell managed to fill the entire storage room where it is faintly smelly now (at first it smelled like something badly rotten).

Here are some hypotheses:

- the silver contained some chloride ions, maybe it was refined from silver chloride and some remained in the metal

- silver nitrate picks up something from the air and reacts with it

- the nitric acid (made from sodium nitrate and sulfuric acid) contained some impurities

- chloride ions could come from water used to make nitric acid - stupid me used just filtered water, not distilled

It's quite a bit of crystals so I think I will recrystallize them and measure pH of the remaining water to confirm presence of an acid.

Any suggestion where that HCl smell could come from would be welcome.

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Sulaiman
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[*] posted on 10-12-2018 at 10:54


I've never associated HCl with 'putrid', sulphur compounds maybe.

The good news is that there is no copper in there- difficult to remove.

The first batch of crystals are clear/white with no odour ?

If so then re-crystalise the second batch leaving some liquid un-crystalized
- which will hopefully contain most of the contaminant(s)




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[*] posted on 10-12-2018 at 10:59


Perhaps you're smelling NO2, which smells like bleach?



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[*] posted on 10-12-2018 at 11:11


NO2 sounds quite plausible to me. Acidic gases tend to have a similar type of odour, and people tend to seem to disagree about the smell of particularly NO2. Trace amounts of nitric acid could lead to this odour. Either way, recrystallisation should fix the problem.



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[*] posted on 10-12-2018 at 11:30


Agreed. Smell isn't a definitive way to identify something; everyone's nose is different.
The only possible source of chloride is from your filtered water, but unless you were filtering salt water this would be in only trace amounts. Plus, even if there were chloride present, the silver would have precipitated it out as AgCl long before it could volatilize into anything smelly. You used nitric acid, so the logical conclusion is nitrogen dioxide.
If you want to "calibrate" your nose, you could prepare a flask of HCl(g) and a flask of NO2 and smell each side-by-side. Remember to waft!
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[*] posted on 11-12-2018 at 04:02


Quote: Originally posted by Sulaiman  
I've never associated HCl with 'putrid', sulphur compounds maybe.


It depends what you are doing with that HCL.
Mix S + Fe and add HCL and you'll definitely get an egg smell.

That was the "do not cut corners" lesson around the time I was 14, trying to make a conductive cement (hence the iron).
I stupidly decided I would not go to the faucet and use any liquid instead. I chose vinegar :-/

Regarding chlorine I really dont believe the contamination comes from water or from your nitric acid distillation.
I dont really care about this when distilling acid or cleaning my glassware for silver and never had any problems.
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[*] posted on 11-12-2018 at 04:40


Quote: Originally posted by Herr Haber  
Quote: Originally posted by Sulaiman  
I've never associated HCl with 'putrid', sulphur compounds maybe.
It depends what you are doing with that HCL.
Mix S + Fe and add HCL and you'll definitely get an egg smell.


S = Sulphur :P




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[*] posted on 11-12-2018 at 06:08


Assuming that NO2 is the gas responsible for the smell. How could it be forming?

You may have created some elemental Ag, from light exposure at one point, which can reliberate electrons, e-, with more light (see http://www.nrcresearchpress.com/doi/pdf/10.1139/v73-375 ):

Ag + hv = Ag+ + e-

Next, the possible consequence of e-(aq) acting on a NO3- ion:

e-(aq) + NO3- = .NO3(2-) (unstable radical anion, created in a fast reaction, see Eq 26 at https://books.google.com/books?id=mckVFtJ7YecC&pg=PA30&a...)

.NO3(2-) + H2O = .NO2 + 2 OH- (see Eq 25, same source, but a very slow reaction)

Note, the nitrate ion (NO3-) is, by itself, not very light sensitive, but Silver metal and many of its salt are!
-------------------------------------------------

A related alternate path could be based on the presence of any Ag2O, where apparently Ag2O is capable of transforming into an efficient and stable visible‐light photocatalyst (see https://onlinelibrary.wiley.com/doi/abs/10.1002/chem.2011010... ). Again the creation of electrons (and electron holes), may account for the NO2 product formation.

