Sciencemadness Discussion Board - silver

silver

Jor - 16-11-2008 at 14:12

I can get a 100g silver button (for 15 euro), with the following composition:

Ag 80.85%
Pb 1.03%
Cu 8.12%
Fe 2.36%
Sn 1.09%
W 4.65%
Se 0.68%
Mo 0.04%

Can this be purified? My goal is pure AgNO3. When dissolving in nitric, I think the Tin, Wolfram and molybdenium don't dissolve. Next I will add excess NaCl and dilute (or maybe HCl, but thats more expensive), and filter AgCl, including the PbCl2.
Next add to a suitable amount of boiling water to dissolve PbCl2, rapidly decant, and add a solution of hydrochinon (or other good reducing agent) to the AgCl, until all white has turned grey. Dissolve in nitric acid again, and boil down.

Is there any other more econominal way? And will this work?

kilowatt - 16-11-2008 at 14:27

Yep, you've got it.

You can also just heat the AgCl until you have molten silver (it decomposes fairly readily with release of chlorine) and re-dissolve that.

Nerro - 16-11-2008 at 14:37

Otherwise ascorbic acid should also work out pretty well.

woelen - 16-11-2008 at 14:41

Yes, it will work, but I think it is a lot of hassle for making silver nitrate. If you take into account all the other chemicals you need, the time, and the uncertainty of the outcome, then I would consider buying a really pure source of silver if you go for cheap silver nitrate.

On eBay there literally are hundreds of auctions for silver bullions (fine silver, .999, just search for 'silver bullion' on eBay) with prices in the 10 to 15 euro range for 1 troy ounce (appr. 30 grams). This of course is 2 to 3 times as expensive as the silver button you mention, but it is very pure and making silver nitrate then is as easy as dissolving this in nitric acid and evaporating the liquid. You also do not have to handle 100 grams at once, you can do 1/3 of that. Such a bullion will make you appr. 40 grams of AgNO3.

So, if your goal is pure AgNO3 I would go for the silver bullions. If your goal is fun with chemistry and the feeling of victory when you master the process of cleaning and purifying crap silver, then go for the impure silver button and as a bonus you may be left with 100+ grams of AgNO3 as well. The choice is yours

kilowatt - 16-11-2008 at 16:12

The only other chemicals besides the nitric acid that you need to purify it to *very* high purity is a bit of sodium chloride and hot water, as well as some good heat or a cheap reductant like sodium bisulfite to decompose the AgCl. It doesn't get much cheaper than that, even when you consider the need for twice as much nitric acid as would otherwise be the case. The time issue may or may not be of any concern, as a matter of opinion, and personally I enjoy doing that sort of chemistry. I know I'd go for it.

I've done this to separate sterling silver too.

not_important - 16-11-2008 at 17:21

As you said, dissolve in nitric acid, using HNO3 of about 25 to 35 percent strength. Dilute that with distilled water to 3 or 4 times the original volume, let sit overnight in the dark and protected from dust, filter. The overnight waiting allows some of the other metal nitrates to hydrolyse, and for fine hydroxides/oxides to clump together.

Then evaporate to dryness, place in a crucible or dish, porcelain or borosilicate glass, heat it until the silver nitrate melts at 121 C. Keep it molten for a few minutes, stirring with a glass rod. Copper nitrate will decompose into the black oxide, iron nitrates should decompose as well. If you have good temperature control you can run the heat up to 375 or, silver nitrate decomposes at 440 C.

Cool, dissolve in D.W. and filter to remove the copper oxide and iron oxides/basic nitrates. The evaporate, fuse, filter sequence can be repeated, and should be if the AgNO3 shows a blue cast from traces of copper.

Lead nitrate may or may not have decomposed, the closer to 400 C you get the more likely it will have decomposed into basic nitrates or oxides.

If you are not out to recover all the silver you could then crystallise from dilute alcohol, Pb(NO3)2 is more soluble in that then AgNO3 is and so will remain in solution.

Another way to deal with this is to melt the button and blow air over it. Most of the impurities will oxidise, including the lead. You could do full cupellation by adding some additional lead, as much as 50% of the total, and using a bone ash dish, or some other container lined with bone ash or even fireplace ashes. The lead oxides dissolve other oxides and then are absorbed in the porous material, leaving pure silver (although fusing the AgNO3 made from the purified silver would still be a good idea)/

Or you could go for electrolytic purification. Hammer the button somewhat into a flat sheet, immerse in dilute nitric acid (roughly 2%), electroplate from that onto a thin flat aluminium cathode (silver foil would be better) using a low voltage, maybe a volt. The Al foil might be able to be stripped from the silver plated onto it, or you can use aqueous NaOH to dissolve the Al. Given the large amount of copper in the button, some copper may plate out with the silver, but fusion of the nitrate will remove it.

