The effect of hydrolysis reactions and pH on solubility

Dissolution to reach equilibrium from undersaturation may also be slow. Dissolution of magnetite at pH 4.5 in 0.1 M NaC104 (25 °C), reached equilibrium only after 20 days (Sun et al., 1998). 

The aquo-Fe ion, [Fe(OH2)6], is the predominant Fe species only at very low pH. As the pH rises above 1, Fe hydrolyses in a stepwise manner to give a series of soluble, positive and (in alkaline media) negative hydroxo species (see Chap. 13). The different hydrolysis species raise the concentration of dissolved iron in equilibrium with the solid at any pH. The equilibrium between the solid oxide and its various hydroxo species in solution is represented by the equation  

Such equilibria and their stability constants are summarized in Table 9.1. The total concentration of dissolved iron (Fe-r) at any pH is given by the sum of the concentrations of the free metal iron and all the soluble hydrolysis species, i. e. 

The solubility plots for lepidocrocite, ferrihydrite and hematite (Fig. 9.2) and for goethite, ferrihydrite and soil-Fe (Fig. 9.3) show only the total Fe activity. They were obtained in the same way as that for goethite using the appropriate constants from Tables 9.1, 9.2 and 9.4. 

Solubility diagrams have nearly always been calculated using solubility and stability constants. Experimental determination of the solubility of iron oxides as a function of pH has been concerned predominately with ferrihydrite. Lengweiler et al. 

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