| Ka = [H3O+] × | [conjugate base] |
| [acid] |
The solution in the titration flask is at equilibrium at all times during the titration. The relationship of Ka to [H3O+] (and pH to pKa) depends on the ratio [conjugate base]/[acid].
Because pH is a logarithmic scale, a change in pH of
one unit corresponds to a ten-fold change in [H3O+]
two units corresponds to a hundred-fold change in [H3O+]
Because pH is defined using -log[H
3O
+], the pH and [H
3O
+] change in opposite directions.
In a solution containing a mixture of acids or bases, the strongest determines the pH. The pH then determines, for weaker acids or bases in the same solution, the ratio [conjugate base]/[acid].
At pH = pKa, [acid] = [conjugate base].
At pH = pKa – 1, [conjugate base]/[acid] = 0.1
At pH = pKa + 1, [conjugate base]/[acid] = 10
Thus if either or both members of the conjugate pair are highly coloured, a change of 2 pH units results in a change in the colour of the solution.