Making silver nitrate from .9999 silver - soln is green/blue

[James Ball]

I bought a bar of 5 t oz silver with .9999 stamped on it from a reputable refiner/seller. I am using this to make pure silver nitrate for my electrolysis cell. The solution is not colorless but has a greenish color to it. It looks like some AgNO3 I made from cement silver. Is this normal? Should I complain to the refiner? I have attached a photo of the solution being dissolved.

 

[Lou]

Boil it. If it doesn’t go colorless, too much copper.

 

[butcher]

Pure Silver dissolved in nitric acid is colorless, the copper nitrate in solution is a blue color, with an excess of the different NOx gases giving the blue copper nitrate solution a green coloring.

 

[FrugalRefiner]

Definitely not 4 9s.

Dave

 

[James Ball]

Boil it. If it doesn’t go colorless, too much copper.

I was in the process of heating the solution to speed up the dissolution reaction. It was taking forever to dissolve the 5 t oz bar (not surprising). I added a stockiometric amount of 70 % HNO3 (density = 1.4 g/cc) and then let it boil vigorously for about 1.5 hr. The bar was almost all dissolved with a tiny bit of silver still bumping around (they gave me 156 g not 155.5 g) so my calculations were spot on. BTY, the soln became colorless and crystal clear. (see photo). I am a retired PhD chemist and do not understand where the color went? Was the color oxidized? Was the blue some type of small particle dispersion/refraction effect that went away when the particles dissolved completely? I would not think copper nitrate would be lost by boiling???

(The red spots in the photo are from the underlying red furnace the beaker was sitting on and got reflected into the photo).

 

[Anil]

I bought a bar of 5 t oz silver with .9999 stamped on it from a reputable refiner/seller. I am using this to make pure silver nitrate for my electrolysis cell. The solution is not colorless but has a greenish color to it. It looks like some AgNO3 I made from cement silver. Is this normal? Should I complain to the refiner? I have attached a photo of the solution being dissolved.

 

[Anil]

Certainly if the glass beaker or flask which you used is not contaminated before putting 9999 silver to react with chemically pure HNO3 ( without any halides) there is no reason it should develop any color. It must be fully colorless and transparent.

 

[Lou]

Pure Silver dissolved in nitric acid is colorless, the copper nitrate in solution is a blue color, with an excess of the different NOx gases giving the blue copper nitrate solution a green coloring.

Indeed this is the exact effect.

When I was starting off in chemistry 20+ years ago, I used to have to distill my own nitric acid from stump remover and sulfuric acid (LiquidFire). Sometimes the red fuming nitric acid would come out and it would have this odd greenish blue/teal color.
N2O4 I suspect is the culprit.

Butcher knew what was up!

 

[James Ball]

Indeed this is the exact effect.

When I was starting off in chemistry 20+ years ago, I used to have to distill my own nitric acid from stump remover and sulfuric acid (LiquidFire). Sometimes the red fuming nitric acid would come out and it would have this odd greenish blue/teal color.
N2O4 I suspect is the culprit.

Butcher knew what was up!

I think your observations regarding disillation of nitric acid and what I saw are consistent. Thanks for your post. N2O4 is colorless, however. Clearly something else is going on but this is not likely to be copper contamination. Thanks to all for their comments and posts.

 

[Lou]

Colorless when cold, apparently. Seems like it's some mixture of the oxides of nitrogen.

http://nitrogen.atomistry.com/nitrogen_tetroxide.html
This article has some interesting details too.

 

[James Ball]

Your article and the one on N2O3 were very interesting. Clearly nitrogen oxide chemistry is quite complex and could explain my observations. Thanks for the article - I have booked marked it.

 

[Lou]

No problem. Welcome to the forum, Doc! We hope you stay here and contribute.