Acid dissociation constant,

 Ka

Equilibrium constant for the following reaction of an acid HB:

where co = 1 mol dm-3 is the standard amount concentration and activity coefficients have been neglected.

Note 1: This constant, because activity coefficients are neglected, is valid at a specified ionic strength. The thermodynamic dissociation constant is found by suitable extrapolation of the conditional constant to zero ionic strength. Note that it is defined as a dimensionless quantity, but sometimes it is given dimensions by omitting the standard amount concentration.

Note 2: Because this constant differs for each acid and varies over many degrees of magnitude, the acidity constant is often represented by the additive inverse of its common logarithm, represented by the symbol pKa (using the same mathematical relationship as (H+) is to pH), viz.:

pKa = -log10Ka

In general, a larger value of Ka (or a smaller value of pKa) indicates a stronger acid, since the extent of dissociation is larger at the same concentration.