The aqueous solution chemistry of many elements is complex because of the number of different oxidation states and the formation of oxyions and hydroxyl complexes in different pH regions. Iron, for example, can exist in aqueous solution in three oxidation states: +2, +3, and +6. The Fe2+(aq) ion hydrolyzes to Fe(OH)+, dissolved Fe(OH)2, Fe(OH)3_, and Fe(OH)42". The relative amounts are determined by pH.Similarly, iron(III) may be present as Fe3+, Fe(OH)2+, Fe(OH)2+, dissolved Fe(OH)3, Fe(OH)4-, and Fe2(OH)2't+. Under certain conditions the dissolved species may be in equilibrium with solids such as Fe(OH)2, cv-FeO(OH), or Fe304. A convenient way to summarize the chemistry of such a system is with a potential-pH diagram, which shows the domains of redox potential and pH in which the various species predominate.
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