Gels from Hydrolysis of Metal Aqua Ions

1 Gels from Hydrolysis of Metal Aqua Ions 

We saw in Section 13.6 that hydrolysis and subsequent polymerization of aqueous metal cations can lead to the precipitation of gels. In the case of Fe(H20)6 in mildly acidic solutions, the polymerization sequence of Eqs. 13.25 and 13.26 and Fig. 13.6 first reversibly forms cationic colloidal spherules, 2-4 nm in diameter, with the structure of 7-Fe0(0H) [double chains of Fe(0,0H)e octahedra] on a timescale of about 100 s. These lose H and harden over several hours and then, over several days, form aged polymer rods, then rafts, and ultimately, after several months, needles of solid goethite [a—FeO(OH)]. Thus, aging is an important feature of hydrolytic polymerization. 

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