Question
For the titration of a dibasic weak acid H2A $\left( p^{K_{a(2)}} - p^{K_{a(1)}} \geq 2 \right)$ with a strong base, pH versus volume of the base graph is as shown in the figure. The value of $p^{K_{a(1)}}$ and $p^{K_{a(2)}}$ may be equal to the pH values corresponding to the points.
Options
Solution
Point A: Buffer H2A + HA–; $P^{H} = P^{K_{a}} + \log\frac{\left\lbrack HA^{-} \right\rbrack_{0}}{\left\lbrack H_{2}A \right\rbrack_{0}}$
Point B: 1st equivalent point HA–; $P^{H} = \frac{1}{2}\left( P^{K_{a1}} + P^{K_{a2}} \right)$
Point C: Buffer HA– + A2–; $P^{H} = P^{K_{a2}} + \log\frac{\left\lbrack A^{2 -} \right\rbrack_{0}}{\left\lbrack HA^{-} \right\rbrack_{0}}$
Point D: 2nd equivalent point, A2–; $P^{H} = 7 + \frac{1}{2}\left( P^{K_{a1}} + \log C \right)$
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