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?

Jan 18, 2016

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

Explanation:

A buffer contains a weak acid and its conjugate base in appreciable quantities. The $p H$ of the buffer is reasonably close to the $p {K}_{a}$ of this weak acid.

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

$p H = p {K}_{a} + {\log}_{10} \left\{\frac{\left[{A}^{-}\right]}{\left[H A\right]}\right\}$

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, ${\log}_{10} \left\{\frac{\left[{A}^{-}\right]}{\left[H A\right]}\right\}$ $=$ ${\log}_{10} \left(1\right)$ $=$ $0$, so $p H = p {K}_{a}$.

In an unbuffered solution, added hydroxide or protium ion is free to express its concentration, and dramatic rises/falls in $p H$ occur. In the buffered solution, added protium ion protonates the base ${A}^{-}$, but this increase in $\left[H A\right]$ is moderated by the logarithmic term. Capisce?