Black film on gold plated wire?

[devin]

I have come across an unusual problem. I have been using nitric acid to seperate my gold foils from misc. e-waste. I have some gold plated pins from old computers from the 80's. When I process these particular pins there is no change in mass, they just get a black film on them. I have searched and searched and can not find out what is going on. Is there some strange metal in the alloy? I have no idea whats going on. Any ideas? I am using only nitric acid at room temp.. I know this is not a cost-effective or environmentally friendly method, I am only doing this for educational purposes. My preferred gold foil recovery method is AP. If the blackened pins are put in fresh HN03 the black film is eaten off and then returns. Any advice would be greatly appreciated. :roll: :roll:

 

[Oz]

If your pins actually are gold plated and you are using technical grade or better nitric, you are using water that has chlorides in it. The presence of chlorides in your water allows the nitric to digest some of your gold and it is cemented back on your remaining base metals.

Putting them in fresh nitric with the same water source will indeed re-digest your black finely divided gold, but if base metals remain they will do the same thing as they did the first time round.

 

[kurt]

Are you using tap water with your nitric? - If so that would cause the reaction as Oz has explaned it.

Kurt

 

[Lino1406]

In the above description, one part is missing:
The violent dissolution of the base metals
which leaves no pins at all?

 

[Harold_V]

If your pins actually are gold plated and you are using technical grade or better nitric, you are using water that has chlorides in it. The presence of chlorides in your water allows the nitric to digest some of your gold and it is cemented back on your remaining base metals.

Putting them in fresh nitric with the same water source will indeed re-digest your black finely divided gold, but if base metals remain they will do the same thing as they did the first time round.

Fact. I refined for more than twenty years, using tap water when I digested base metals, even in digesting inquarted gold. Yes, my water was chlorinated---but I experienced no losses. The minuscule traces of chlorine were either rapidly expelled, or what little value that may have been dissolved was quickly cemented on the base metals that were present. I was very good about testing my solutions for values, so I am very comfortable with the idea that I lost little, if any. Way too much emphasis is placed on chlorinated water, although, for processing silver, it does create some silver chloride, I admit. That can be annoying, but certainly not fatal.

Harold

 

[devin]

That makes so much sense. I indeed have been using tap water! I experimented last night with distilled H20 and no black film formed. Thank you all so much! Is there such a thing as leaving pins in nitric solution for too long? Am I correct by saying that the nitric will infact slowly disolve the gold foils slowly if left in for over a week? Thanks guys.

 

[Geo]

nitric acid alone will never dissolve gold. it needs some form of chloride to form auric chloride.

 

[Oz]

Glad it helped you devin.

As Geo pointed out having your pins in nitric is safe as long as there are no chlorides introduced, no matter the duration. It is possible with a quality, thick gold plating some pins may not have the base metals digested if the nitric cannot find a weakness in the gold plating in order to penetrate, however it is rare.