Sciencemadness Discussion Board

Crystallisation of AgNO3 with HNO3 present

Sulaiman - 11-7-2015 at 15:01

I realise that this is a common topic but I could not find a few specific details,
please forgive me if i missed it elsewhere on SM.

Some time ago I reacted a US 1oz silver (99.9%) coin with nitric acid (69%) to make silver nitrate.
The (glass) distilled (and 'polished') water and the nitric acid were from APC pure (though the water was labeled from Atom Scientific) so should be low in contaminants.
I boiled off the liquid to crystallise the silver nitrate,
it was light sensitive so I dissolved the crystalline AgNO3 in distilled water and added a little HNO3 to prevent light sensitivity.

Now I want to try crystallisation again, this time by slow natural evaporation.
(heating the solution produced an 'oily' liquid phase)

From Wikipedia; at 20C the vapour pressure of water is c17.5 mm Hg and pure HNO3 is 48 mm HG.
I realise that the azeotrope is c69% HNO3 but I can't work out what will happen to the solution as it evaporates ...

1) Will the HNO3 evaporate more quickly than the H2O OR will the solution tend towards the azeotrope ?

2) Can anyone point me to a method to determine this, or just explain it for me ?

3) I have read that PURE AgNO3 is NOT light sensitive,
so was the 0.1% 'other' in the coin enough to cause the light sensitivity?

annaandherdad - 11-7-2015 at 16:10

There's a thread on silver and nitric acid, make sure you read it.

http://www.sciencemadness.org/talk/viewthread.php?tid=27851

The US mint doesn't make any coins that are 99.9% siliver (and never did). Do you mean a coin made by some US company? A real US silver coin is worth more for its collector value than for the silver it contains.

If you have a solution of AgNO3 and HNO3 don't boil it. Get a plastic container (like the kind used in kitchens with a plastic lid), put a beaker of your solution in there, and a second beaker with sodium hydroxide (solid, not solution). Leave it in the dark for several weeks. The HNO3 will evaporate along with the water, and both will get absorbed by the NaOH. You might open it up every so often to replace the NaOH.

You could recrystallize it to purify it; after the HNO3 is gone you could use CaCl2 as the drying agent.

[Edited on 12-7-2015 by annaandherdad]

BromicAcid - 11-7-2015 at 17:03

Quote: Originally posted by annaandherdad  
The US mint doesn't make any coins that are 99.9% siliver (and never did).


*Cough* *Cough* American Silver Eagle *Cough* *Cough*

I made my silver nitrate in the same simple method, 99.9% silver coin chucked into nitric acid, evaporated to dryness but I ended up with colorless flakes, no oil.

Sulaiman - 11-7-2015 at 17:08

The coin was a 1oz Silver Eagle which is a bullion coin https://en.wikipedia.org/wiki/American_Silver_Eagle
I thought I had read all of the silver nitrate threads
but after your admonition I re-read and found your note on dessication using NaOH in two threads ... oops ... sorry.

the beaker of AgNO3 solution is now in a (nearly) airtight container with a beaker of NaOH.
(actually commercial drain cleaner NaOH with Na2CO3)


Thanks

the original link above did not work so I put a link to Wikipedia

[Edited on 12-7-2015 by Sulaiman]

annaandherdad - 12-7-2015 at 08:56

Quote: Originally posted by Sulaiman  
The coin was a 1oz Silver Eagle which is a bullion coin https://en.wikipedia.org/wiki/American_Silver_Eagle
I thought I had read all of the silver nitrate threads
but after your admonition I re-read and found your note on dessication using NaOH in two threads ... oops ... sorry.

the beaker of AgNO3 solution is now in a (nearly) airtight container with a beaker of NaOH.
(actually commercial drain cleaner NaOH with Na2CO3)

[Edited on 12-7-2015 by Sulaiman]


Yes, and I must apologize, I didn't know about the American Silver Eagle. It's apparently still 0.1% copper, did you see any blue color when you dissolved it? Or maybe it's just guaranteed to be 99.9% Ag, while in actuality it's much better than that. I'm just guessing.

I did this with a sterling coin, had to get rid of the copper. One method advocated is to add metallic copper to the AgNO3, Cu(NO3)2 solution, to reduce the silver and leave the copper in solution. I found, however, that there were traces of copper left in the silver after I did this, so I went the AgCl route to separate the two.

aga - 12-7-2015 at 14:45

Silver
http://www.ebay.co.uk/itm/Johnson-Matthey-1-Troy-ounce-999-f...

Ag Ag Ag Ag Ag

https://www.youtube.com/watch?v=wTDUuBWGtpU