I have a quantity of old silver coins that I would like to refine into pure silver. The coins are approximately 80% silver and 20% copper. Would some
knowledgeable person be able to show the appropriate stoiciometry to derive an appropriate amount of 70% Nitric Acid to use to dissolve all the silver
and all the copper?
I remember a bit of basic stiocoimetry but the last time I used it was about 40 years ago.CHRIS25 - 24-7-2014 at 01:52
Hi, very briefly then:
Ag + 2HNO3=AgNO3+NO2+H2O
61 mLs of 70% Nitric acid = 1 mole
108g silver = 1 mole
Dus, for example, if you have 54g silver coins (you can ignore the copper content because this only means you add less nitric acid per coin, but you
will be adding excess acid anyway so forget the 20% non-silver); 54g silver you need 60 mLs acid at 70%. But I would dilute by 1/3 and add another
10% extra acid of whatever volume you choose to use.
Hope this helps
[Edited on 24-7-2014 by CHRIS25]gdflp - 24-7-2014 at 05:53
Remember that copper nitrate will form in the +2 oxidation state, not the +1. This gives the equation of Cu + 4HNO3 -->
Cu(NO3)2 + 2NO2 + 2H2O. Also in the case of silver, that reaction will only occur when the acid is hot
and concentrated. If the nitric acid is dilute, then the following reaction occurs(extremely slowly), 4HNO3 + 3Ag --> 3AgNO3
+ NO + 2H2O. Therefore, if we assume the coins weigh total 100g, then we need (20/63.546)*4 + (80/107.87)*2 = 2.742 mols of nitric acid.
Assuming the nitric acid is 15.6M, a reasonable approximation for concentrated nitric acid, then (2.742/15.6)*1000 = 175ml of concentrated
HNO3. Adding too much water will affect the stoichiometry, but depending upon the actual concentration of your acid if it is too
concentrated it will passivate the copper. You can remedy this by adding a few mls of distilled water.
Stoiciometry
charris1950 - 24-7-2014 at 22:56
Thank you gdflp and CHRIS25. Most kind of you to reply. I sincerely appreciate your generosity in sharing your skill and expertise. It seems so
difficult to gain access to such information these days. Very few are willing to impart information on any subject whatsoever for fear of (a) being
sued for it or (b) somehow losing revenue and then having to sue or (c) feeling that anything remotely related to chemistry automatically makes me a
bloody bomb making suicide terrorist...
So I shall now that I am adequately primed with useful information, I shall, with such dignity as befits an old man, repair to the Man Cave to
prosecute Stage 1 of the Sodium Formate Process for refining silver.CHRIS25 - 25-7-2014 at 05:09
Thank you gdflp and CHRIS25. It seems so difficult to gain access to such information these days.
Tell me what sort of information/data/learning/references you would like to read or know about and I might be able to send you some links if you would
like.charris1950 - 28-7-2014 at 03:35
I shall indeed. One of the problems I have is the sourcing of raw materials reasonably cheaply. An example would be silica gel for the production of
sodium silicate. Looked everywhere for it only to discover (after I'd bought the lab grade stuff at nine million bucks a kilo) that the "Magic
Moisture Absorbing Crystals that your pussycat will adore" is in fact silica gel. Available at any supermarket on the planet, except perhaps those in
Somalia, for ten bucks a bucket full. However, this has nothing to do with refining silver and I shouldn't be wasting electrons... Cheers and thanks
again.
charris. .... How the hell do the cat crap specialists know what a cat is thinking when it takes a dump anyhow?AJKOER - 29-7-2014 at 13:06
A novel electrochemical approach may be to employ galvanic corrosion of a Ag/Cu electrode, which should be attempted on a smalll scale as there are
potential complications and possibly low yield, but, at least, no need for expensive reagents.
This suggestion is based on my observation of the formation of copper hydroxide, copper oxychloride,...from the copper metal electrode in the so
called bleach battery. This galvanic cell also produces aluminum hydroxide, aluminum oxychloride,..from the aluminum electrode along with good amounts
of chlorine gas (perform outdoors) even if one is attempting to keep the reaction in a closed cell.
The galvanic cell previously detailed on SM by myself requires a Cu and Al electrode in an electrolyte of aqueous NaCl plus HOCl. The latter is
conveniently prepared by adding 1.4 parts 5% vinegar to 1 part of 8.25% chlorine bleach (NaClO).
