Calc.  SED & PREDOM / Ionic Strength

Activity corresponds to the concentration for a solution behaving ideally. It has the same units as concentration.

The activity coefficient, γi, gives the relationship between concentration and activity:

γi = ai / mi

where ai and and mi are the activity and molality, respectively, of the aqueous species “i”.

In aqueous solutions the activity coefficients depend on the ionic strength.

When writing equations, concentrations are usually written with brackets, for example “[H+]”, while activities are often written with braces, “{H+}”.