James Richard Fromm
There are a large number of different types of equilibrium constant which are distinguished by the specific form of equilibrium to which the constant applies and the approximations to activity made in the use of the constant. These types are indicated by a subscript following the general symbol for an equilibrium constant K. A list of some of the more common types of equilibrium constant is given in a following section in which many of them are introduced.
Equilibrium constants have nominal units, although these are unfortunately left to be understood, which depend upon their form as well as upon any incorporation of constants into the reported value. The numeric value of a constant, and the nominal units given to it, must be consistent.
Equilibrium constants have values which depend upon the conditions under which they are measured. In most cases the values are measured under convenient laboratory conditions of 25oC and 100 kPa (one bar) pressure or corrected to these conditions. The values of equilibrium constants given anywhere in chemistry discussions are all always assumed to be at 25oC and one bar pressure, either in the gas phase or in aqueous solution, unless otherwise specified. Older works may still use one atmosphere pressure, which differs from the bar by only about 1%.