Sciencemadness Discussion Board

Unexpected aggressive reaction build up with silver nitrate

CHRIS25 - 12-10-2012 at 00:25

Without the photographs I will try to make this as visually explanatory as possible.

Making silver nitrate with sterling silver and 30% nitric acid.

1. After dissolving 11.6 grams of silver into 36mls of HNO3 it turned the expected colour of copper sulphate a bit lighter though.

2. Excellently clean Solid Copper with good surface area suspended 3/4 way into the solution so that silver could drop off.

3. Silver immediately began to become displaced by the copper, dropped to the bottom of beaker gently.

4. Gray Gunge started to collect rather quickly on the copper, became thick very quickly, even though I was tapping the copper constantly against the side of the beaker, also the exposed copper above the beaker lips became very oxidised, and even collected a bit of the gunge that was actually in the solution, but a very very thin layer, but not evenly covered.

5. After about a full 3 or 4 minutes the reaction started to get more aggresive, fizzing on the surface, bubbling and quite an increase in exothermic activity overall with the result being that I had to quickly filter out the stuff?! at the bottom and leave myself with just an empty solution once again. Thereupon I cleaned and made sure that the copper was totally clean again and re-immersed into a clean solution.

6. The same thing happened but this time without the silver. Gray gunge was precipitating out of solution so unbelievably fast, mildly aggressive and fizzing.

7. Decided to stop reaction, mix everything together and add about 50ml of distilled water. The 2 beakers one with the filtered gunge containg silver, the other gray gunge containing some silver I imagine were re-mixed. I mixed the two again and place the beaker outside for the night, in the freezing cold. Covered of course in a metal container.

8. This morning to my total surprise there was still fizzing, bubbling, but most of all the majority of the gunge has re-dissolved, the solution is near blue again, except for what appears to be quite a lot of silver, colour of solution is not the same as the beginning, but certainly blue again.

At this point I am not sure what is going on, I really did not think that I could foul this one up.
[Edited on 12-10-2012 by CHRIS25]

[Edited on 12-10-2012 by CHRIS25]

[Edited on 12-10-2012 by CHRIS25]

[Edited on 12-10-2012 by CHRIS25]

bbartlog - 12-10-2012 at 06:17

So, presumably your blue color is nitrous acid (though copper is an alloying agent in sterling silver, so cupric nitrate is also possible). Your stoichiometry looks OK to me - something like 107mmol silver and 160mmol acid, 2:3 ratio. How do you know that what was precipitating was silver?
The fizzing seems likely to be NO gas. Nitrites in acid solution will produce NO when reduced. On standing, nitrous acid also disproportionates, net reaction being (per Wikipedia) something like 3 HNO2 → HNO3 + 2 NO + H2O; so that one might expect that some further quantity of nitric acid was produced overnight, allowing redissolution of your precipitate.
If I were to try this I think I'd attempt to neutralize the nitrous acid before attempting to reduce and precipitate the silver. Its presence seems to allow for a lot of different and interesting reactions, which in your case you don't want.

CHRIS25 - 12-10-2012 at 10:37

hi bbartlog. Well that's just it, silver adheres to the copper and then drops down to the bottom of the solution. Copper displaces silver in solution. the copper metal added to the nitric acid that is after the silver has dissolved. I was just very surprised to see so much gunge so quickly forming and dropping, it looked nothing like a couple of videos I saw, even though they were using pure silver and the other one using silver alloy. Their reactions proceded gently and gradually as opposed to mine that seemed to take off like a rocket and simply became aggressive - which in the videos never happened. And the gunge does not look right, even now I am waiting for the remaining bubbling to stop and there has been no solid copper in the solution for about 20 hours. I am not sure where to proceed from here, maybe I will just add a fresh piece of copper suspended and let it do its thing until something stops happening.

Just put the copper pipe back into the solution that had turned from Blue to Blue/Green to Green. Before the copper was placed back into the solution the tiny silver granules could be clearly seen at the bottom of the beaker. Everything else had re-dissolved. Now the copper is back, the thick heavy granules that are more like clumps have pre-cipitated back out of the solution and are now laying on top of the finer granules that obviously did not re-dissolve. I could go back and forth like this forever, it does not make sense to me at the moment what is happening.

