How do pH buffer systems work?

1 Answer
Aug 16, 2017

Answer:

When appreciable quantities of a weak acid and its conjugate base are mixed in solution......

Explanation:

When appreciable quantities of a weak acid and its conjugate base are mixed in solution, a buffer solution is formed that tends to resist gross changes in #pH#. How?

Because the conjugate base of a strong acid DEMONSTRABLY does not compete strongly for the proton...................whereas the conjugate base of a weak acid DOES compete to some extent for the proton.......

And thus we can write the acid-base dissociation reaction:

#HA(aq) + H_2O(l)rightleftharpoonsH_3O^(+) + A^-#

And from this equilibrium expression we can derive an expression for #pH# of the buffer......

#K_a=([H_3O^+][A^-])/([HA])# (as you know #[H_2O]# is effectively a constant).

And if we take #log_10# of BOTH SIDES, we get........

#log_10K_a=log_10[H_3O^+]+log_10{([A^-])/([HA])}#

Which on rearrangement gives........

#underbrace(-log_10[H_3O^+])_"pH"=underbrace(-log_10K_a)_(pK_a)+log_10{([A^-])/([HA])}#

And thus we can write the so-called buffer equation............

#pH=pK_a+log_10{[[A^-]]/[[HA]]}#

And as we know, when #[A^-]=[HA]# at half-equivalence, #pH=pK_a#. Do you agree? Why?

#pH# could not be so moderated UNLESS #A^-# competed somewhat for the proton (and if it did not it would mean that the parent acid, #HA#, was a STRONG acid).

Anyway, is this what you want?