3.6:
Solution Composition During Acid/Base Titrations
Recall that the titration of acetic acid with sodium hydroxide produces sodium acetate and water, resulting in a buffer.
The relative concentration of the different species in the buffer solution during various points of the titration is indicated by the alpha values.
While the curved line represents titration of acetic acid and sodium hydroxide, the straight lines labeled alpha-knot and alpha-one denote the relative equilibrium concentration of acetic acid and acetate ions, respectively.
Initially, the alpha-knot value of 0.987, suggests that 98.7% of the acetic acid is undissociated, with the remaining 1.3% constituting acetate ions.
At the cross-over point or the half-equivalence point, pH equals pKa. Accordingly, the solution has an equal concentration of acetic acid and acetate ions.
As the titration proceeds, the concentration of the acetic acid, alpha-knot, decreases to nearly zero at the equivalence point. Alternatively, the concentration of the acetate ions, alpha-one, increases and approaches unity.
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 pKa 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.