# Why is my silver not dissolving in the nitric acid?



## pljohnson (Oct 26, 2010)

Ive got 10 ounces of sterling I am trying to dissolve. So far ive added almost 2 liters of 70% nitric acid solution and its been about 2 hours. It starts to fume a small amouont each time I add it, but quickly stops and the solution is green(which from reading another post, may mean theres no more free nitric). Did I get a bad batch of nitric?


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## martyn111 (Oct 26, 2010)

Try diluting your nitric 50/50 with distilled water.


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## rusty (Oct 26, 2010)

martyn111 said:


> Try diluting your nitric 50/50 with distilled water.



The nitric I use is only 39 baum and still works diluted 50/50 with water, I heat my beaker.


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## qst42know (Oct 26, 2010)

Green says nickel to me are you certain this is sterling.


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## pljohnson (Oct 26, 2010)

Yes i tested all the pieces that were not marked sterling and put them in.


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## qst42know (Oct 26, 2010)

pljohnson said:


> Yes i tested all the pieces that were not marked sterling and put them in.



Tested, how?


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## pljohnson (Oct 26, 2010)

Just using a silver acid testing solution. Some of the items that were even marked sterling are not dissolving. Ive put way over the 150ml acid per ounce of silver in.


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## qst42know (Oct 26, 2010)

It's very common to find silver plated nickel silver. It would be uncommon to find unmarked sterling silver items.


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## pljohnson (Oct 26, 2010)

When I say unmarked, i meant they came from a scrap silver lot with weights and such removed.


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## qst42know (Oct 26, 2010)

Can you be more specific as to what this scrap is?

As a test take a small piece of this material and put it in AR. A high silver alloy will be immune and a high nickel alloy will be dissolved.


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## goldsilverpro (Oct 26, 2010)

If you start with 70% nitric, dilute it 50/50 with distilled water, and heat it, it will dissolve sterling silver 100% of the time, without fail, without exception. Actually, it's not even necessary to heat it, although it does speed things up. If the metal does not dissolve, it is not sterling silver. And, like qst intimated, if it isn't marked sterling, it most likely isn't sterling.

The answer to your original question, "Why is my silver not dissolving in the nitric", is either because your sterling silver isn't really sterling silver or your nitric isn't really nitric.


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## pljohnson (Oct 27, 2010)

Its definately sterling, the light pieces of sterling partially dissolved while the heavier pieces were barely even touched by the nitric. Im going to call the company I got the nitric from...


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## philddreamer (Oct 27, 2010)

Did you incinerate your material before running thru acid? It makes a difference.


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## Juan Manuel Arcos Frank (Oct 27, 2010)

I agree with GSP...you have to heat the acid solution and silver must be in shot form or laminated because the process needs a hig superficial area to dissolve silver,I mean,if you dip in nitric acid a bar of sterling silver it will not dissolve fast.
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Regards.

Manuel


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## rusty (Oct 27, 2010)

pljohnson said:


> Its definately sterling, the light pieces of sterling partially dissolved while the heavier pieces were barely even touched by the nitric. Im going to call the company I got the nitric from...



What color is your nitric?


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## pljohnson (Oct 27, 2010)

The nitric is a clear light yellow. Its def 70% nitric. Forgive me for my lack of knowledge but what do you mean by shot and laminated. I thought from the reading ive done nitric would dissolve any sterling silver.


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## 4metals (Oct 27, 2010)

Shot is small little BB's of metal made by melting the sterling and pouring it into cold water, pouring straight into water from a crucible makes cornflake shot, which is big and ugly with a lot of surface area, (a good thing) pouring it through a crucible with small holes drilled in it to assure a fine stream makes BB's. Both are good, make whatever is easier. 

I have digested 500 ounce bars of sterling in 50% nitric and they digested completely, just slow because the surface area wasn't there, but I did it so as not to overload the scrubber with the red cloud. 

Make sure you are using chloride free water! Take a few drops of silver nitrate and add it to the water you are using, if it forms any white cloud the water has chloride in it and it will stop the reaction, done finito, nada! If it is sterling, and it is nitric, and the water has no chlorides, the sterling will dissolve, and the acid will be a beautiful blue (from the copper).


