Attached is a summary of some experimental results which puzzle me regarding Phenidone (in the case the Dimezone-S derivative) oxidation products, pH and sulfite.
I don't know how many Dimezone-S oxidation products there are, but we know at least one of them is coloured. If we assume the concentration of this oxidation product is directly related to the depth/chroma of the solution colour, I'm surprised by some of my observations:
1. Sulfite appears to partially decolourize the solution. According to Haist, with benzene-derived developing agents, sulfonation of oxidation products by sulfite apparently explains the decolourizing action. But sulfite is not supposed to for sulfonates with Phenidone oxidation products.
2. Bisulfite appears to have the same effect on colour as sulfite. I tested this by first lowering the pH of the sulfite-free solution to ~4 using acetic acid, which lightened the solution colour significantly, and then adding sodium bisulfite/metabisulfite, which further lightened the colour to that observed for an alkaline sodium sulfite-Dimezone-S solution.
3. Reducing the pH of the sulfite-free solution to a neutral state also appears to have a decolourizing effect, though to a slightly lesser degree than the addition of sulfite. On the other hand I suppose it is possible this effect is the result of some reaction between the Dimezone-S oxidation product and acetic acid, rather than the pH change per se.