Ageing and isomorphous substitution

Other characteristics of the solid that influence its solubility are whether it has been freshly precipitated (formerly this material was termed active ) or whether it has been aged ( inactive ) and also whether foreign ions are present in the structure. Fer-rihydrite precipitates from highly supersaturated solutions as fine (2-5 nm) particles with a poorly ordered structure. With time, however, the solubility decreases (see Fig. 9.4). 

The equilibrium solubility of an Fe oxide can be approached from two directions -precipitation and dissolution. The first method involves precipitating the oxide from a supersaturated solution of ions with stepwise or continuous addition of base und using potentiometric measurements to monitor pH and calculate Fej- in equilibrium with the solid phase until no further systematic change is detected. Alternatively the oxide is allowed to dissolve in an undersaturated solution, with simultaneous measurement of pH and Fejuntil equilibrium is reached. It is essential that neither a phase transformation nor recrystallization (formation of larger crystals) occurs during the experiment this may happen with ferrihydrite which transforms (at room temperature) to a more condensed, less soluble phase. A discussion of the details of these methods is given by Feitknecht and Schindler (1963) and by Schindler (1963). 

Solubility products may be calculated using the free energies of formation of the oxide, the free metal ion, OH and water (Tab. 9.1 and 9.2). For goethite, for example. 

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