Why is my silver not dissolving in the nitric acid?

[Harold_V]

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

 

[goldsilverpro]

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.

 

[goldenchild]

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.

 

[Oz]

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.