Acid Dissociation Constant by Potentiometric Titration

In this application note, the pKa determination of acetic acid by acid/base titration is described, and three different approaches named Half-neutralization value HNV-quick procedure, potentiometric method I and II are compared.

Background:

The determination of the acid dissociation constant K_a is an important step in the development of new pharmaceutical products since this physico-chemical parameter can affect the efficacy of the product.

What are the characteristic properties of an acid?

When dissolved in water, H₂O, the acid HA dissociates by releasing a hydrogen ion, H⁺:

𝐻A + 𝐻₂𝑂  ⇆   A⁻ + 𝐻₃𝑂⁺

Acids have different dissociation strengths. For example, hydrochloric and nitric acid are stronger than acetic acid because they are completely dissociated. The extent of dissociation is described by the equilibrium constant K:

𝐾=               𝑎_A⁻ ∙ 𝑎_𝐻3𝑂⁺

                             𝑎_𝐻A ∙ 𝑎_𝐻2𝑂

where ax are the activities of H₃O⁺, A^-, HA and H₂O. In dilute solutions, activities are to a good approximation equal to the concentrations [X]. Moreover, the water concentration remains constant compared to those of the other reaction partners. Therefore, its value can be included in K:

K^. [H₂O]  = K_a   = [H₃0⁺]^. [A^-]

                              [HA]

K_a characterizes the acid strength of acid HA, and in general, its negative logarithm is used:

            PK_a  = -log(K_a)

Download the application for step wise procedure for the determination of the Acid Dissociation Constant by potentiometric titration using HNV-quick procedure, potent. method I and II.