What is Predominance and distribution diagrams?

The predominance diagram of a couple is a pH axis divided at $pK_a$: below, the acid HA predominates ($[\ce{HA}] > [\ce{A-}]$), above, the base $\ce{A-}$. The distribution diagram shows the fraction of each form, $[\ce{HA}]/c$ and $[\ce{A-}]/c$, against the pH; the two curves cross at $\mathrm{pH} = pK_a$.

For a couple HA/A, $\mathrm{pH} = \mathrm{p}K_a + \log([\mathrm{A}]/[\mathrm{HA}])$: the acid predominates ($[\mathrm{HA}] > [\mathrm{A}]$) for $\mathrm{pH} < \mathrm{p}K_a$, the base for $\mathrm{pH} > \mathrm{p}K_a$. The predominance diagram is the pH axis divided at each $\mathrm{p}K_a$ into the domains where each form predominates. The distribution diagram gives the fraction of each form, $[\text{form}]/C$, as a function of pH.