1.4.2 Buffers

Unit 1

1.4.2 Buffers

Buffers are an important concept in acid-base chemistry. Here's a look at what buffers are and how they function.

 A buffer is an aqueous solution that has a highly stable pH. If you add acid or base to a buffered solution, its pH will not change significantly. Similarly, adding water to a buffer or allowing water to evaporate will not change the pH of a buffer.  A buffer is made by mixing a large volume of a weak acid or weak base together with its conjugate. A weak acid and its conjugate base can remain in solution without neutralizing each other. The same is true for a weak base and its conjugate acid.  When hydrogen ions are added to a buffer, they will be neutralized by the base in the buffer. Hydroxide ions will be neutralized by the acid. These neutralization reactions will not have much effect on the overall pH of the buffer solution.  When you select an acid for a buffer solution, try to choose an acid that has a pKa close to your desired pH. This will give your buffer nearly equivalent amounts of acid and conjugate base so it will be able to neutralize as much H+ and OH- as possible. Refer figure1.31 The curves have similar shapes but are shifted vertically along the pH axis. The pH at the equivalence point of each titration (where the equivalents of OH- added equal the equivalents of HA initially present) is 7 because of the reaction of A- with H2O to form [HA] = [OH]; similarly, each initial pH is 7. The pH at the midpoint of each titration is numerically equal to the pK of its corresponding acid; here, according to the Henderson–Hasselbalch equation, [HA] = [A-]. The slope of each titration curve is much less near its midpoint than it is near its wings. This indicates that when [HA] nearly equal to [A-], the pH of the solution is relatively insensitive to the addition of strong base or strong acid. Such a solution, which is known as an acid–base Figure 1.31 Acid–base titration curves acetic acid, H2PO4-, and NH4+ by of 1-L solutions a strong base. of At 1M the buffer, is resistant to pH changes because starting point of each titration, the acid form of the conjugate small amounts of added H+ or OH-, acid–base pair overwhelmingly predominates. At the midpoint respectively, react with the A- or HA present of the titration, where pH = pK, the concentration of the acid is without greatly changing the value of log([A- equal to that of its conjugate base. Finally, at the end point of ]/[HA]). the titration, where the equivalents of strong base added equal Buffers Stabilize a Solution’s pH The ability of a buffer to resist pH changes with the equivalents of acid at the starting point, the conjugate base is in great excess over acid. The shaded bands indicate the pHadded acid or base is directly proportional to ranges over which the corresponding solution can function the total concentration of the conjugate acid– effectively as a buffer. base pair, [HA] + [A-]. It is maximal when pH = pK and decreases rapidly with a change in pH from that point. A good rule of thumb is that a weak acid is in its useful buffer range within 1 pH unit of its pK (the shaded regions of figure 1.31). Above this range, where