since logarithms can only be taken of pure numbers. must also be a pure number. On the other hand, the reaction quotient at equilibrium does have the dimension of concentration raised to some power (see , below). Such reaction quotients are often referred to, in the biochemical literature, as equilibrium constants. For an equilibrium mixture of gases, an equilibrium constant can be defined in terms of partial pressure or fugacity. An equilibrium constant is related to the forward and backward rate constants, kf and kr of the reactions involved in reaching equilibrium: Types of equilibrium constants Cumulative and stepwise formation constants A cumulative or overall constant, given the symbol β, is the constant for the formation of a complex from reagents. For example, the cumulative constant for the formation of ML2 is given by M + 2 L ML2; [ML2] = β12[M][L]2 The stepwise constant, K, for the formation of the same complex from ML and L is given by ML + L ML2; [ML2] = K[ML][L] = Kβ11[M][L]2 It follows that β12 = Kβ11 A cumulative constant can always be expressed as the product of stepwise constants. There is no agreed notation for stepwise constants, though a symbol such as K is sometimes found in the literature. It is best always to define each stability constant by reference to an equilibrium expression. Competition method A particular use of a stepwise constant is in the determination of stability constant values outside the normal range for a given method. For example, EDTA complexes of many metals are outside the range for the potentiometric method. The stability constants for those complexes were determined by competition with a weaker ligand. ML + L′ ML′ + L The formation constant of [Pd(CN)4]2− was determined by the competition method. Association and dissociation constants In organic chemistry and biochemistry it is customary to use pKa values for acid dissociation equilibria. where log denotes a logarithm to base 10 or common logarithm, and Kdiss is a stepwise acid dissociation constant. For bases, the base association constant, pKb is used. For any given acid or base the two constants are related by , so pKa can always be used in calculations. On the other hand, stability constants for metal complexes, and binding constants for host–guest complexes are generally expressed as association constants. When considering equilibria such as M + HL ML + H it is customary to use