keeping the reaction temperature constant and by using a medium of relatively high ionic strength as the solvent. It is not unusual, particularly in texts relating to biochemical equilibria, to see an equilibrium constant value quoted with a dimension. The justification for this practice is that the concentration scale used may be either mol dm−3 or mmol dm−3, so that the concentration unit has to be stated in order to avoid there being any ambiguity. Note. When the concentration values are measured on the mole fraction scale all concentrations and activity coefficients are dimensionless quantities. In general equilibria between two reagents can be expressed as {\mathit{p}A} + \mathit{q}B <=> A_\mathit{p}B_\mathit{q} in which case the equilibrium constant is defined, in terms of numerical concentration values, as The apparent dimension of this K value is concentration1−p−q; this may be written as M(1−p−q) or mM(1−p−q), where the symbol M signifies a molar concentration (). The apparent dimension of a dissociation constant is the reciprocal of the apparent dimension of the corresponding association constant, and vice versa. When discussing the thermodynamics of chemical equilibria it is necessary to take dimensionality into account. There are two possible approaches. Set the dimension of to be the reciprocal of the dimension of the concentration quotient. This is almost universal practice in the field of stability constant determinations. The "equilibrium constant" , is dimensionless. It will be a function of the ionic strength of the medium used for the determination. Setting the numerical value of to be 1 is equivalent to re-defining the standard states. Replace each concentration term by the dimensionless quotient , where is the concentration of reagent in its standard state (usually 1 mol/L or 1 bar). By definition the numerical value of is 1, so also has a numerical value of 1. In both approaches the numerical value of the stability constant is unchanged. The first is more useful for practical purposes; in fact, the unit of the concentration quotient is often attached to a published stability constant value in the biochemical literature. The second approach is consistent with the standard exposition of Debye–Hückel theory, where , etc. are taken to be pure numbers. Water as both reactant and solvent For reactions in aqueous solution, such as an acid dissociation reaction AH + H2O A− + H3O+ the concentration of water may be taken as being constant and the formation of the hydronium ion