Several details about the titration curves in Fig. 2-17 should be noted: 1. The curves have similar shapes but are shifted vertically along the pH axis. 2. The pH at the midpoint of each titration is numerically equivalent to the pK of its corresponding acid; at this point, [HA] ⫽ [A ⫺ ]. 3. The slope of each titration curve is much lower near its midpoint than near its wings. This indicates that when [HA] ⬇ [A ⫺ ], the pH of the so- lution is relatively insensitive to the addition of strong base or strong acid. Such a solution, which is known as an acid–base buffer, resists pH changes because small amounts of added H ⫹ or OH ⫺ react with A ⫺ or HA, respectively, without greatly changing the value of log([A ⫺ ]兾[HA]).