pH at Equivalence

[nothing123]

For an weak acid/strong base titration, the pH at equivalence can be estimated by pH = pKa + pHtitrant/2. Can someone explain where that equation comes from?

Thanks.

[Hitman2-2]

For an weak acid/strong base titration, the pH at equivalence can be estimated by pH = pKa + pHtitrant/2.

Can you provide a source or link for that?

Because I don't think that's quite true; I tried your formula on an example weak acid/strong base problem in my analytical chemistry text and didn't get the right answer.

[Borek]

I suppose what nothing refers to is

[H^+] = \sqrt {K_a C_a}

Just take the log. But it is only approximation and there are several assumptions done on the way to this equation. As shown it will work only for the titration of a weak base (Ka is for conjugated acid).