Addition of #"sodium hydroxide"# to an #"acetic acid/sodium acetate buffer"# only marginally increases the #pH# of the solution. Why so?

1 Answer
Jun 1, 2017

Answer:

Because #[HO^-]# is moderated by the capacity of the buffer..........

Explanation:

From here, we learn that for a buffer solution...........

#pH=pK_a+log_10{[[""^(-)OAc]]/[[HOAc]]}#

And thus addition of hydroxide ion results in rapid formation of #"acetate ion"#..................likewise addition of #H_3O^+# would give rise to formation of #"acetic acid"#.

Given the equation, and the logarithmic term, during the titration, #pH# should remain tolerably close to the #pK_a# of #"acetic acid"#. At half-equivalence #pH=pK_a#, because #log_10{[[AcO^-]]/[[HOAc]]}=log_(10)1=0#, i.e. #[HOAc]=[""^(-)OAc]#. And such titrations are one way to measure #pK_a# for a weak acid, if you take #pH# measurements during the titration.

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