Sodium bicarbonate has a pKa of 6.4 or 10.32, depending on the type of reaction. The pKa is derived from the acid dissociation constant, Ka, through the formula pKa = -log(Ka). Both measurements are used to describe the strength of an acid.
The pKa of an acid describes how it reacts when combined in an aqueous solution. A pKa below -2 describes a strong acid that completely dissociates into its ions when added to a solution. Acids with a high pKa, such as sodium bicarbonate, dissociate less. They are known as weak acids. The pKa is used to calculate the pH of an acid or base, which is another calculation to describe the acidity of a compound.