[Klamp]

Hello everyone  . 

I have an issue at work that is quite puzzling me, mainly because my chemistry knowledge is limited.  I have a hard time convincing people with my theory because I can't explain a few "holes" in it.

Problem

-We make 30 liters batches in stainless steel tanks of an aqueous solution of EDTA and sodium Metabisulfite (its a mixture of bisulfite and metabisulfite according to the supplier).
-Once everything is dissolved the pH is around 4.5 and we need to adjust it to 5.4 using NaOH 0.2 N.
-The is quite basic stuff really and they've been doing it for a few years now and everything worked fine.

However, we've been running in some issues lately.  Seems like sometimes its impossible to adjust the pH to 5.4.  We add some NaOH to 5.4 but it goes right back to the original 4.5 (takes maybe 10-30 mins).  No matter how much NaOH we add it will not stabilize (worst I've heard was around 2.5 L NaOH total). 

Facts


- We are bubbling nitrogen in the tank to avoid oxidation but it might not be optimal and the headspace of the tank is wide open, so the solution is still in contact with air.
- I'm assuming its oxidation of the bisulfite by dissolved oxygen.
- I tested it in the lab and the only way I could stabilize the pH to 5.4 was to bubble nitrogen, cover the flask to prevent air from entering in contact with my solution.  When I uncovered the flask, the pH started to drop again almost immediately (maybe 30 seconds after).  This is the test that convinced me its oxidation.
- Batches that are problematic have a lower concentration of SO4 (or maybe SO2, cant remember) when compared to batches that "worked".

Unanswered questions I'm being challenged on

- I cant explain why we use to be able to adjust the pH and now we can't.  Tanks are the same, procedure is the same etc.  I also looked into the raw materials supplier's documentation but didn't find any noticeable changes (same manufacturer, same quality)   
- They tested the efficiency of the nitrogen bubbling by measuring dissolved oxygen in the tank using only water and it was efficient (don't have any concentrations to report but I've been told it was next to nothing).

Questions (finally!!)

-Can the oxidation reaction be that violent and noticeable in a matter of seconds?  Would it even be possible, considering the nitrogen bubbling?
-Do you have any possible explanations besides oxidation that would explain the observed pH drop?
-I'd appreciate any idea explaining why the problem only appeared recently or any other input you think could help me solve this problem.

Cheers!

Klamp.