Weak Acid-Strong Base Titration Curves

Start adding strong base to a weak acid like acetic acid, CH_3COOH. Each drop of OH- reacts with the acid to make its conjugate base, acetate. Early on, you have a mix of leftover acid and newly made conjugate base sitting in the same flask: that's a buffer, and buffers resist pH change. That resistance is why the curve goes nearly flat for a wide stretch before equivalence, instead of climbing steadily the way a strong acid curve does from the very start.

The flat stretch is centered on the half-equivalence point, where exactly half the original acid has been converted to its conjugate base. At that instant [acid] = [conjugate base], so the Henderson-Hasselbalch equation collapses to pH = pKa. That single fact gives you a fast way to read pKa straight off any weak acid titration graph: find the volume that is half of the equivalence volume, go up to the curve, and read the pH.

Past half-equivalence, acid runs out and the buffer stops working, so the curve swings sharply upward at the equivalence point, just like a strong acid curve does. But the equivalence point itself lands above pH 7, not at 7, because the solution at that point is just the conjugate base (acetate) dissolved in water, and acetate is a weak base that generates some OH-. The equivalence volume, though, is untouched by any of this: it depends only on moles of acid reacting with moles of base, so a weak acid and a strong acid at the same concentration and volume hit equivalence at the identical volume of titrant.