Phenidone Oxidation Chemistry

I received from Rudeofus (thanks) a Kodak paper "The autoxidation of phenidone", W E Lee & Eva Zahorian, Photo Sci Eng 12 no5 1968 p251-262,
In most developers except those like POTA the phenidone gets regenerated by a secondary agent and the work described does not apply, and in stock solutions of phenidone in (say) 70% isopropanol the pH is about 6 which is below the range of pH 7.8 to 9.2 considered in the paper. So I just summarize a few points from this paper, [with comments by me in brackets].

The authors elucidated the mechanism of oxidation of phenidone by oxygen in the pH range 7.8-9.2.
Phenidone is first ionized and the phenidone anion reacts with O2 to give the semiquinone and the HOO. radical.
The semiquinone then disproportionates giving phenidone and 1-phenyl-3-hydroxy-pyrazole [this latter is the usual oxidation product of phenidone and is the ionized form of pyrazolone mentioned in Mason's book Photographic Processing Chemistry]
[IIRC the oxidation of ascorbate proceeds in a similar way]

The HOO. radical probably forms hydrogen peroxide and O2.
Hydrogen peroxide reacts with phenidone giving colored oxidation products.
If sulfite is present [as in POTA] it probably mops up hydrogen peroxide and slows down this previous reaction.[This is presumably the way in which sulfite acts as preservative for phenidone in POTA].
Copper (II) and Iron (III) catalyse the oxidation, they may form complexes with phenidone which can bind molecular oxygen more easily than phenidone. [May again be like ascorbate].

It seems likely that in phenidone/dimezone-S + ascorbate developers the ascorbate may also mop up the hydrogen peroxide, in addition to the sulfite doing this as mentioned above. So, little color from the phenidone or dimezone-S is formed while there is still ascorbate left and there is little warning the stuff may be oxidized.
If something that oxidizes but does not form hydrogen peroxide, thereby possibly allowing a change of color, could be added to such developers it might give some indication that the stuff should be thrown out.
I don't have any predictions from theory as to what might be added but would just guess hydroquinone might be worth a try as it seems to do this in MQ and PQ developers.

Michael, with a balanced mixture of TEA and Salicylic Acid instead of Carbonate you would have a good buffer in the pH region you want, and you'd take care of all the copper and iron in your test setup.

Also, there could be a substantial difference between Dimezone-S and Phenidone at higher pH, because the latter suffers from hydrolysis, therefore the mix of oxidation products could be quite different. At lower pH this difference should be much reduced.

@Alan: I wonder whether one could test small samples of Xtol with a controlled amount of oxidizer which would first consume all remaining Ascorbate, then turn the Dimezone-S into a range of coloured compounds.

Rudeofus said:

I wonder whether one could test small samples of Xtol with a controlled amount of oxidizer which would first consume all remaining Ascorbate, then turn the Dimezone-S into a range of coloured compounds.

Click to expand...

I used 10g/L potassium permanganate solution as oxidiser.
First, a concentrated solution of Dimezone-S in 70% isopropanol was diluted with an equal volume of water and the permanganate solution added. This produced a red color.
Second, Xtol 1+0 was used and adding 3x its volume of permanganate solution did not produce any significant color.I daresay this is due to the permanganate reacting with sodium sulfite in the Xtol.

Alan Johnson said:

Second, Xtol 1+0 was used and adding 3x its volume of permanganate solution did not produce any significant color.I daresay this is due to the permanganate reacting with sodium sulfite in the Xtol.

Click to expand...

Darn, I forgot about the Sulfite
It may still be possible to test a small sample of Xtol with a Silver Thiosulfate complex (mix Formulary Residual Hypo Tester with Carbonate/Metaborate/Lye and rapid fixer), since this complex is AFAIK not reduced by Sulfite, but definitely by Ascorbate and Dimezone-S, at least in broad daylight.

The oxidation route of phenidones may vary quite a bit depending on pH.

PE

I will leave any further investigation of these colors to someone else as it was not my idea.
Hydrogen Peroxide from a Pharmacy should work as oxidizer maybe.

The articles suggest (anyone else interested: please contact me through PM), that the structure of these coloured compounds was thoroughly investigated, and despite concerted efforts no conclusive answers were found. The research teams working on this had equipment that we amateurs can only dream of, so we might well be looking at a dead end here. Phenidone and its derivatives were of incredible importance to the photographic industry, so they're among the most well investigated compounds.