cemented silver differnet color

[flyfish]

Hello All,
My first post...be gentle. After extensive reading of this forum and internet I started dissolving coin and sterling silver in nitric acid about 3 weeks ago. Have been doing good. I've been using about 180 ml of nitric and 180 ml of dist water for every 5 ozt of silver. I was drying the last batch and it changed to a dark grey color. The color of the solution when I was cementing it with copper wire was a lot more green than the other blue-ish color I had been seeing. What did I mess up? The batch solution that I'm cementing as I type is doing the same thing (turning more green). This solution is also "active" (don't know what to call it). It's not bubbling but there's definitely motion. My inexperienced gut says there was too much nitric (measured incorrectly), and that the extra copper is making it green. what do I do with the dark silver powder that I have, and this batch that seems to be heading my way?

Thanks for your time.

 

[butcher]

Excess nitric acid can make the solution green.
When dissolving a metal in nitric acid, there can be a delayed reaction when metal is first added, as gases in solution form and the heat of the reaction builds this reaction can get vigorous to violent, (after this initial reaction I will normally add heat), as the reaction progresses and before all of the nitric acid is consumed by dissolving metals into solution making dissolved metals salts, the reaction will slow to a crawl, with heat and stirring you can help the acid complete its reaction with metal in a much shorter time.

Palladium can also color nitric green to brown, but with coins or sterling that is not likely the problem here (testing your solutions could prove this).


Normally with heat (after initial reaction), it takes around 1.22 ml of 70% HNO3 diluted with equal volumes of water to dissolve a gram of silver.

And about 3.6 ml of 70% HNO3 diluted with equal volume of water to dissolve approximately a gram of copper or base metal.

So if my math is right, I cannot see where you would have a very large volume of excess nitric acid, did you use heat, did you have any metal left undissolved?

 

[Harold_V]

Follow butcher's lead, and test the solution with stannous chloride.

If you no longer have any of the solution (it must be the green solution, not the solution from which all the values have been extracted), dissolve a small sample of the cemented silver, then test. If you have palladium present, it will report in the solution (tested with stannous chloride or DMG). If the unknown substance happens to be platinum, it most likely will not be dissolved, but you will find it is present in the way of very finely divided black material.

The discoloration you speak of tends to be from the platinum group, most likely palladium, but it's possible it's platinum, too. Both of them will yield a greenish colored solution (when mixed with copper and silver), which will remain until all of the silver is down, then it slowly cements as well, with a black deposit. When you have achieved a full recovery, the solution should then be blue.

The cementation of the platinum metals discolors the cement silver, shifting its color towards dark gray, getting darker and darker with the amount of platinum group present. I suggest you do not guess, as the value of the platinum metals exceeds the value of silver by a generous margin, making it worth recovering (in the silver cell).

It might help for you to understand that silver melted with platinum present will dissolve some of the platinum, so even though you have been working with just nitric (not aqua regia, which is normally required to dissolve platinum), you may have some platinum present from an unknown source. Palladium, of course, readily dissolves in nitric.

Harold