I'm with the previous two posters - easier to make up a new batch. If it's yellow, time to dump it.
But if you really had to, UV Spectro is a pretty standard method for determination of nitrate in water (probably can be extended to ethanol as the medium instead).
Acid/base titration would be appropriate - nitrate concentration would be save as H+ conc.
The concentration of the nital etch is related to the time of etching, depending on the material. For instance, if you have a 2% nital concentration, your etch time might be x minutes (or seconds in some cases), but at 2.5% concentration, it might be x-y. In short, the etchant concentration and timing are somewhat experience driven and the concentration of the etchant is less important than the necessity to reveal the particular feature of the microstructure that you want...so experiment a little and be less concerned with the mathematical/chemical absolutes than what you observe.
I concur with the above posts that there really isn't a problem with Nital concentration as most etching of samples requires a visual interpretation. It doesn't matter whether a Nital solution is 1.9% or 2.1%. Generally technique comes with the above mentioned experience.
Use reagent grade materials and store in a dark bottle and allocating a small ammount for each job and dump the unused portion.
I rinse with demineralized water, dry with reagent grade acetone and completely dry the sample with a heat gun, which in our case was doggie hair dryer over 50 yrs old.
