Question

When acid or base is added to water, the `pH` changes dramatically. Why, when acid or base is added to a buffer solution, does the `pH` change only marginally?

Answer 1

Because in distilled water, the effect of added acid/base is unmoderated. Buffers act to resist gross changes in `pH`.

Explanation:

A buffer contains a weak acid and its conjugate base in appreciable quantities. The `pH` of the buffer is reasonably close to the `pK_a` of this weak acid.

If we use the buffer equation, we can appreciate this property:

`pH = pK_(a) + log_10{([A^-])/([HA])}`

At undergraduate level, you should be able to derive this equation. When the concentration of the acid is equal to that of the conjugate base, `log_10{([A^-])/([HA])}` `=` `log_10(1)` `=` `0`, so `pH = pK_a`.

In an unbuffered solution, added hydroxide or protium ion is free to express its concentration, and dramatic rises/falls in `pH` occur. In the buffered solution, added protium ion protonates the base `A^-`, but this increase in `[HA]` is moderated by the logarithmic term. Capisce?