The solubility behavior of an element's higher oxidation state (relative to its lower oxidation state) in oceanic redox couples depends on whether the higher-oxidation-state species forms an insoluble oxide/hydroxide precipitate or a soluble oxyanion.
For Fe: Fe(III) forms highly insoluble oxide/hydroxide precipitates, so solubility DECREASES relative to soluble Fe(II). For Cr: Cr(VI) forms the soluble chromate/dichromate oxyanion, so solubility...
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