I am having a difficult time understanding what makes a buffer a buffer.
Buffers in my textbook are defined as a solution of a weak acid or base and their conjugate acid/base. So if I were to just dissolve acetic acid (a weak acid) in water why would this not be defined as a buffer. Acetic acid incompletely dissociates into an acetate ion (the conjugate base) and a Hydrogen ion. Why is it that for a base to be made the acetic acid would have to be mixed with something like Sodium acetate if acetic acid incompletely dissociates into a conjugate base in the first place? Could someone please help me understand why this is the case?
Answer
The term "buffer" implies a purpose of resisting changes in pH when small amounts of acid or base are added. The buffer will be most effective when the pH equals the pKa of the acid. This is when there are equal amounts of acid and conjugate base. For each pH unit the system deviates from pH=pKa, the buffer is about a factor 10 less effective.
If you just dissolve acetic acid in water, yes it is true that there will be a small fraction that ionizes to yield the conjugate base, acetate. However, the pH will be far from the pKa of acetic acid, and the system will not be very effective as a buffer.
In summary, "buffer" is not a very precise term, but it implies that there are significant amounts of both the acid and the conjugate base.
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