As a retired chemist I have some advice.
In this case and with even dilute HNO3 pH paper is useless. PH is only useful for weak acids. If your solution is greater than pH 0.5 then your acid is depleted.
While you may still have a lot of protons from the acid available to react with your brass two things may be effecting your efforts.
- obvious reduction of acid concentration (but not even close to zero) recognizing that concentration and reaction rate is not a linear relationship.
- Presence of other ions (all the metal dissolved by your processes that is now metal nitrate salts) that will physically and significantly interfere with the acid getting next to the metal you want to ‘clean’.
Three things you might do to extend your pot life.
- Easiest, heat the solution up (rule of thumb 10 degrees doubles the reaction rate)
- Mix the solution with a magnetic mixer, don’t use air as the bubbling will release noxious gases into your studio.
- You might try freezing the solution to remove reaction salts…HNO3 should not be affected and the nitrate crystals might pop out (unfortunately Nitrate salts are usually the most soluble) haven’t done this myself??