Stable and Metastable Solid Phases

It is then clear that under a given set of conditions, metastable and/or stable phases can exist. To find the conditions for precipitation/dissolution of a stable solid, one can use the corresponding thermodynamic constants as shown in the following example. 

FIGURE 5.8. Formation of metastable (super index m) or stable (super index s) phases by precipitation of the solid S from an aqueous solution. 

Example 5.5 With the following data, establish the four main equilibrium equations involved in the solubility/precipitation of iron (III) hydroxide, in the form log [X] =/(pH). Then, plot the complete solubility diagram for this substance. Lastly, show the predominance zone of the insoluble Fe(OH)3. 

The corresponding plots are shown in the solubility diagram for iron (III) oxide, see below. In order to assign the different species to the correct side of their corresponding equilibrium lines, one can do the following reasoning for each (based on Le Chatelier s principle)  

If one moves along the pH axis in the diagram toward higher pH values (which means higher [OH-]), then the equilibrium shifts towards the left, and the solid Fe(OH)3(v) predominates. 

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Fig. 1. Scheme of the Si-B-C-N concentration tetrahedron with stable and metastable solid phases indicated... 

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