An old way of converting AgCl to silver is to fuse the chloride with flux of sodium borate and carbonate or bicarbonate. You want more of the carbonate than is needed to take up all the chloride as NaCl; in the fused mixture there is some exchange to form NaCl2 and silver oxide which immediately decomposes into metallic silver.

chemrox - 16-11-2008 at 18:13

@Nerro .. ascorbic should work pretty well???? for dissolving metals??
@Jor: what locale are you in? "wolfram" "hydrochinon" we mostly call the latter "hydroquinone" and most us call the former "Tungsten". I've heard of wolframite .. even prospected for it as a geochemist ... trying to locate Tungsten depsosits. Anyway, I have to ask, is it cheaper to make AgNO3 than buying it ready made? I agree with others at least start with 999 silver becaue the 80% stuff will add more than double the cost in reagents alone.

Nerro - 16-11-2008 at 18:17

No, for reducing Ag<sup>+</sup> to Ag.

Fleaker - 16-11-2008 at 18:42

Quote:

Originally posted by kilowatt
Yep, you've got it.

You can also just heat the AgCl until you have molten silver (it decomposes fairly readily with release of chlorine) and re-dissolve that.

This is a very bad idea and isn't the way of getting quality silver. It doesn't readily decompose by any sense of the word (otherwise the Miller chlorine process would be kind of silly). Much of the silver chloride will end up in the air, volatilizing and ending up all about the lab, and possibly in your lungs. Also, metabisulfite (or SO2 for that matter) does not effectively reduce silver chloride. Works a treat for Au(III) though!

not_important's instructions look the best to me except that electrolytic parting probably wouldn't work in this case (electrolyte would foul and fail rapidly). Electrolytic separation via Thum cell works best when the silver is already of high fineness. I've tried running sterling and it wasn't an effective method, I had to alloy it up.

Personally, I would just dissolve the lot in nitric as per not_important's recommendations letting it sit as well. Filter that, then add a chloride source being careful not to use excess chloride (silver will complex to AgCl2-). Filter the AgCl and PbCl2, rinsing with water until the filtrate runs clear. Boil it 2-3 times in distilled water and filter hot each time. The water from your last boil should be clear. You can check for lead with a crystal of NaI if you'd like (yellow precipitate). That should remove most of the lead and also any other solubles like the iron and copper nitrates.

Once you have reasonably pure silver chloride, reduction to silver is simple:
the carbonate-AgCl method recommended by not_important
formation of Ag2O and decomposition to silver using heat. Done by simply adding NaOH to AgCl (again not in excess).
Tollens-type reduction by basification and reduction with suitable aldose sugar.

Dissolve that in nitric and heat it as per not_important's instructions.

Or if you want to make it simple, just buy some silver nitrate off of ebay or a photography supplier. If that's anathema to you, then do Woelen's route and go to your local coin shop and buy some silver bullion. What you pay in premium, you save in time.

Wrote this when I was tired. Edited for clarity.

[Edited on 17-11-2008 by Fleaker]

S.C. Wack - 16-11-2008 at 19:36

If someone is selling 2.6 oz. 80% silver to you for $18.80, you need to buy all you can, but not for silver nitrate. US coin silver (the 90%, balance Cu variety) bags are popular here and far more expensive to buy presently than the button.

Since this has turned into silver purification rather than silver nitrate really, two methods from coinage, the classic method and an acid one.

Pure Silver from Coin Metal

Melt together, by means of a slightly luminous flame from a blast lamp, about 0.3 g. of silver coin and 1 g. of pure lead in a flat cavity, made as smooth as possible, in a piece of blowpipe charcoal. Press some bone-ash firmly into a small porcelain crucible with a pestle so that at the top there is a slight hollow with a coherent, perfectly smooth surface. Place the metallic button by means of pincers upon the bone-ash which is to serve as a cupel. Support the porcelain crucible on a clay triangle, or embed it in sand, with its top slightly inclined towards the blast lamp; and direct the point of the oxidizing flame toward the vicinity of the button, and some of the time directly upon it. At first keep the metal but barely melted, as otherwise it is likely to spirt. The lead oxide, as fast as it is formed, runs off and is absorbed by the bone-ash; in order that it may not all run into the same place change the inclination of the crucible from time to time. Continue the heating until the size of the button no longer diminishes; towards the end apply a higher heat, whereby films of metallic oxide run across the button in the direction of the oxidizing draft, and lead oxide separates out on the opposite side in the form of dark-brown crystals containing copper. The end of the process is reached upon the disappearance of the oxide film, which at the last shows for an instant a rainbow-like play of colors. This is known as the "blick." After this point is reached, further heating causes a loss of silver by volatilization. During the process a part of the lead is volatilized.

