# Nitric dissolving Silver problems



## MysticColby (Sep 12, 2011)

Hey, so I've done some homework and come up with a pretty procedure for purifying silver, but I haven't got it to work well yet, and I have a couple questions I haven't been able to find answers for:
what temperature should nitric and silver be heated to?
how long might it take to completely dissolve 204 g sterling silver in 256 ml 65% nitric? (provided decent surface area; about 5mm thick)
I've also heard to add nitric and water step wise (20 ml every hour or so), does this help?

I make the nitric, and I've frequently questioned it's reality. There just usually seems to be less nitric in the end than there should be (usually end up with 150 ml of non-precipitant instead of ~250). On the plus side, I always get really clear nitric, hardly a hint of yellow.
I've tried it a couple times, and the silver is always barely dissolved (end up with about 10 g per batch via cementing on copper)
when filtering out non-dissolved silver pieces, there's this grey powder at the bottom - what is that? I've read that silver nitrate reacts with sulfuric acid to make silver sulfate, which has low solubility...
I'm thinking about increasing the DI water when making nitric to 250 ml so it doesn't need to be as hot when adding sulfuric (less loss due to decomposition/evaporation), thoughts?

My procedure:
256 ml 65% Nitric Acid
200 ml DI (deionized water) in 500 ml beaker
Heat to 100ºC (boil)
Add 348.5 g Sodium Nitrate (4.1 mol)
Stir until dissolved
Cool to 80ºC
While stirring, SLOWLY add 111.1 ml 95% Sulfuric Acid (2 mol)
OUTSIDE, fumes are toxic
NO BOIL (65% Nitric boils at 121ºC)
2NaNO3 + H2SO4 → Na2SO4 + 2 HNO3
4HNO3 + heat → 4NO2 + 2H2O + O2
Cool to RT in water bath, stirring occasionally
Let sit for 1 hour
Decant into 500 ml bottle
Repeat sitting and decanting until no more liquid
Put bottle in freezer overnight
Decant through filter into 500 ml Beaker
About 256 ml of about 65% Nitric Acid
Use immediately or Store at -20ºC

Purify Silver
Add less than 204 g Impure Silver to 1 L Beaker on hotplate (<2 mol)
Add 256 ml of 65% Nitric Acid
Add 20 ml at a time + 20 ml DI
OUTSIDE, fumes are toxic
NO BOIL
Ag + 2HNO3 → AgNO3 + NO2 + H2O
Heat for several hours
Dilute 1:1 with DI
Cool to RT
Decant through filter into 1 L Beaker or 1 L Bottle
Suspend more than 63.5 g Copper in solution (1 mol)
2AgNO3 + Cu → 2Ag + Cu(NO3)2
Let sit for a few days, agitate occasionally
When silver no longer precipitates, decant through filter
Collect powder in crucible
Add some borax
Melt with propane torch
Pour into ingot molds
About 188 g of 99.0+% pure silver

Solubility in Water (g/100 ml)
Compound	MW	Density Color	0ºC	20ºC	100ºC
95% Sulfuric Acid (H2SO4)	98.079	1.84 g/ml 
50% Nitric Acid (HNO3)	63.01	1.513 g/ml 
Sodium Nitrate (NaNO3)	84.9947 White/Clear	73	87.6	180
Sodium Sulfate (Na2SO4)	142.04 White/Clear	4.9 19.5	42.5
Silver Nitrate (AgNO3)	169.87	White/Clear 122	216	733
Copper Nitrate (Cu(NO3)2)	187.56 Blue	137.8	125	247
Silver Sulfate (AgSO4)	311.799	Black solid	4.9 19.5	42.5
Copper Sulfate (CuSO4)	249.7	Blue	23.1	32	114
Silver (Ag)	107.8682 Gray/Brown 
Copper (Cu)	63.546	Copper 
Distilled Water (DI) 1 g/ml 

Safety
Nitrogen Dioxide
Toxic by inhalation
Corrosive to skin, eyes = Wash with water
Sulfuric Acid
Corrosive to skin, eyes, lungs = Wash with water
Carcinogen
Nitric Acid
Corrosive to skin, eyes = Wash with water
Sodium Sulfate = Irritant = Wash with water
Sodium Nitrate = Irritant = Wash with water
Silver Nitrate = Irritant = Wash with water
Copper Nitrate = Irritant = Wash with water

Thanks for the help
-Colby


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## samuel-a (Sep 12, 2011)

MysticColby, you have done your homework indeed.
It's nice to know a person really puts his 'back in to it' before asking the forum.

