酸塩基滴定曲線

Acid-base titration can be performed to determine an unknown concentration of an acid using a known concentration of a base or vice versa.  When titrating an acid with a base, the titrant is gradually added to the acid, raising the pH. The pH change is plotted against the volume of the base to create a titration or pH curve. The titration curves are S-shaped, though differences exist depending on whether the acid is strong or weak. The pH at the start of the titration will be acidic but higher for a weak acid, assuming equal initial concentrations of acid. The equivalence point is the point on the titration curve at which the moles of base equals the moles of acid in solution. In the case of a strong acid, the equivalence point is reached when a strong base neutralizes all the hydronium ions produced by the strong acid; therefore, it always occurs at pH 7. In contrast, a weak acid’s equivalence point occurs at a pH greater than 7 and is reached when all the weak acid is converted into its conjugate base. Near the equivalence point, there is a sudden jump in the pH of the solution. However, in a weak acid titration, this rapid pH shift is not as steep as in a strong acid titration. The pH after the equivalence point on a titration curve for both weak and strong acids gradually increases due to excess strong base. Other features of a weak acid titration curve, but not a strong one, are the buffer region and the half-equivalence point. When a strong base is added, a weak acid produces its conjugate base creating a buffer. The half-equivalence point lies within this buffer region. This point is when half of the volume of base needed to reach the equivalence point has been added. Here, the concentration of weak acid equals its conjugate base, and the pH of the solution equals the pKa. The titration curves of a strong or a weak base with a strong acid as the titrant is an inverted S-shaped, where the pH starts high and decreases with the addition of the strong acid.