[Edited on 11-12-2018 by AJKOER]
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[*] posted on 11-12-2018 at 06:53


So the time trusted equations;

3Ag + 4HNO3 (cold and diluted) = 3AgNO3 + 2H2O + NO
and
Ag + 2HNO3 (hot and concentrated) = AgNO3 + H2O + NO2

are probably correct ?




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[*] posted on 11-12-2018 at 08:34


Quote: Originally posted by Sulaiman  
So the time trusted equations;

3Ag + 4HNO3 (cold and diluted) = 3AgNO3 + 2H2O + NO
and
Ag + 2HNO3 (hot and concentrated) = AgNO3 + H2O + NO2

are probably correct ?


Good! And may be correct as well.

One issue is that the Ag is not in the same form as in my scenario, a very fine suspension, whereas your Silver is at its smallest, an Ag powder, which if unreacted, could be easily separated off. Second issue, the 1st equation producing NO is most likely, but escaping NO does not immediately convert to NO2 (like takes around a 100 seconds), which may preclude a quick smell check near the solution, but is in agreement with a smell developing in a larger area (in your favor)!

As to the details of the prep, only Nimgoldman may know for sure.
-------------------------------

I would also now put on the table the possibility of a metal impurity (like Pb) including a sulfide presence (leading to H2S as suggested also by Herr Haber).

[Edited on 11-12-2018 by AJKOER]
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[*] posted on 11-12-2018 at 11:35


Quote: Originally posted by Sulaiman  
So the time trusted equations;

3Ag + 4HNO3 (cold and diluted) = 3AgNO3 + 2H2O + NO
and
Ag + 2HNO3 (hot and concentrated) = AgNO3 + H2O + NO2

are probably correct ?
I've seen both, but never "cold+conc" nor "hot+dil". Is the rxn stoichiometry and NOx byproduct actually dependent on conc or temp?



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[*] posted on 11-12-2018 at 13:50


as far as I know, it does not depend on temperature, but these reactions do depend on concentration and follow the reaction schemes as shown by Sulaiman.
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[*] posted on 11-12-2018 at 14:43


Quote: Originally posted by Tsjerk  
as far as I know, it does not depend on temperature, but these reactions do depend on concentration and follow the reaction schemes as shown by Sulaiman.
So textbooks that added the temperatures (hot/cold) as rxn conditions are misleading right?



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[*] posted on 12-12-2018 at 17:33


Quote: Originally posted by Sulaiman  
So the time trusted equations;

3Ag + 4HNO3 (cold and diluted) = 3AgNO3 + 2H2O + NO
and
Ag + 2HNO3 (hot and concentrated) = AgNO3 + H2O + NO2

are probably correct ?


Here is my radical path analysis (after all, both •NO and •NO2 are radicals!):

Ag --> Ag+ + e-

2 HNO3 = 2 H+ + 2 NO3-

H+ + e- = •H (see Eq (6) at https://books.google.com/books?id=mckVFtJ7YecC&pg=PA6&am... )

Ag+ + NO3- = AgNO3

NO3- + •H = OH- + •NO2 (as •H = H+ + e-, more precisely may proceed via e- + NO3- = •NO3(2-) a fast reaction, see Eq 26 at https://books.google.com/books?id=mckVFtJ7YecC&pg=PA30&a... followed by the addition of H+ and slow breakdown of an intermediate to OH- + •NO2 ; complexity confirmation, Eq 27 citing only 'Products')

Net: Ag + 2 HNO3 --> AgNO3 + H2O + •NO2

which seemingly confirms the hot and concentrated scenario. I say seemingly, as the plausible intermediate compound (I cited above) could have a minor decomposition pathway (like to •OH + NO2-), which would imply that the net equation's stoichiometric coefficients above may just be approximately accurate (sorry Sulaiman, 'time trusted equations' perhaps, and note, at least I did not pursue a rigorous following of Eq 27 and declare the entire net reaction creates but 'Products'!).
-------------------------------------------------------------------------------

Now, for different conditions with NO only formation, I first scale by a factor of two:

2 Ag + 4 HNO3 --> 2 AgNO3 + 2 H2O + 2 •NO2

•NO2 + •NO2 = N2O4 (see Eq 21, reference below)

