Why isn't there a third buffering zone in the pH curve of this titration? I'm in chem 30 right now and we were learning about pH curves and buffering zones. I was looking at a question on the titration of $\ce{H3PO4}$ with $\ce{NaOH}$. The curve showed two qualitative reactions occurring as seen in the two sudden changes in pH. It also showed two buffer zones. Shouldn't there be 3 buffer zones not 2? My understanding is that once the reaction of $\ce{HPO4^{-2}}$ and $\ce{PO4^{-3}}$ is occurring, a third buffer zone would occur.
According to this, we have three pKas, as you observed: 2.12, 7.21 and 12.67 (at 25°C).
Your values may be different due to temperature.

The third value (12.67) can't show up in your graph because it doesn't go beyond pH 12.