Nitric acid is a long term stable solution in storage. That being said, I think I remember you mentioning plastic?!? Nitric will oxidize almost all plastic in time. PTFE is about the only one long term stable. Pp and hdpe are only considered as resistant and not impervious. Glass or stainless steel is a better choice for concentrated nitric. Most industrial storage containers are made of stainless. Just posted for information.
Also if your gold turned “ black “ you might not have had enough nitric to put your gold in solution. The black color is sponge gold cemented out by the base metals in your original sample. If you have a positive stanis test and no drop cementing it out and starting over is usually necessary to move along. If you have iron fooling you then you should be properly confused and you need to pay more attention to your starting material. Many people starting out find iron to be a major disappointment on their road to spending much on a little gold.