The metal still contains a small amount of copper. Melt it again with 1 g. lead, and cupel the greater part of this lead away as before, using a fresh cupel. Free the button from bone-ash by hammering it into a cube (holding it in pincers), and remove the last traces of lead by heating it on a third cupel. Weigh the silver button that is finally obtained. Yield, 96 to 97% of the actual silver content of the coin.

- Biltz and Biltz

Weigh half a dime to two or three decimal places, depending upon the type of balances available. Place in a 250 ml. beaker and add 15 ml. of dilute HNO3. After covering with a watch glass warm gently until all of the metal goes into solution.

Wash the liquid from the underside of the watch glass into the beaker, dilute to approximately 100 ml., and add 5 ml. of dilute HCl. Maintain the solution just below the boiling point and stir frequently until the precipitate of AgCl has become coagulated and the supernatant liquid is clear.

After allowing the precipitate to settle, decant the supernatant liquid, add 50 ml. of water, stir, let settle, decant and repeat the process until the decantate is free from Cu. The test for Cu may be made by adding an excess of NH4OH to a few ml. of the decantate and observing the formation of a blue color if Cu is present. Usually the third decantate is free from Cu.

Add 100 ml. of water to the precipitate and 10 ml. of 6N NaOH and boil the resulting mixture for 15 minutes. The result is a brown suspension of Ag2O. Add to this suspension 10 g. of cane sugar and boil 15 minutes. The reduction of the Ag2O to free silver causes the precipitate to darken in color and to appear partly spongy and partly crystalline in appearance.

After filtering and mashing twice with cold water, remove the filter paper and funnel and place in a depression previously prepared in a piece of blowpipe charcoal. Heat with the blowpipe until all the paper is burned off and the silver has fused together into a single globule. Allow to cool and then weigh.

- J. Chem. Ed. 225 (1949)

watson.fawkes - 16-11-2008 at 20:18

Quote:

Originally posted by S.C. Wack
The end of the process is reached upon the disappearance of the oxide film, which at the last shows for an instant a rainbow-like play of colors. This is known as the "blick."

A curiosity satisfied! I'd read about blick before in cupellation, but I'd never seen a reason for it. This doesn't exactly say either, but it's clear enough that it's thin-film diffraction.

[Edited on 17-11-2008 by watson.fawkes]

chief - 17-11-2008 at 01:15

It's a good price for silver: 80 g == 2+2/3 oz; 1 oz == 10-15 $; I would buy as much as I can get of it, maybe 1 kg ...

Has anyone tried to get the silver out of mainboards (solder) ? Shouldn't be more complicated than to just put the entire mainboard into HNO3; only the metal would dissolve ; maybe yielding some Cu(NO3)2, ...

The silver is bound to Sn in the soldering ...

May be definately worth to do it: Just a 10-liter plastique-container, and everymainboard into it; after a while yielding some precious metals ...

[Edited on 17-11-2008 by chief]

Jor - 17-11-2008 at 02:04

I think I will just go for it. I'm gonna buy it. Woelen, you suggest buying the pure silver. i know this would make an easier progress to AgNO3, but I think the purification is FUN!

I'm not sure what method I'll use as many are described here. But I have enough time until the silver arrives.

The only real concern is that it is a single piece of metal, and that sounds like a lot of NO2

I have a fume hood now, but it has to come out somewhere ...
So washing the gass with 2 wash bottles, one with NaOH, and the second with NaOH/H2O2, would that absorb 99% of the nitrous vapours?

kilowatt - 17-11-2008 at 02:58

If you dissolve the metal in a tall column (preferably a few feet deep, but whatever you can do will help) with fairly dilute acid (say 10%), and bubble air through it (better yet pure oxygen, but air will do), you can recycle some or most of the NO2 given off to nitric acid in a manner similar to the final step of the Ostwald process. Then just run the output of that through your fume hood, or even through an alkaline scrubber (make sure it is set up in a way that it can't suck back!).

woelen - 17-11-2008 at 04:13

Jor, good to read that you choose the way of purifying the 80% silver. Please report back on your results when you have done the purification and explain all the steps taken in the process.

Scrubbing NO2 is easily done with NaOH (giving NaNO3 and NaNO2), but I'm not sure about the NO. Even NaOH+H2O2 probably will not do the trick. NO simply does not dissolve well enough.

