I have very little knowledge of chemistry, I'm not even sure I'm posting in the right subforum, or if I use terminology correctly, so my apologies and please bear with me.
I am a jeweler who uses nitric acid to test the authenticity of sterling silver. I have been buying the stuff on eBay and it does not come with a "born on", "best used by" or expiration date, but I have read that nitric acid does have a shelf life. The acid I have seems to work as expected now, although I don't know if I should be getting deeper or more saturated dark red than I currently do for a positive result, or if it is working perfectly fine. My questions are:
1) What is the shelf life or nitric acid (I've read 4 years?) , and what does that mean? i.e. when it "goes bad" is it just less potent and less likely to give clear positive results for sterling silver?
2) Are there inexpensive sources I can purchase from that provide those dates on their products when purchased?
3) How can I test if nitric acid is still good? (or, how do I know when it's gone bad?) Is it a simple pH test? If so, I've read that pH is not simple and depends on the concentration, but I don't understand what my concentration is, or what it should be, or the math required to make sense of it.
If anyone can help me to determine whether my nitric acid is still good or not for its purpose, I would greatly appreciate it. Thanks.