N2O4 + H2O = HNO2 + HNO3 (reaction not rapid and likely temperature sensitive, see Eq 23 at https://books.google.com/books?id=mckVFtJ7YecC&pg=PA30&a... )

Assuming the decomposition of Nitrous acid proceeds by:

3 HNO2 = HNO3 + 2 NO + H2O (Per Wikipedia, in “warm or concentrated solutions of HNO2”, link: https://en.wikipedia.org/wiki/Nitrous_acid )

Rescaling: HNO2 = 1/3 HNO3 + 2/3 NO + 1/3 H2O

Implied Net: 2 Ag + 8/3 HNO3 --> 2 AgNO3 + 2/3 NO + 4/3 H2O

Rescaling: 6 Ag + 8 HNO3 --> 6 AgNO3 + 2 NO + 4 H2O

Rescaling: 3 Ag + 4 HNO3 --> 3 AgNO3 + NO + 2 H2O

Which actually also agrees with:

“3Ag + 4HNO3 (cold and diluted) = 3AgNO3 + 2H2O + NO”
except for the “cold and dilute” HNO3 does not seem to apply to “warm or concentrated solutions” of HNO2 (?).
--------------------------------------------------------

Here are comments from a more recent 2006 article:

"The reaction between silver and nitric acid is of electrochemical nature. Because of considerable conductivity of the metallic layer, there is no resistance to charge transfer through the solid region. The only remaining resistance is thus related to the NO3- and Ag+ ion transfer through the aqueous electrolyte that fills the porous region of the solid phase. This resistance is considered by the inter-diffusion coefficient De in the present system."

Also:

"...further measurements (Fig.2). The extraction rate decreases, for example, by increasing of the size of the particles dissolving in the nitric acid solution as shown in Fig.2. This result is in obvious contradiction with chemical reaction mechanism reported by previous authors[6]."

Source: S.K. Sadrnezhaad, et al, 'Kinetics of Silver Dissolution in Nitric Acid from Ag-Au0.04-Cu0.10 and Ag-Cu0.23 Scraps', in J. Mater. Sci. Technol., Vol.22 No.5, 2006. Link to download free pdf at https://www.researchgate.net/publication/259560528_Kinetics_...

So apparently, not a 'time trusted' chemical reaction either!
---------------------------------------------------------

[EDIT] Just found a source article: 'Temperature Dependence of Hydrogen Atom Reaction with Nitrate and Nitrite Species in Aqueous Solution' by Stephen P. Mezyk
Research Chemistry Branch, AECLWhiteshell Laboratories, published in J. Phys. Chem. A, 1997, 101 (35), pp 6233–6237, DOI: 10.1021/jp970934i. Link: https://pubs.acs.org/doi/abs/10.1021/jp970934i .

To quote from the abstract:

"Arrhenius parameters for the reaction of hydrogen atoms with NO3-, HNO2, and NO2- in aqueous solution have been determined by the use of pulse radiolysis and electron paramagnetic resonance free induction decay attenuation measurements. At 25.0 °C, the calculated rate constants for these compounds are (5.61 ± 0.51) × 10^6, .....
Computer modeling of these systems suggests that ·H atom reaction with either anion directly produces hydroxide anion and the corresponding NOx radical."

So this 1997 source (which is several years after the book referenced above) provides a confirmation of my claimed slow reaction with no mention of a minor path, however, did cite a temperature dependency.

[Edited on 13-12-2018 by AJKOER]

[Edited on 14-12-2018 by AJKOER]
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[*] posted on 14-12-2018 at 06:34


The yellow stain could very well be leftover HNO3 decomposing and giving off NO2.

The rotten smell is indeed unusual for both NOx and potential HCl. HCl has a faintly bleach smell but more penetrating and pungent but definitely not putrid. NOx smells like car exhaust/smog.

The only known rotten smell is associated to sulphur compounds. How was your starting silver powder? Was it dark grey/black? Silver has a strong affinity for H2S and gets covered in Ag2S with time. Perhaps you obtained some sulphur byproducts:

Ag2S + 4HNO3 --Z 2AgNO3 + 2NO + S + 2H2O

This could in turn further react in the strongly acidic/oxidizing environment and give off SOx. Did you not smell anything beside NOx during your synthesis?
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[*] posted on 14-12-2018 at 13:00


Quote: Originally posted by Tsjerk  
as far as I know, it does not depend on temperature, but these reactions do depend on concentration and follow the reaction schemes as shown by Sulaiman.


