Black color solution

[sayf]

Hello every one i hope you are all doing well ,
I got some military and medical gold plated pins about 500 grams
When i added diluted 25% nitric acid to them, the reaction started but the color of the solution is not its usual green-blue color, its deep black this time, can any one explain to me why?

 

[Lino1406]

No black nitrate is known, something other than nitrate...

 

[sayf]

No black nitrate is known, something other than nitrate...

You are right, so that i was wondering lino, any way it turned blue after 5 minutes, i read some where about a compound called nitroso ferrous sulphate and its black solution as i recall

 

[orvi]

Sometimes when alloy contain the element, which is not soluble in the acid used, very small particles of that metal could be produced as the acid eat it´s way through the metal alloy. Very common with gold or PGM alloys with lower content of values. However, in this case, it is questionable.
Nitroso ferrous sulfate. Very interesting compound, never heard about that kind of stuff but from where the sulfate anion comes ?