Defining the equilibrium problem

The chemical problem has to be reduced to an algebraic one in order to calculate the variables required. To do this a number of constraining equations are needed to define the system. To solve any problem the number of equations needed must equal the number of unknowns. 

Perhaps the best way to introduce the way for organising equilibrium problems is to introduce the concepts with a number of simple examples. 

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The concept of chemical components. Each species can be expressed as the product of a set of chemical components that define the equilibrium problem and a formation constant. Morel (1983) has expressed the definition of the chemical components as a set of chemical entities that permits a complete description of the stoichiometry of the system . For the example of the hydroxy-aluminium species given above Al3+ and H+ are the chemical components. As will be seen in the section on surface complexes the components are not necessarily elements or species. The components concept is important for understanding how to set up chemical equilibrium problems with various computer models. 

The PBE can be thought of as a proton reference level relative to the aqueous solution components chosen to define the equilibrium problem. A more detailed discussion of the PBE is given by Morel (1983) and Pankow (1991). 

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