Interesting test, Alan.
A naive question if I may (I know some stuff but I'm not a chemist so bear with me), from an oxidation/stability perspective, is there any difference between Sodium Ascorbate and Sodium Isoascorbate (which is the ingredient in XTOL)?
Also, while slightly off topic, I wanted to add something related to XTOL oxidation. One of the things people complain about is that they can't tell if XTOL is failing by observing a colour change. However has anyone noticed an odour when it goes bad? I'm asking because I think I have. This was not XTOL which I had mixed and stored, but rather a few batches that had an odour as soon as I mixed them. Normally I don't notice XTOL having an odour of any significance. But around two years ago I received a mail order of 5 or 6 packets (the large ones). Based on the stamped expiry dates they should have been fine, but each time I mixed one of those packages there was an odour. Unfortunately I can't remember with 100% certainty whether the smell was present after dissolving part A or if it came about after adding part B, but I'm reasonably sure the odour was there when part A went into solution, and didn't go away after adding part B. The colours were normal (slight "tawny" colour after mixing part A, which went away after adding part B). And sure enough, while it wasn't "dead", these batches were consistently weaker than normal, requiring longer developing times. I only had that problem with those particular batches. Never had it happen before or since. But I still remember that strange odour. Is an odour perhaps something people could check for before potentially using oxidized XTOL, rather than looking for a colour change?