This chart shows a titration of a diprotic acid with a strong base. A few important points are worth noting.

1) The pH starts low and goes up because base (high pH) was added to acid (low pH).

2) Around 5.5 mL and 9.0 mL NaOH added there are inflection points in the plot. Because there are two inflection points, one can tell that the acid has two protons to lose, or is diprotic.

3) The inflection points are where the protonated and deprotonated forms of the acid are in equilibrium. Thus, by measuring the pH at these points, equilibrium constants K₁ and K₂ can be determined.

4) The final portion of the graph has a relatively straight, increasing slope. This happens because the end point has been exceeded. Once the endpoint is passed, excess base only increases the pH of the solution and serves no real purpose.