A known scrubber for NO is a solution of FeSO4. This gives a dark complex with NO. If release of the gas is not too fast and bubbles are made very fine with a pumice stone (you can get those things at aquarium shops), then a series connection of a bottle with NaOH and a bottle with FeSO4 (cheap impure stuff from a gardening supplier or ceramics supplier is OK) will filter out almost all NO2+NO and what remains can be blown away with the fume hood.

kilowatt - 17-11-2008 at 05:10

NO can also be oxidized to NO2 and then to nitric acid by bubbling air through a column. 4NO2 + O2 + 2H2O --> 4HNO3 However the oxidation of NO is, I believe, favorable at higher temperatures, so you might have trouble getting a good conversion. Silver may be a good catalyst however.

[Edited on 17-11-2008 by kilowatt]

not_important - 17-11-2008 at 05:13

I must note that the dry thermal reduction I suggested for AgCl does not use charcoal or other form of carbon. It uses the thermal instability of silver oxide to get metallic silver, most other metals will remain as chlorides or oxides under the reaction conditions and dissolve in the flux. Indeed, a little NaNO3 can be added to the fusion mixture.

S.C. Wack provided nice details on two classic methods, the wet reduction with alkali and sugar is a good one when you intend to dissolve the metal again.

Most references seem to suggest using HNO3 of 10 to 30 percent to dissolve the metal.

If you use the precipitation of AgCl it can be helpful to use very dilute HCl as the wash solution, just a few drops of the concentrated acid per liter of water; in the same way of you use NaCl to precipitate the AgCl add a drop of HCl to the solution before boiling and filtering.

Magpie - 17-11-2008 at 09:22

Jor I have made AgNO3 from silver bullion. I melted down a 1-ounce commemorative coin in my muffle furnace. I then drip poured this into a deep bucket of water per Ammen.
This fragments the silver into mossy silver. This can then be dissolved in nitric acid. As I recall the dissolution took several hours with a somewhat slow evolution of NO2. It certainly was of no concern in my fume hood.

You can read more about this here:

http://www.sciencemadness.org/talk/viewthread.php?tid=2772&a...

Jor - 17-11-2008 at 09:28

NO2 is no concern in my hood at all. None gets out, everything is sucked away. But... I live in aneighbourhood, so i don't want to release the NO2 from 100g of silver, that's like a small gas attack. So wash-bottles should be used.

I cannot melt the silver, as fumes of Pb and Se will also be formed, and this is something I don't want. Possibly you ahd 100% silver, but mine conatains other (toxic) metals.

But it's an option. Haven't looked up the mp/bp of lead yet, as that's a reason I really don't wanna do a melt.

dann2 - 17-11-2008 at 13:26

Hello,

This may be slightly off topic. It was a method for purifying Sterling Silver. Never used it myself.

Purifying sterling silver
Dissolve your sterling silver in nitric acid; a bit of heat will help if your acid is dilute. Next, precipitate the silver as the chloride, AgCl, by adding an excess of NaCl solution. Wash your precipitate and then reduce it to metallic silver by one of the following methods:
1) Add an excess of NaOH solution and heat to boiling/ nearly boiling. Add either glucose (dextrose) or formaldehyde solution, a little at a time, until all the AgCl is reduced to metallic silver. (The resultant silver powder is grey. not shiny) This method only takes less than an hour.

2)Add some pieces of zinc to your AgCl under water. The zinc will slowly reduce the AgCl to silver metal. The excess zinc can be dissolved in HCl. This method takes a few days.

3) Dry your silver chloride and intimately mix it with an excess of Na2CO3 (soda ash). Heat this mixture with a propane torch (this is normally done on a block of charcoal) until the silver melts to form a button of pure silver.

Dann2

[Edited on 17-11-2008 by dann2]

Fleaker - 17-11-2008 at 16:50

Dann,

Your cementation methods works nicely (#2), but it works much faster in the presence of dilute sulfuric acid, going in a few hours instead of days. You can use iron or zinc.

Not_important,

Charcoal is only used to keep the silver from dissolving excess gas (mainly oxygen); Ag doesn't seem to hold onto CO/CO2. Since you did not mention the use of charcoal I'll edit my post so as to not misquote you. I inadvertently included that because it is so routine.

not_important - 17-11-2008 at 16:59

Fleaker, you'd be correct on using charcoal if the metal was for casting, for the very reason you said. But as this was for metal to be redissolved, and it will be well covered with flux, dissolved gases aren't a large concern.

