3.6 : Solution Composition During Acid/Base Titrations

The titration of a weak acid with a strong base results in the formation of water and the conjugate base of the acid. For instance, titrating acetic acid with sodium hydroxide leads to the formation of water and sodium acetate. A solution of acetic acid and sodium acetate constitutes a buffer whose relative concentration at different stages of the titration is indicated by the α values, which represent percentages of the weak acid and its conjugate base.

The α₀ and α₁ values represent the relative equilibrium concentration of acetic acid and acetate ions, respectively. Before adding the base, the α₀ and α₁ values are found to be 0.987 and 0.013, respectively. This suggests that the solution comprises 98.7% acetic acid and 1.3% acetate ions. With the addition of sodium hydroxide during the titration, α₀ decreases while α₁ increases. At the half-equivalence point, where the pH is equal to the pK_a of acetic acid, the value of α₀ is equal to α₁. The solution at this stage is a mixture of 50% acetic acid and 50% acetate ions. As the titration proceeds, α₀ decreases, and its value drops to zero at the equivalence point. On the contrary, the value of α₁ becomes unity.