I would hammer your Ag/Cu coins into a thin plate to replace the normal copper metal electrode.
So, with time and shaking to recycle the chlorine, one may be able to transform a good amount of the copper metal to copper salts, which, at the
conclusion of the cell reaction (when all of the Al is consumed), I would treat with vinegar to help remove insoluble Cu and Al salts leaving mostly
unreacted Ag. Well, at least, that 's the speculation. Physical separation of the Ag may also be possible.
------------------------------------------------------------------
Here is a detailed rehash of the steps involved in the so called Bleach battery:
1. Whip up some Hypochlorous acid by mixing bleach (NaOCl) and vinegar (which contains Acetic acid HAc) in the volume ratio 1.4 parts of 5% vinegar to
one part of 8.25% extra strength chlorine bleach.
2. To the HOCl add a piece of copper metal which will function as the cathode and an Aluminum source to act as the anode (finely cut up Al foil, for
example, will also work).
3. Lastly, add a little (or perhaps more, see discussion below) salt to act as the electrolyte to get things started.
My take on the chemistry:
The creation of Hypochlorous acid:
NaOCl + HAc --> HOCl + NaAC
And, on the major electrochemical reactions:
6 H2O <--> 3 H3O+ + 3 OH-
At anode:
Al + 3OH- ⇒ Al(OH)3 + 3 e-
At the cathode (copper):
3 HOCl + 3 H3O+ + 3 e- ⇒ 3/2 Cl2(g) + 6 H2O
for an implied net reaction of:
3 HOCl + Al --> Al(OH)3 (s) + 3/2 Cl2 (g) Eo net = 3.93 V
This battery cell is theoretically capable of generating 3.93 volts.
A possibly less effective version of this attempt on applying cathodic corrosion to remove copper would be to substitute plain NaOCl for HOCl, and I
would suggest an extra amount of sea salt as the electrolyte. Then, no chlorine generation will occur!
In either embodiment, one of my tested recent improvements to speed up the cell reaction was to substitute for Aluminum foil, Al foil that has been
heated to a red glow in a flame. The result is Al2O3/Al where the underlying Aluminum is now much more prone to chemical attack (research suggests a
weakened gamma form of Al2O3 has been formed).
[Edited on 29-7-2014 by AJKOER]charris1950 - 29-7-2014 at 19:42
Thank you AJKOER for your timely contribution to the business of refining silver at home in the Man Cave. The indicated method sounds interesting and
highly do-able, the constituents being easily obtainable. Oddly enough I have a bit of glacial Acetic Acid. Well maybe it's not so glacial anymore but
still useable. I shall add this method to the notebook for trial in a week or two. Once again, thanks for showing interest.Metacelsus - 29-7-2014 at 20:47
I would think that in AJKOER's proposed reaction the aluminum would dissolve instead of the copper. A better idea might be to leave
the Al out entirely. Hypochlorous acid is powerful enough to oxidize copper by itself.
Also, mixing glacial acetic acid and bleach might be hazardous, due to chlorine gas production.
[Edited on 30-7-2014 by Cheddite Cheese]charris1950 - 29-7-2014 at 22:23
I am afraid that I am a bit too dim to be able to comment authoritatively about AJKOER's methodology. But I appreciate the fact that he... or perhaps
she... whatever, devoted the time to tell me about it. When I get back from a little photo job in Tasmania I shall set up the experiment and post the
result. Thanks for your interest Cheddite Cheese. Most decent of you to comment. I appreciate it.MrHomeScientist - 30-7-2014 at 12:37
It's generally advisable to ignore most of what Joker posts. He believes bleach and vinegar can do literally anything, and touts this method
forum-wide despite repeated attempts to point out flaws in his methods. His use of the word 'novel' usually translates to 'nonfunctional.' Were he
able to provide experimental results to back up his claims, I would happily reverse my position.
Fleaker is the resident precious metals expert, so searching for posts by him would be valuable. I think gdflp is on the right track, and IIRC the
youtube user 'indeeditdoes' has some good videos on precious metals refining that would be worth a look.aga - 30-7-2014 at 13:12
I would disagree.
AJ's posts tend to contain links to some useful references, so ignoring all of his/her posts would be folly.
Ignoring the Chemistry contained in the posts, well, maybe wise.