[Edited on 18-11-2008 by not_important]

Fleaker - 17-11-2008 at 17:16

Oh I know :-), I'm not daring to dispute what you said especially when gave sound advice. It was more ''for the record'' in case anyone here does any casting of ingot or other items.

One thing I am curious about are the conditions you recommend for electroprocessing silver at his level of purity. Does it work well and have you used it; is the aluminum absolutely necessary (Al ion is a nuisance for any refining process)? I've only ever got it to work using a conc. (almost saturated) silver nitrate/nitric acid bath and plating onto silver.

not_important - 17-11-2008 at 20:59

I suggested Al because I doubted they had a handy piece of silver foil to plate onto, and because it would be easy to remove the Al from the deposited silver. I doubt much Al would have a chance to make it into solution.

And yes, it might not work well enough to really clean the silver up as the impurity concentration is fairly high. Certainly some copper would come over, but copper is easy to remove via fusion of the nitrates. I'd expect W, Mo, and Sn to ppt out as oxides; Fe might be a problem. As for Pb, that's tough, I'm guessing it wouldn't plate over. Perhaps it was just an evil plot on my part to try to get someone else to do the actual hard work and confirm or disprove the idea ;-)

But the commercial electrorefining doesn't use that strong of a electrolyte; I'd have to dig around and get actual values. A coherent layer of Ag isn't needed, after all, which makes the process a bit less challenging.

Jor - 26-11-2008 at 13:03

I just dissolved the metal. it was 25g instead of 100g, a mistake by the seller, but I got 75% of my money back.

I put it a RB-flask, and added slowly with a dropping funnel 65% nitric acid. One of the necks was connected to a wash bottle containing 10-15% NaOH solution. There was a safety bottle in between, so suckback couldnt occur.
At first there was no reaction (?), but when I heated very vigorous NO2 evolution started. I kept heating, and adding 65% HNO3 until nomore NO2 was evolved. I was not sure if the HNO3 was gone, so I added 2mL more, and heated. I just got a lot of white fumes, and stopped heating as my fumehood-blower doesn't like nitric acid fumes too much

. The solution is thus very acidic, as I added those 2mL and it didn't react.
What suprises me is the amount of nitric acid needed. I only needed about 20-25mL.

I then diluted about 6-8 times, and I'm letting sit overnight. No basic nitrates will form, as the solution is too acidic, but I diluted to make filtering possible, not having to worry about the HNO3 attackign my filter.
There is quite some black precitipate on the bottom... I wonder what it is. I can only think of that it is metal, but I wonder how that's possible, as it wouldn't dissolve anymore.

I will continue tomorrow.

unionised - 29-11-2008 at 10:07

I wonder if the black powder is tungsten.

Jor - 2-12-2008 at 08:07

Yes I also think it is tungsten. After I decanted the dark blue solution, I saw a piece of metal in the black powder. So that is why I only used such a small amount of acid. I recall there were a lot of crystals in the solution before I diluted it. Those probably covered the surface of the metal, making it impossible for the nitric to react with the metal.

I continued dissolving it, by adding nitric and heating. The solution I already had, wich probably contained most of the silver, was being processed, by first adding sodium chloride solution until precitipte stopped to form. This took about 9g, so there really is not much metal left as I would need about 11g NaCl in theory if all metal would have dissolved.
I heated the solution to make the AgCl coarser, making it easier to decat. Next I decanted most motherliquid and washed 3 times with distilled water, and next, washed the snow-white precitipate with 2x 60-70mL of hot (70-80C) water. To both these washings were added a crystal of KI, and in the first washing there was quite a lot of lead, in the second not much. On cooling the second, there was a beautiful crytal formation, it looked like the classic golden rain.

When they both cooled to 10C (the temperature in my lab), I decated the liquid, and flushed away (contained hardly any lead), and the precitipates in both were collected as heavy metal waste.

This is as far as I am right now. Next I am going to heat the AgCl in NaOH-solution (less than 1.eq right, for every eq. of AgCl?), and add sugar. I have 2 sugars: Normal sugar, the stuff you eat, wich is saccharose, and pure lab-grade glucose. Wich one is best?

I must say, in dissolving 25g of metal, a LOT of NO2 is formed. Without a hood, this is quite hazardous. As I live in a neighbourhood, I scrub the gas through NaOH. The final dissolving part, wich i am doing right now, I am doing that without a srubber, but I do it in steps, so add a ml or 2 of conc. nitric, dissolve and wait for 10 mins, there is no smell of NO2 in the gardens. I walk outside now and then, and no NO2 is obvserved.

[Edited on 2-12-2008 by Jor]

[Edited on 2-12-2008 by Jor]