Editing Chemical equilibrium (section)

===Concentration quotients===

In aqueous solution, equilibrium constants are usually determined in the presence of an "inert" electrolyte such as [[sodium nitrate]], NaNO<sub>3</sub>, or [[potassium perchlorate]], KClO<sub>4</sub>. The [[ionic strength]] of a solution is given by
:<math> I = \frac12\sum_{i=1}^N c_i z_i^2 </math>
where ''c<sub>i</sub>'' and ''z<sub>i</sub>'' stand for the concentration and ionic charge of ion type ''i'', and the sum is taken over all the ''N'' types of charged species in solution. When the concentration of dissolved salt is much higher than the analytical concentrations of the reagents, the ions originating from the dissolved salt determine the ionic strength, and the ionic strength is effectively constant. Since activity coefficients depend on ionic strength, the activity coefficients of the species are effectively independent of concentration. Thus, the assumption that [[Gamma|''Γ'']] is constant is justified. The concentration quotient is a simple multiple of the equilibrium constant.<ref>{{cite book|first1=F. J. C. |last1=Rossotti |first2=H. |last2=Rossotti |title=The Determination of Stability Constants |publisher=McGraw-Hill |date=1961}}</ref>
:<math> K_\mathrm{c} = \frac{K}{\Gamma} </math>
However, ''K''<sub>c</sub> will vary with ionic strength. If it is measured at a series of different ionic strengths, the value can be extrapolated to zero ionic strength.<ref name="davies"/> The concentration quotient obtained in this manner is known, paradoxically, as a thermodynamic equilibrium constant.

Before using a published value of an equilibrium constant in conditions of ionic strength different from the conditions used in its determination, the value should be adjusted.