Courbes de titrage acide-base

The equivalence point is where moles of added base equal moles of acid in solution. For strong acids titrated with strong bases, this occurs at pH 7. For weak acids, the equivalence point occurs at pH greater than 7 because the conjugate base produced hydrolyzes, making the solution basic. This point marks a drastic pH change on the titration curve.

Strong acid titration curves show a steeper pH jump at the equivalence point compared to weak acid curves. Weak acid titrations feature a buffer region and half-equivalence point where pH equals pKa, absent in strong acid titrations. Both curves are S-shaped, but weak acid curves start at higher initial pH values when concentrations are equal.

When a strong base is added to a weak acid, it produces the conjugate base, creating a buffer solution. The buffer region occurs before the equivalence point and contains both unreacted weak acid and its conjugate base. At the half-equivalence point within this region, the concentration of weak acid equals its conjugate base, and pH equals the pKa.

Before the equivalence point, pH increases gradually as titrant is added and the acid is consumed. The solution contains both unreacted acid and its conjugate base reaction product. This gradual increase contrasts with the sudden pH jump occurring at the equivalence point, where the solution transitions from acidic to neutral or basic.

After the equivalence point, pH is determined by the amount of excess strong base titrant added. Both strong acid and weak acid titrations behave similarly at this stage because excess base dominates the solution composition. The pH gradually increases as more titrant is added, creating the upper portion of the S-shaped curve.

At equal concentrations, weak acids have higher initial pH values than strong acids because weak acids only partially ionize, producing fewer hydronium ions. Strong acids completely ionize, generating more hydronium ions and lower pH. This difference in initial pH is reflected at the start of their respective titration curves.

Titration curves plot pH against titrant volume, allowing identification of the equivalence point where moles of base equals moles of acid. By knowing the base concentration and volume at equivalence point, you can calculate the unknown acid concentration using stoichiometry. The curve's shape also reveals whether the acid is strong or weak.