Activity, ionic strength and dielectric constant

The activity of ions in a solution is governed by the dielectric constant of the medium they are dissolved in and by the total concentration of ions in solution. For solutions of electrolytes in water with concentrations 0.5 M the activity of the ions present in solution is usually approximated to their individual concentrations. The mean activity coefficient for an ion in solution is defined as  

Although activity is regarded as 1 in dilute solutions this is still an approximation. The activity of an electrolyte solution in water can be estimated from the following equation  

This slight difference between the pH determined from activity and from concentration is usually ignored. However, from the equation used to calculate the activity coefficient it can be seen that the activity decreases with decreasing ionic strength. In addition the constant (-0.509 for water) increases with decreasing 

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