Species Distribution in Freshwaters

The activity of acid initially in solution, C, termed the analytical activity, is equal to the sum of the equilibrium activities of the acid and its conjugate base. 

Equations (3.37) and (3.38) can be combined to give an expression for the activity of the conjugate base in terms of the equilibrium constant, the analytical activity, which is also a constant, and the hydrogen ion activity. 

Alternatively, the activities of acid and conjugate base can be represented as a fraction of the analytical activity. 

The equilibrium pH can be obtained directly from the log ra. pH plot by adding lines showing log H and log OH and by using an expression for charge balance. 

For a solution containing a weak acid, the charge balance relationship (3.47) is satisfied at the equilibrium pH (Example 3.5). The same log vs. pH plot can also be used to determine the equilibrium pH of a solution containing the salt of a weak acid, e.g. NaA, by using the appropriate charge balance expression. 

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