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Speciation of ions refers to the changing concentration of varying forms of an ion as the pH of the solution changes.
The pH of a solution of a monoprotic weak acid can be expressed in terms of the extent of dissociation. After rearranging the expression defining the acid dissociation constant, and putting pH = −log10, one obtains
This is a form of the Henderson-Hasselbalch equation. It can be deduced from this expression that
It follows that the range of pH within which there is partial dissociation of the acid is about pKa ± 2. This is shown graphically at the right.
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