Ferric - Hydrolysis of Iron(III) and Rust

Hydrolysis of Iron(III) and Rust

In water, ferric iron forms compounds that are often insoluble, at least near neutral pH. A salt of ferric iron hydrolyzes water and produces iron(III) oxide-hydroxides while contributing hydrogen ions to the solution, lowering the pH. In contrast, typical Na+ salts (e.g. NaCl) dissolve in water without lowering the pH. The differing behavior of Na+ vs Fe3+ ions reflects the effect of charge: water when bound to Fe3+ is highly acidic, inducing the formation of iron(III) hydroxides, which polymerize via the process called olation. Aluminium(III) (Al3+) behaves similarly to ferric ion.

Rust, a mixture of ferric hydroxide compounds, illustrates the low solubility of ferric ions in water. Various reagents cause rust to dissolve even at neutral pH. These ligands include EDTA, which forms a chelate complex with the ion, displacing the hydroxide and oxide ligands that comprise rust. For this reason, EDTA is often used to dissolve iron deposits or to deliver soluble iron in fertilizers. Citrate also solubilizes ferric ion at neutral pH, although its complexes are less stable than those of EDTA.