A buffer consists of a weak acid and its salt or weak base and its salt. When the ratio of weak acid and its salt in a buffer (or the ration of weak base and its salt) is equal to 1, we say that the buffer capacity is maximum.

What happens when buffering capacity is reached?

Once the buffering capacity is exceeded the rate of pH change quickly jumps. This occurs because the conjugate acid or base has been depleted through neutralization. This principle implies that a larger amount of conjugate acid or base will have a greater buffering capacity.

Where is the maximum buffering capacity on a titration curve?

As the slope of the titration curve is the smallest at the acid : base ratio 1:1, the buffer capacity is maximal at the same point (the second derivative of the titration curve is equal to zero at the same point).