Hydroquinone sulfite effects

It has been apparent for some time that the inhibition of sulfite autoxidation by organic compounds is due to their reactions with free radical intermediates. Further, it seems likely that the chemical effects associated with S02 autoxidation are due to these radicals or, possibly, peroxymonosulfate. Now that the reactivities of the likely free radical intermediates are known, the mechanism of these effects can be begun to be understood. For many organic compounds, like hydroquinone and other phenolic species, reaction with S03 and SO " is possible. Indeed, they prove to be the most efficient inhibitors of S02 autoxidation and the order of... 

These reactions involve proton and electron transfer and kinetic data suggest that the mechanism involves a protonated intermediate followed by base-catalyzed deprotonation of the ring [32]. In black-and-white developers the amount of sulfite present is usually very high at about 60 g L of sodium sulfite (0.5 m), and almost all the oxidized developer formed is immediately removed from the reaction site. (At low sulfite levels the effects of accumulated oxidation products can have a significant effect on the development rate, particularly for hydroquinone developers see the section on Lith development ). In the case of hydroquinone the reaction with sulfite is similar to that of p-phenylenediamine (Eq. (22)). 

In order for hydroquinone to be effective in regenerating Metol by the mechanism proposed, it is necessary that the quinone reacts with sulfite more rapidly than does methylquinoneimine. 

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