pH of buffer

[joonkimdds]

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if buffer region is where pH = pKa, why is pH of buffer = pKa + log (salt/acid) instead of just pKa?

 

[yuppers]

a buffer region occurs when the addition of an acid or base does not yield a large change in pH.
pH = pka when the [acid] = [salt] so using hypothetical numbers
pH = pka + log ([.250]/[.250])
pH = pka + log (1)
pH = pka + 0
so pH = pka

 

[joonkimdds]

well...then why do they even bother using + log (salt/acid) if we know that it will become 0 anyway? and there was a question in destroyer with diff concentraion of salt and acid and I don't know how these two can be different and we can still call it a buffer.

 

[UndergradGuy7]

When pH = pKa you have equal amounts of acid and the conjugate base (or a base and a conjugate acid).

So, a buffer works best if you have a 1:1 mixture of an acid and its conjugate base (like at pH=pKa). Because then it can resist changes if you add an acid, since it will react with the conjugate base. Or if you add a base it will react with the acid.

Lets say you add HCl to a 1:1 buffer solution. The HCl will react with the conjugate base and won't affect the pH of the solution much because the conjugate base neutralized the acid. If you add too much HCl you will use all of the conjugate base and then the pH will increase a lot more.

So the equation pH = pKa + log ...
when the acid = conjugate basic concentration the log 1/1 = 0 because log 0 = 0. If you do not have equal amounts of acid and conjugate base the log term will not be 0 and this affects the pH, but you will still have a buffer! Anytime you have an acid and a conjugate base it is a buffer. Say you have 4 mols of and acid and 2 mols of its conjugate base. It is still a buffer. It is not as good since you don't have equal amounts of both, but still a buffer.

A buffer region is not just pH = pKa. pH = pKa is where the best buffer is. The entire buffer region is from the region pH-1 to pH+1.

Check this picture http://facstaff.gpc.edu/~mkim/C1211&1212Lec/16_BufferRegion_TitrationCurve.jpg
The best buffer is at pH 4.77. But there is a buffer region anywhere from pH 4 to pH 6.

 

[yuppers]

well...then why do they even bother using + log (salt/acid) if we know that it will become 0 anyway? and there was a question in destroyer with diff concentraion of salt and acid and I don't know how these two can be different and we can still call it a buffer.

You use the equation because the [salt] is not always equal to [acid].

 

[joonkimdds]

You use the equation because the [salt] is not always equal to [acid].

Oh~ OK.
I thought buffer means salt = acid always~