Buffer capacity is defined as the amount of strong acid or base that can be added with a unit change in pH; it depends on what characteristics of the buffer?

Buffer capacity is defined as the amount of strong acid or base that can be added with a unit change in pH; it depends on what characteristics of the buffer?

• A. The amount of a weak acid or base and the pH near pKa
• B. The total concentration of buffer components only
• C. The concentrations of HA and A− and how close the pH is to pKa; maximum near pKa
• D. The buffer capacity remains constant for a given buffer

Answer: (C)

Buffer capacity is all about how much strong acid or base the system can neutralize before the pH changes by a unit. This capacity comes from how much of the buffer components are present and how balanced they are around the pH where the buffer works best. The two components are the weak acid HA and its conjugate base A−, so having significant amounts of both gives you more material to convert one into the other when acid or base is added. But the key is the pH relative to the acid’s dissociation constant, pKa. When pH is near pKa, the ratio [A−] to [HA] is about 1, meaning both species are present in appreciable amounts and can readily absorb changes in either direction by shifting between HA and A−. This makes the buffer most effective, i.e., it has maximum capacity. If the pH drifts far from pKa, one form dominates and the buffer loses much of its ability to counteract added acid or base, even if the total amount of buffer components is large. So the best description is that buffer capacity depends on the concentrations of HA and A− and how close the pH is to pKa, with maximum capacity near pKa.

Buffer capacity is all about how much strong acid or base the system can neutralize before the pH changes by a unit. This capacity comes from how much of the buffer components are present and how balanced they are around the pH where the buffer works best. The two components are the weak acid HA and its conjugate base A−, so having significant amounts of both gives you more material to convert one into the other when acid or base is added. But the key is the pH relative to the acid’s dissociation constant, pKa. When pH is near pKa, the ratio [A−] to [HA] is about 1, meaning both species are present in appreciable amounts and can readily absorb changes in either direction by shifting between HA and A−. This makes the buffer most effective, i.e., it has maximum capacity. If the pH drifts far from pKa, one form dominates and the buffer loses much of its ability to counteract added acid or base, even if the total amount of buffer components is large. So the best description is that buffer capacity depends on the concentrations of HA and A− and how close the pH is to pKa, with maximum capacity near pKa.