[Edited on 12-10-2012 by CHRIS25]

AJKOER - 12-10-2012 at 16:18

Quote: Originally posted by bbartlog  
So, presumably your blue color is nitrous acid (though copper is an alloying agent in sterling silver, so cupric nitrate is also possible). Your stoichiometry looks OK to me - something like 107mmol silver and 160mmol acid, 2:3 ratio. How do you know that what was precipitating was silver?
The fizzing seems likely to be NO gas. Nitrites in acid solution will produce NO when reduced. On standing, nitrous acid also disproportionates, net reaction being (per Wikipedia) something like 3 HNO2 → HNO3 + 2 NO + H2O; so that one might expect that some further quantity of nitric acid was produced overnight, allowing redissolution of your precipitate.
If I were to try this I think I'd attempt to neutralize the nitrous acid before attempting to reduce and precipitate the silver. Its presence seems to allow for a lot of different and interesting reactions, which in your case you don't want.


OK, some of what's going on could also be related to the formation/decompostion of Silver nitride (AgNO2) by the action of HNO2 on freshly precipitated Silver. To quote one source ( http://silver.atomistry.com/silver_nitrite.html ):

"Sodium nitrite and the corresponding salts of potassium and barium react with Silver nitrate to form Silver nitrite, AgNO2. It crystallizes in long, greenish-yellow, rhombic needles, the density at 0° C. being 4.542, and between 21° and 31° C. 4.453. At 15° C. its percentage-solubility is 0.2752, and at 18° C. 0.0216 gram-molecules dissolve in 1 litre of water.

In the moist condition the salt is readily reduced by organic matter. When heated rapidly in vacuo, it is completely decomposed into silver and nitrogen peroxide. Slower heating in air causes side-reactions in accordance with the equations

Ag + 2NO2 = AgNO3 + NO;
AgNO2 + NO2 = AgNO3 + NO.

Several double salts with alkali-metal nitrites have been described. Double compounds with ammonia of the formulae AgNO2,NH3; AgNO2,2NH3; and AgNO2,3NH3 have also been prepared."

Also, in the event of the formation of any Cupric nitrite, comments include (http://copper.atomistry.com/cupric_nitrite.html ):

"The nitrite is only known in solution, prepared by addition of lead nitrite to cupric-sulphate solution. Exposure of its dilute solution to air causes slow formation of nitrate. On evaporation of a concentrated solution over sulphuric acid, there is partial decomposition in accordance with the equation

3Cu(NO2)2 = Cu(NO3)2 + 2CuO + 4NO.

It forms a number of complex nitrites with other metals."


[Edited on 13-10-2012 by AJKOER]

ScienceHideout - 12-10-2012 at 16:30

Hmm, was your silver pure, or was it sterling?

CHRIS25 - 13-10-2012 at 04:16

AJKOER, thanks for the links, reading with interest. However I am not dealing with silver nitrite, and definitely do not see that I am producing any even by accident.
Quote: Originally posted by ScienceHideout  
Hmm, was your silver pure, or was it sterling?


It was 925, and that was the idea, seperate the copper so that I can then re-dissolve the silver to make silver nitrate. Now I have to say that I have just finished the process. I now have 5.7grams of solid silver. Theoretically way under what I should have. I decided to allow the reaction to finish, collected the gunge and all. then charcoaled a piece of wood and placed the gunge on the wood. By the way I added about 250mls of distilled water into the reaction, and it still precipitated out the gunge, remove the copper pipe, and the gunge would re-dissolve after 15 hours. nevertheless after washing the whole mass in filter paper with plenty of distilled water I placed the mass onto the wood and subjected it three times to 1000 degrees of heat. The gunge decomposed leaving the silver behind. So my theory for when I repeat this experiment is to allow only the tiny granular nmass to settle at the bottom of the beaker and when I start to see the gunge appear, remove the copper pipe and allow it to re-disslove, take out the fine granular sand composed of silver and work on that. Unfortunately I still want to know what is happening here. The final waste solution turned from its original blue to a nice fine turquoise green which I have kept just in case I have missed something. Anyway I need to test the purity of the silver that I now have.

[Edited on 13-10-2012 by CHRIS25]