The activity of an ion is its ‘effective’ concentration in solution. The reason that the effective concentration of an ion is different than the actual concentration (as measured in a water sample) is because ions form uncharged pairs in solution (see Figure 1). The extent of ion-pair formation increases with salinity. There is little difference between the activities and concentrations of most ions at salinty’s below about 3 ppt.
There are several software packages that can be used to to calculate activity coefficients for ions based on Debeye-Huckel theory or Pitzer equations. The Pitzer approach is most suitable when salinity’s are in the seawater range.« Back to Glossary Index