Acid
Buffer regions and midpoints

A titration curve for a diprotic acid contains two midpoints where pH=pK. Since there are two different K values, the first midpoint occurs at pH=pK and the second one occurs at pH=pK. Each segment of the curve that contains a midpoint at its center is called the buffer region. Because the buffer regions consist of the acid and its conjugate base, it can resist pH changes when base is added until the next equivalent points.
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