Themes > Science > Chemistry > Inorganic Chemistry > Acids and Bases > Acids and Bases Index > Buffer range


A buffer is only useful across a certain pH range. Past a certain pH, there is not enough of the acid or the conjugate base to react with added acid or base. This can be seen graphically if you consider the fraction of acid or conjugate base in the buffer at various pHs.

  • f(HA) = [HA]/([HA] + [A-])
  • f(A-) = [A-]/([HA] + [A-])
f is the fraction of the species at any given pH- a fraction of f(HA) = 0.8 means that the buffer is 80% HA and 20% A-. If we graph f(HA) and f(A-) we see curves that look similar to those below. (The example below has a acid with a Ka = 1.0*10-5

Acid/base fractions at various pH

Looking at the above, we note that at pH 5.0, the acid and conjugate base are in equal amounts. By the time we move beyond pH 6, virtually all the acid is gone and there is only the conjugate base A-. As we move towards more acid, beyond pH 4.0 almost all the conjugate base is gone and the buffer is almost all acid.

This tells us that a buffer is usually only effective for about + or - 1 pH unit from its midpoint. For a pH 5 buffer, below pH 4 all there is in solution is acid and the solution cannot counteract additional acid. In the same fashion, above pH 6 there is only conjugate base and the system cannot absorb more base.


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