To your problem, i could only suggest you to purchase a quart of thechnical grade nitric acid and see how it behaves with s. silver.
Then maybe refine your Nitric making process... Unless you have a very large Tin contamination in your sterling (which is unlikely), i would question the quality of your home made nitric acid.


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## MysticColby (Sep 12, 2011)

I'm glad I impressed you  was worried I might get the 'there is a search button' response.
It is possible there is tin in the silver (it's scrap silver purchased from eBay). Actually, I started doing this because the castings I made from the silver I purchased didn't act like sterling silver should (too brittle), so I wanted to turn it into pure silver and formulate my own sterling so I know it's good.
Last night, I made a batch of nitric and when I took it out of the freezer today, it looks better than previous batches. I tried starting with 250 ml of DI, and there is about 350 ml after removing precipitates, which is the expected amount (though it's expected to be 54%). I'm trying it on some silver and I"ll let you know what happens (so far it's bubbling around the silver with redish gas).
I don't know of any suppliers of nitric in my area, but I'll look around and give that a try.
So, I work in a biology lab, and I had a thought today that I could bring a bit of this nitric to work and titrate it with 5M NaOH to determine it's concentration. I think it should work - just measure how much NaOH it takes to reach pH 7, then calculate how much HNO3 there was and divide by the 1 ml volume, convert to percentage. I think this would be the equation: HNO3 + NaOH -> NaNO3 + H2O. which would make it a 1:1 relationship. hehe, this would be a very exothermic reaction which I'd do very slowly in fume hood.


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## MysticColby (Sep 13, 2011)

alright, I did the titration. It worked out very well. no energetic reaction, yet still fun because I knew what it meant. The results (batch where I started with 250 ml DI) = 46.84%
Expected: 54% Nitric Acid
So, it's close! discrepancy could be due to inexact measurement of total nitric volume (graduations on side of beaker).

Hmmm, what if it's just dilute sulfuric acid instead?
I have no way of knowing the difference...gerrrr. there's exactly as much reactivity in the sulfuric acid as how much nitric that sulfuric makes. quick google search says that you can do what I did, then heat a different small sample for 15 minutes, then repeat the test (nitric evaporates whereas sulfuric doesn't?), difference in NaOH required is how much nitric there is... does this sound reliable?

So I've had this nitric working away on the silver for a few days now... I've kept it at around 90ºC with a funnel to catch evaporated stuff. Seems to still be doing stuff: still has strong acidic odor, has a deep blue color, no longer making any red gas, I can hear bubbles. I pushed some of the silver around and it looks black (silver oxide?). I've read that this happens, but adding water relieves it, and it's already about half water that I added.
I really want to find a place that sells nitric. Is it legal to buy it and have it delivered?
So I just found some places to buy it online, not sure if you need a permit to buy it or not, I'll find out in a few hours. But I'm most surprised with how cheap it is- it only costs like 50% more than the ingredients to make it yourself. That savings is just not worth that headache and uncertainty.


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## butcher (Sep 15, 2011)

make sure you calculate shipping costs, it can be four times as much to shjp as the bottle of acid costs.


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## MysticColby (Sep 19, 2011)

hehe, yeah. Shipping was expensive, but not unreasonably so. 2.5 L of 70% for $35 + $25 S&H from www.chemical-supermarket.com. I don't use very much, so this should last me a couple years (should dissolve like $80,000 worth of silver...), at which point it's shelf life might be up.

