[kennymcmack]

I have 500 gallons of 30% nitric acid.  When it was new 10 years ago, it would dissolve a penny in minutes.  Now, it doesn't do anything to a penny, even after a day of sitting in the bath.  The specific gravity and temperature test we do says it's at 27% and the pH paper says it has a pH of 0. 

What could be inhibiting the acid's performance?  Is that even possible?  If so, how is it doing that? 
Is the specific gravity and temperature test the best test for determining concentration?
What is the pH measurement useful for?  It doesn't seem to correlate with performance. 
Can the nitric acid be filtered or treated to make it new again?  I don't want to dispose it if I don't have to. 

Thanks, Kenny.

[Arkcon]

Briefly ... no, its not likely your bath is the same that it was 10 years ago.  If you've been using it to ... bathe ... things then reactions are happening that are changing it into ... something else.  I can't be more specific, because you haven't given us enough information.

Is the specific gravity and temperature test the best test for determining concentration?

No.  Specific gravity is a fair workhorse analysis, so you can monitor it from day to day, but it can be tricked by something other than nitric acid in your bath.  Which is likely the case.  The pH test is likewise, not a really quantitative measure of what you have in your bath, if it started at 30%.

You can probably ask a chemist in-house to titrate a diluted sample against standardized alkali.  That will let you know just how much acid is available.  You might also want to submit a sample for elemental analysis, to see how much metal has become dissolved in your bath.  Although I don't know how much is "too much" for typical applications.  You may have to check your standards for your industry.