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?

1 Answer
Jan 18, 2016

Answer:

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

Explanation:

A buffer contains a weak acid and its conjugate base in appreciable quantities. The #pH# of the buffer is reasonably close to the #pK_a# of this weak acid.

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

#pH = pK_(a) + log_10{([A^-])/([HA])}#

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{([A^-])/([HA])}# #=# #log_10(1)# #=# #0#, so #pH = pK_a#.

In an unbuffered solution, added hydroxide or protium ion is free to express its concentration, and dramatic rises/falls in #pH# occur. In the buffered solution, added protium ion protonates the base #A^-#, but this increase in #[HA]# is moderated by the logarithmic term. Capisce?