While Xtol keeps for a long time in full bottles, it oxidizes in part full bottles. When a bottle is only 10% full the ratio of the volume of air to the volume of developer is over 80 times that when the bottle is 90% full, so the oxidation can go ever faster as the level in the bottle falls.
I ran some tests on ingredients of Xtol. 2 lots of 12g/L sodium ascorbate without and with addition of 20 g/L sodium sulfite were exposed to the air and the color change noted (see attachments).
After 37 days the ascorbate alone had oxidized to an orange color, possibly dehydroascorbate as dehydroascorbic acid is reported to be orange. The ascorbate plus sulfite solution showed much less color change.At 130 days the colors had darkened.I cold not get rid of the orange color by adding sodium sulphite.
I am inclined to favour this explanation for these color changes:
In presence of sodium sulfite the ascorbate does not oxidize to the orange compound, probably dehydroascorbate, but to a relatively colorless substance which may be an ascorbate-sulfite compound.
So it is the sodium sulfite which in some cases leads to the oxidative failure of Xtol with minimal color change.
Thanks for comment.
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?
Ascorbate=L-ascorbate is a mirror image of isoascorbate=D-araboascorbate=erythorbate.
AFAIK the photographic properties of the two are the same as they do not have any reactions in this case where the mirror image type has any effect.