Hydrolytic Polymerisation of Iron(III)
Potentiometric and other studies by a number of investigators have implicated a binuclear complex, [Fe2(OH)2]4+, as the predominant hydrolysis species in solutions of iron(III) even at pHs less than 29–13. Continued hydrolysis of iron(III) solutions readily leads to the formation of various poorly characterised polymeric precipitates and, eventually, to the formation of the insoluble brown polymerisation product, iron(III) hydroxide (Ksp = 10−38.7, 25°, 3 M NaClO414. Although this familiar brown colloid has usually been assumed to be amorphous and ill defined, hydrolysates that are of small particle size (<10 nm diameter) but of sufficient crystallinity to give X-ray powder patterns have been isolated recently 15,16 The iron(III) oxide and iron(III) oxyhydroxide systems contain several crystalline phases: α-, β-, γ-Fe2O3, Fe3O4, and α-, β-, γ-, δ-FeOOH. Their structures and interconversions have been extensively studied17,18.