[eRReR]

Hi,

I was wondering why, when drawing a pH curve for a titration (e.g. strong base to weak acid), the pH has an initial "upswing" instead of having a gradual increase at the beginning.

[rabolisk]

Because of buffering. The nearly flat part is about 1 pH around the pKa of the weak acid, where the solution effectively resists pH change due to addition of strong base. In another words, pH rises very gradually even though strong base is added. Outside of this range, pH rises more dramatically as each drop of base is added, which is why the graph looks like an upswing rather than gradual.

[Borek]

I don't think that's what the OP asks about, although the question is rather ambiguous. There is a substantial difference between titration curve of the weak and strong acid - strong acid curve is mostly flat, weak acid curve has a well visible upswing at the very beginning. Technically buffering can be used to explain flat part of the weak acid curve, but there is no buffer in the strong acid solution - yet curve around 50% is not much different, if not flatter.

[eRReR]

Thanks for the replies. I do apologize. My question was too ambiguous. With the pH curves posted above (e.g. weak acid and base), my question is about the very first (i.e. left) part of the graph: not the buffer region where pH is approximately pKa. Why is there that initial upswing, when in the strong acid-strong base titration there is none?