Solubility products of the various oxides

The dissolution reactions for goethite are given in eq. (9.1) and (9.4). The log Kso value of 1.4 + 0.8 listed here was obtained in 3 M NaC104 by Schindler et al. (1963) using the dissolution method. In fact the value lay between 0.6 and 2.2. The value of Bigham et al. (1995) obtained by precipitation agreed with this value but showed less variation between triplicates. Hsu and Marion (1985) also using the precipitation technique (and a 16 year equilibration time) obtained similar results. 

Researchers in the aluminium industry have investigated the solubility of goethite in sodium aluminate and NaOH solutions. Basu (1983) found, using samples of natural goethite, that the equilibrium solubility of goethite in sodium aluminate solution was close to zero at room temperature and increased exponentially as the temperature rose above 100 °C. She also found that the isothermal solubility was greater in 5 M NaOH than in 5 M sodium aluminate solution at 150 °C, for example, [Fej] was 20 and 50 mgL , respectively. 

Only three values of log Kso are available for lepidocrocite and these are not in very good agreement. Hashimoto and Misawa (1973) measured the solubility of lepidocrocite produced by anodic deposition from Fe solution on a platinum electrode and obtained a value for log Kso of 2.50. This agrees with the value calculated from the free energy of formation (Blesa et al., 1994). Mohr et al. (1972) quote a value of 2.72 and Lindsay (1979) gives 1.59. 

The log Kso value which is the only value available to date was found to be -3.04 + 0.05. The free energy of formation (-752.7 kj moC ) calculated from this Kso is very much lower than that of goethite which does not accord very well with the observed behaviour of the two oxides this suggests that the equation chosen by Biedermann and Chow to describe their reaction may not in fact apply to akaganeite. 

Fox (1988) suggested that the K o for ferrihydrite should be written as [Fe] [OH ] instead of [Fe] [OH ] as is customary. This suggestion was based on the following linear relationship (pH range 2-7), 

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