Acid dissociation constant

Acid dissociation constant (also known as Ka) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions.

Definition

The acid dissociation constant is defined by the following equation:

Ka = [A-][H3O+] / [HA]

where [A-] is the concentration of the acid anion, [H3O+] is the concentration of the hydronium ion, and [HA] is the concentration of the acid. The larger the Ka, the more the acid dissociates and the stronger the acid.

Determination

The acid dissociation constant can be determined experimentally by measuring the pH of a solution containing the acid and using the Henderson-Hasselbalch equation. It can also be calculated from the standard Gibbs free energy change for the dissociation reaction.

Applications

The acid dissociation constant is used in many areas of chemistry, including analytical chemistry, environmental chemistry, and biochemistry. It is particularly important in the field of acid-base chemistry, where it is used to predict the outcome of reactions and to calculate the pH of solutions.

See also

• pKa
• Henderson-Hasselbalch equation
• Acid-base reaction theories
• pH
• Acid strength