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## Oz (Oct 27, 2010)

If I am reading this thread correctly pljohnson has been using 70% nitric and not diluting it. If he does have sterling under those conditions it will be slow to react. If he starts to add heat it will start to react and as the nitric concentration comes down it will become far more efficient. So efficient as to possibly become a runaway reaction (depending on the available surface area of his silver) since it is an exothermic process. It is a bad day having silver nitrate foaming out of your beaker onto who knows what.

With that caution out of the way, I have never needed a stronger nitric for silver than 70% doubled in volume with distilled water (what most here call 50/50 nitric). 4metals makes a good point on chlorides, some city waters may actually be high enough in it to cause a pacification layer of silver chloride that would bring the reaction to a near stop.


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## pljohnson (Oct 27, 2010)

Thanks for your response Oz. I was not using enough water, my silver is dissolving rapidly now without heat.


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## Oz (Oct 28, 2010)

pljohnson said:


> Thanks for your response Oz. I was not using enough water, my silver is dissolving rapidly now without heat.



No problem, I am glad to hear I could help. My greater concern was that if I was right about what you had and you were preparing to add heat to drive the reaction based on advice you had been given, that it would result in a runaway reaction spilling your acid and silver all over the place if you had not diluted your 70% nitric. In my haste to reply I actually did a rather poor job of explaining it in terms that could be beneficial to others with similar circumstances. 

Many people assume that just because something is stronger or more concentrated it will do the job better or faster. That often does not hold true with chemistry. The simple explanation in this case is that plain old water “allows the room” in a nitric solution for a reaction to occur. Concentrated nitric is a far slower reaction than 50/50 nitric and it can surprise you unexpectedly if it becomes diluted at a given temperature as it consumes your nitric, then starts to heat up due to a more efficient reaction that greatly speeds things along even faster.

Frankly 50/50 nitric is not the most efficient way to digest sterling silver, it is however a good balance between expediency of time vs. cost of nitric. Typically if I have sterling that I wish to digest in nitric I do the math as to how much 50/50 nitric it will require before anything else. I then place the silver in an appropriately sized beaker and add the distilled water and one third of the nitric needed. Keep adding nitric when the reaction slows, then finish off with heat. You will find that added heat is not required in the beginning as the chemical reaction generates plenty on its own (exothermic). 

There are a few good reasons to follow the above. It minimizes the risk of runaway reactions even if digesting shot or cemented silver that you need to remove platinum group metals from, and it economizes on nitric acid consumption. With the greater portion of water compared to nitric in your beginning solution it allows some of your red NO2 gas to be reabsorbed into solution (that extra water) to do additional work instead of being wasted as a gas and being a toxic byproduct that should be scrubbed. To be fair, there are many tradeoffs between time and cost in refining but taking the slow path at first is always the best route until you have gained hands on experience. 

Just to be clear in case you or another think my advice is condescending in nature. I myself practice the multiple additions of nitric because I have more time than money for reagents at this moment. If the situation came up that time became of the essence, I would drive reactions hard and fast the same as you will if the need arises once you gain experience.


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## Harold_V (Oct 28, 2010)

For starters, the very idea that concentrated nitric is more effective simply isn't the case, at least when digesting silver. Oz said it well. However, my experience in making electrolyte for my silver cell, which was done in one fell swoop, yielded results that are very different from that which may be expected. Note that my comments apply to silver alone---I had more than my share of run-away experiences when processing eye glass frames for gold. 

When I made my electrolyte, I used 30 troy ounces of silver crystals from my cell, generally those that were harvested towards the very end of an anode's life, where copper concentration was high. It was my way of proving that my silver was, or was not, pure. At any rate, I'd place the 30 ounces of silver in a 4,000 ml beaker, to which I'd add a liter of concentrated nitric, then a liter of distilled water. It was then placed on heat. I wanted the fastest reaction I could get, for to me, time was money. 

Even with applied heat, I never had a problem with a runaway solution. It would foam up to roughly 3 liters in the beaker, then stay steady. I heated the solution to a boil, to insure that I had consumed the free nitric. I always used a trace more silver than was required, which was simply dumped to the silver cell, where it was combined with the next harvest. 