I would guess temperature matters; at higher temperature, NO2 boils away more quickly, but at lower temperatures, it remains in solution and oxidizes Ag while being reduced to NO. It can also disproportionate in aqueous solution:

3 NO2 + H2O >> 2 HNO3 + NO

A more complete explanation would be that both NO2 and NO are always produced -- they interconvert! -- but hotter/more concentrated shifts the products towards NO2 while colder/more dilute favors NO.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 15-12-2018 at 04:07


Quote: Originally posted by Tsjerk  
as far as I know, it does not depend on temperature, but these reactions do depend on concentration and follow the reaction schemes as shown by Sulaiman.


Quote: Originally posted by AJKOER  

.......
Here are comments from a more recent 2006 article:

"The reaction between silver and nitric acid is of electrochemical nature. Because of considerable conductivity of the metallic layer, there is no resistance to charge transfer through the solid region. The only remaining resistance is thus related to the NO3- and Ag+ ion transfer through the aqueous electrolyte that fills the porous region of the solid phase. This resistance is considered by the inter-diffusion coefficient De in the present system."

Also:

"...further measurements (Fig.2). The extraction rate decreases, for example, by increasing of the size of the particles dissolving in the nitric acid solution as shown in Fig.2. This result is in obvious contradiction with chemical reaction mechanism reported by previous authors[6]."

Source: S.K. Sadrnezhaad, et al, 'Kinetics of Silver Dissolution in Nitric Acid from Ag-Au0.04-Cu0.10 and Ag-Cu0.23 Scraps', in J. Mater. Sci. Technol., Vol.22 No.5, 2006. Link to download free pdf at https://www.researchgate.net/publication/259560528_Kinetics_...
....................................................

Just found a source article: 'Temperature Dependence of Hydrogen Atom Reaction with Nitrate and Nitrite Species in Aqueous Solution' by Stephen P. Mezyk
Research Chemistry Branch, AECLWhiteshell Laboratories, published in J. Phys. Chem. A, 1997, 101 (35), pp 6233–6237, DOI: 10.1021/jp970934i. Link: https://pubs.acs.org/doi/abs/10.1021/jp970934i .

[Edited on 14-12-2018 by AJKOER]


Also, per a related another work (‘Kinetic Characteristics of Silver Dissolution in Nitric Acid Solutions in the Presence of Ammonium Nitrate’ by A. B. LEBE, D. YU. SKOPIN a nd G. I. MALTSEV, in Chemistry for Sustainable Development 20 (2012) p. 391-396), link: http://www.sibran.ru/upload/iblock/6b1/6b1d39caf4be374dc4a01... ) to quote:

“Studying the effect of the silver disk area demonstrated that the dissolution rate of the metal in nitric acid solutions is not dependent on the presence of ammonium nitrate in the system, whereas it is proportional to the area of the working surface (S) being in contact with the solution.”

Assuming the electrochemical nature of the reaction, to be correct, means that the dissolution rate of the Silver metal is NOT a function of the concentration of Silver relative to HNO3 (as long as there is some Ag presence, but the surface area of the silver (see comment by Lebe relating to working area solution contact), along with the relative surface area in the presence of other metals, I would add as factors, note also, a particle size comment above by Sadrnezhaad).

Also, the composition of the product mix, per my analysis, suggests a HNO3 concentration dependence (as is well known), and the article by Mezyk cites a temperature effect (which is also true for electrochemical reactions). The effect of starting concentration of HNO3 is also depicted in Figure 2, Graph (b) by Lebe.

I do support Sulaiman's stoichiometric coefficients, at least in the long term, for the action of HNO3 on Silver metal, but in the short-term, especially for the case of dilute nitric acid, I would not be surprised if they are observational disagreements on the volume of NO gas evolution.

As the precise characterization of this reaction has been wrong for so long, I would be surprised if there is an agreement with my comments.

[Edited on 15-12-2018 by AJKOER]
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