The ionic strength and non-ideality

The ionic strength is a special type of summation over all charged species in solution, and is defined as  

Zi is the algebraic charge on each type, i, of ion in solution, i.e. it includes the sign of the charge. The Debye-Htickel theory (see Sections 10.7, 10.10.1 and 10.10.2) allows a calculation of the activity coefficient, y, for any ion from the known ionic strength. 

This calculation shows the dramatic increase in the ionic strength which occurs as the charge on the ions increases, despite all the solutions being of the same concentration. Non-ideality will be expected to increase in line with increasing ionic strength. 

These are activity coefficients calculated for the same value of the ionic strength. The 

Gathering all the calculations together gives the following table  

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