Introduction

The initial solution (before any base is added), only contains weak acid. So the concentration of H₃O⁺ ions can be predicted using

Between the start of the titration and the equivalence point, the weak acid is only partially neutralized, so there is a mixture of both the weak acid and conjugate base. Since both parts of the conjugate pair are present, a buffer is formed (which is where the name “buffer region” comes from), and the concentration of H₃O⁺ ions can be predicted with the Henderson-Hasselbalch equation.

At the equivalence point, the original weak acid has been fully neutralized and converted into its conjugate base. This process creates a slightly basic solution. To calculate the concentration of H₃O⁺ ions, the amount of OH^– ions must be calculated first. The auto-ionization constant for water can then be used to determine the concentration of H₃O⁺ ions using an equation similar to the one that describes the relationship between K_a and K_b for a conjugate pair.

After the equivalence point, the concentration of H₃O⁺ ions is determined by the excess amount of OH^– ions added from the titrant (for example, sodium hydroxide, NaOH, or potassium hydroxide, KOH). The weak base remaining from the equivalence point does have an effect, but it is usually small enough that it can be ignored in the calculations for this area of the titration curve.

The initial solution consists of a weak base.

The Henderson-Hasselbalch equation can still be used to calculate the concentration of H₃O⁺ ions, although the pK_a of the conjugate acid must be used.

At the equivalence point, all of the weak base is neutralized and converted into the conjugate acid.

After the equivalence point, the concentration of H₃O⁺ ions is determined by the excess H₃O⁺ from the titrant.