Bottom line---silver may be one of the metals that will dissolve with few issues. As I said, I do not recall ever having any problems with silver. 

As I've made it known on a regular basis, I am not a chemist. However, consideration should be given to the hydronium ion, which may explain why silver dissolves readily in the presence of water and nitric. There's more going on than creating "space" in the solution. 

Harold


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## goldsilverpro (Oct 28, 2010)

For what it's worth.

I basically dissolved silver the same way Oz outlined in his last post, with no added heat. I ran many lots of large contact points which averaged about 81% Ag, with most of the remainder Cu and a lesser amount of Cd. I put the weighed points, usually in 5# lots, more or less, in a 5 gal plastic bucket. I then calculated the approx. amount of 70% nitric it would take and measured that out (or, at least, kept track of it). One gallon of nitric will dissolve about 100 oz of Ag or 30 oz of the base metals (Cu, Cd, etc.). For exactly 5# of 81% Ag points, this figures out to be about 1.05 gallons or 4 liters of nitric. Also, it will take an equal amount of distilled water. If the material I was dissolving was 5# of sterling silver, it would take about .85 gallons of nitric (3.2 liters), plus an equal amount of water.

Under the hood, I added enough of the water so the water level was about 1", or a little less, above the points. I then added about 750 ml of nitric. This immediately started reacting and, in short order, the solution got quite hot. I allowed the reaction to die down somewhat and added more nitric. This was repeated until I had added all of the nitric except for about 200 -300 ml. All together, it took maybe an hour to add the nitric. It is best to add the nitric at such a rate so that the solution doesn't cool off - don't add some and then go to lunch - stay with it until it's in. I then added the rest of the water, covered the bucket, and allowed the reaction to continue overnight. 

The next morning, except in very cold weather, all of the nitric was reacted and spent. I then poured off all of the solution into another bucket. Since I had held back some of the nitric, there was always a few oz of undissolved silver left in the bucket. I transferred this to a beaker and dissolved it with the remaining nitric and an equal amount of distilled water. I usually heated this a little to make it go faster. Sometimes, I didn't need all of the nitric and sometimes I needed a little more. The reason for doing it this way (saving back a little nitric) is so that no excess nitric is used. Since the next step is to cement out the Ag with Cu, no Cu (or nitric) will be wasted. When all of this Ag is dissolved, combine this solution with the rest of the solution.


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## goldenchild (Oct 28, 2010)

Oz said:


> Frankly 50/50 nitric is not the most efficient way to digest sterling silver, it is however a good balance between expediency of time vs. cost of nitric.



Oz,

Are you saying this concentration of nitric is not the most efficient or that there is a totally different technique for digesting silver? The first thing that came to mind was sulphuric acid but if there is something else I would like to know. I'm sure everyone would like to save on nitric.

Edit

I didnt read the posts after yours. I guess some would favor an eletrolytic cell over nitric digestion. So like you said there are pros and cons to both techniques.


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## Oz (Oct 28, 2010)

I was not referring to silver cells, sulfuric, or any process other than digestion of silver or sterling with nitric. Simply put if you are willing to have a slower reaction time you will need slightly less nitric to digest your metal. If you use 50/50 nitric with heat you will generate a rather good red cloud. If you use a more dilute nitric or add it in steps you get very little red gas (NOX) coming off of your solution. This is because with the greater quantity of water the reaction is slowed down enough that some of this gas is reabsorbed into solution to do work again instead of going into the scrubber. 

Try it yourself in a controlled side by side reaction in 2 beakers. To be sure you have consumed all of your nitric you will need to have excess silver remaining in the bottom of your beakers and finish both with heat close to a boil. As long as you have used equal amounts silver, water, and nitric in both beakers you will find that the slower reaction will be more efficient at digesting the silver for a given amount of nitric, and of course the faster reaction starting with 50/50 nitric and heat will be more efficient as to time. 

Typically time is more important if you are doing this for a living, but I mention this because many here are hobbyists that pay as much as $75 for a liter of 70% nitric online. For them it may be worth considering. If you want the best of both worlds and do not mind the equipment cost you could just use 50/50 nitric with heat but run your fumes through a condenser, reflux them back into solution, or bubble them into H2O2 to reclaim wasted nitric due to a hard fast reaction.


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