[zwik]

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.

 

[fledarmus]

What sort of nitric acid are you using, and how precise do your results need to be? Could you describe the test you use? Is this a threshold test (if x happens, then the metal is sterling, if not it isn't) or is it a quantitative test (the material tested is 98.64% silver)?

The shelf life of anything is highly variable on storage and use conditions. You might be interested in this page http://en.wikipedia.org/wiki/Fuming_nitric_acid for a very basic introduction to the different types of nitric acid available. For some solutions of nitric acid, especially the fuming ones, you may be changing the concentration of the acid every time you open the bottle. For the azeotropic nitric acids, they may be stable as long as the temperature is relatively well controlled.

If you are looking for precise results and need an accurate determination of the acid content, you can titrate your acid with a standard base solution.

[zwik]

I'm going to have to contact the manufacturer, I don't know what kind it is. I thought it was "just" nitric, now I'm not sure what it is.... didn't realize it was not so simple.
Typically I test using a stone (although I have applied directly onto the piece), I rub streaks onto stone or file into the piece to get below any plating, then apply a drop to each streak... .925 dark red, .800 is brown, .500 green.

Thanks, if I have any questions after contacting mfgr with q's, I'll come back here.

[zwik]

Turns out it is a mix of nitric and muriatic acid.   

[Stepan]

Mixture of Nitric and Hydrochloric acids is unstable and quickly looses it property (several weeks at the best). In lab environment, people mix it every time before use. For your purposes, by Nitric acid (usually it comes as 50-60% solution) and Hydrochloric acids separately. When apart they have almost unlimited shelf life. Keep Nitric acid at room temperature and away from sun light (in old time in my lab we wrapped bottles with black paper). Do not buy concentrated Nitric acid - it is too dangerous to ship and work with. 

[vmelkon]

Mixture of Nitric and Hydrochloric acids is unstable and quickly looses it property (several weeks at the best).

What happens to it?

[Arkcon]

When you mix hydrochloric and nitric acids, noting happens at first.  Then the solution turns yellow, then brick-red, and the nitrogen oxide fumes out.  This is active aqua regia, with all of its properties.  As the nitrogen oxides fume off over time, the solution turns clear again.  At which point, it no longer has the properties of aqua regia.  Usually, I would notice this state of affairs the next day (not a term of weeks,) and dump it into the acid waste container.