in the reaction A + B ⇄ C + D at equilibrium (that is, no net change in concentrations of A, B, C, or D), the concentrations of the four components are related by the equation
Keq = [C][D]/[A][B];
Keq is the equilibrium constant If any component in the reaction has a multiplier (for example, H
2 ⇄ 2H), that multiplier appears as an exponent in the calculation of
K (for example,
Keq = [H]
2/[H
2]). When this equation is applied to the ionization of a substance in solution,
Keq is called the dissociation constant (
Kd) and its negative logarithm (base 10) is the p
Kd.
See also:
Henderson-Hasselbalch equation,
mass:action ratio.