The Haber–Weiss reaction generates •OH (hydroxyl radicals) from H2O2 (hydrogen peroxide) and superoxide (•O2−) catalyzed by iron ions.
[1] This reaction has long been studied and revived in different contexts, including organic chemistry, free radicals, radiochemistry, and water radiolysis.
As Fe3+ is about 100 times less soluble than Fe2+ in natural waters at near-neutral pH, the ferric ion concentration is the limiting factor for the reaction rate.
At high pH, under alkaline conditions, the reaction considerably slows down because of the precipitation of Fe(OH)3 which notably lowers the concentration of the Fe3+ species in solution.
Moreover, the pH value also directly influences the acid-base dissociation equilibrium involving the hydroperoxyl and the superoxide radicals (pKa = 4.7)[5] as mentioned above.