Anyways, regarding that grey powder. At this point I'm fairly sure it's silver sulfate (which is supposed to be black, but w/e). Reason being that I added water, some of it got dissolved, I filtered the solution into another bottle, added copper, and silver cemented out. I believe that copper + silver sulfate = silver + copper sulfate, same as silver nitrate cementing, but silver sulfate has low solubility. This leads me to believe that my 'nitric acid' had some nitric in it, but was mostly sulfuric acid. I'm not sure where I went wrong when making it (first guess would be to have it be hotter when adding sulfuric acid), but I'm just glad I won't have to deal with it anymore.


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## Harold_V (Sep 20, 2011)

MysticColby said:


> (should dissolve like $80,000 worth of silver...),


Yes, it should, but it won't. With a little luck, 2.5 liters will dissolve about 75 troy ounces of silver. 

I think you'll find that small amount of nitric won't last long unless you aren't doing anything. 

Harold


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## MysticColby (Sep 20, 2011)

oops. so close yet so far.
I figured that stoichiometrically, 2.5 L of 70% nitric will dissolve 2007 g sterling (=64.6 troy ounces)
my mistake was multiplying by $40/troy ounce = $80,280 rather than by $1.2/g = $2,408.
That sounds more like the real world.
It's good to hear it'll actually dissolve a bit more (probably from capturing and reintroducing the NO2?)
I don't plan on working with much more than ~500 g of silver for the short future (I do small castings - rings and such).


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## Geo (Sep 20, 2011)

if you do your process and reflux your acid back to your solution it will go much farther than just letting it off gas into the air.


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## Harold_V (Sep 21, 2011)

MysticColby said:


> oops. so close yet so far.


For the record, I'm basing my comments on the fact that I used to use 30 troy ounces of silver crystals to make the electrolyte for my silver cell. I'd use 1 liter of nitric, and dissolve everything that would dissolve--which was nearly all of it. It was done hot, and covered, so little was lost to the atmosphere. In almost all cases, traces of silver remained un-dissolved, so my suggestion that you could dissolve 75 ounces was likely high---but my point was that you had false expectations in thinking you could dissolve so much. I see you addressed that issue, which is a mistake that's easy to make!

Harold


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## MysticColby (Sep 21, 2011)

yeah, I got the hard part of the math right but botched the easy conversion to money.
oh, as an update, I received the nitric yesterday and started using it. it's much more reactive than those homemade batches I did. Very exothermic, too. The solution is still turning deep deep blue. I guess my 'sterling' probably has a lot of copper in it. a few of the pieces I initially melted were probably silver plated copper. I need to be more careful of that when buying scrap off eBay in the future.

I'm not sure I approve of their packaging. plastic bottle inside zip-tied thick plastic bag inside zip-tide thick plastic bag inside bubble wrap inside cardboard box with no hazard labels on outside of box. Probably didn't do hazardous shipping, so FedEx didn't know what was in it, which isn't a good situation. I'm torn between liking them for low cost, but I think I'd prefer paying more for the peace of mind that things are done right (if they skimp on shipping costs, where else do they cut corners?)
www.chemical-supermarket.com = 2.5 L of 69.5% for $35 + $25 S&H = $60
The other place I considered was www.dudadiesel.com = 2.5 L of 61.4% for $75 + $40 S&H = $115
It looks like they are more responsible. are there any other favorites sources for nitric acid?


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## goldsilverpro (Sep 21, 2011)

By my calculations, in practice, 2.5 liters of 69.5% nitric will dissolve about 1700 grams of sterling, assuming it averages 91.5% silver, which may be stretching it..


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## MysticColby (Sep 22, 2011)

One thing I've been wondering about:
when cementing silver with copper, should the copper be free of copper oxide?
I've been doing a quick soak in lemon juice to remove the oxide coating (then rinsing it) before adding to silver nitrate, but is that necessary?
I can't seem to find what is created in this reaction:
Cu2O + 2AgNO3 --> ?
is it Ag2O + 2CuNO3?


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## Geo (Sep 22, 2011)

a dip in hcl will remove copper oxide quickly. ahh, didnt see SILVER nitrate. sorry. i think ive been drinking too much coffee. :lol:


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