What is a #"buffer solution"#, and how is it prepared?

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Answer 1 · 2015-10-31T22:56:07

A buffer solution is a mixture of a weak acid and its conjugate base in appreciable concentrations; it acts to resist gross changes in #pH#.

For the weak acid #HA#, we may write its dissociation as:

#HA(aq) rightleftharpoons H^+ + A^-#

And, #K_a = ([H^+][A^-])/([HA])#. This is an equation that we can manipulate. We take #log_(10)# of both sides.

#log_(10)K_a = log_(10)[H^+] + log_(10)(([A^-])/([HA]))#.

Upon rearrangment,

#-log_(10)[H^+] = -log_(10)K_a + log_(10)(([A^-])/([HA]))#.

But by definition, #pH = -log_(10)[H^+]#, and #pK_a = -log_(10)K_a#.

Thus:
#pH = pK_a + log_(10)(([A^-])/([HA]))#

This is a form of the buffer equation, which tells us that at 1/2 equivalence, #[H^+]=[A^-], log_(10)(([A^-])/([HA])) = 0#, and #pH = pK_a#

Acid - Base Equilibria | Buffer Solution.