- EDTA slows down the Fenton reaction, but less so than other chelating agents. Note that in the experiments described in this paper, an Ascorbate solution lost 3% of its Ascorbate after 15 minutes in a phosphate buffer (pH = 7.0) "protected" by EDTA. Other chelating agents appeared to protect the Ascorbate much better, DETAPAC (which we commonly refer to as DTPA) is 10 times as effective at pH 7. Without chelating agent 30% of the Ascorbate was gone after 15 minutes!
- The same paper also claims that the effectiveness of a chelating agent is strongly dependent on pH. What this paper postulates for pH 7 is completely different in acidic environment, and may again be different at the pH of your dev. Note that DTPA gets better and better compared to EDTA at higher pH and that Xtol is known to be stable for about half a year.
- Another experiment discovered that EDTA becomes less effective after 24 hours in solution, strongly dependent on which other ions are in solution.
If you ask me, given these numbers, I would put a good iron chelating agent into your soup, at least during testing. Note that you pondered about much smaller changes in AA amount than 30% when you formulated and tested your dev, yet I do not think that you always tested your dev within 1 minute of mixing it.
PS: Please note that what I wrote above is no peer reviewed literature survey from an expert in the field, it's just an internet forum posting after a brief online search. Other papers I am unaware of may well state very different results and with my limited level of knowledge in the field I wouldn't be able to judge which one was right. So take these with a grain of salt, maybe more knowledgeable people in the forum could share their opinion on this.