Ion concentration and activity

The ion concentration can be accurately measured by performing calibration and measurment using an ISA. 
The following describes the relationship between the ion concentration and activity and the reason for adding an ISA.

If specific ions exist in the sample solution, the ion selective membrane of the ion selective electrode generates a potential that corresponds to the concentration of those ions. The generated potential is measured as the potential difference on the basis of the reference electrode by using an ion meter or meter with an ion measurement function. A proportional relationship exists between the measured potential difference and the logarithm of the concentration of ions to be measured in the sample solution which can be represented by the Nemst equation.

The activity is a concentration of ions in the solution that is corrected using a coefficient called the activity coefficient, and the relationship between them can be expressed with the following relational expression:

Activity (a) = Activity coefficient (y) • Ion concentration (C)

The following table shows the relationship between the ion concentration and the activity coefficient.
From this table, you can see that when the ion concentration becomes lower than 1 x 10^-3 mol/L, the activity coefficient becomes a value close to 1, and as it becomes higher than 1 x 10^-3 mol/L, the activity coefficient becomes lower than 1.
If the activity coefficient becomes lower than 1 (activity < ion concentration), an error occurs in the potential measurement. As ISA is used to reduce this error. An ISA is generally selected  so that it does not react with  the ions to be measured and does not affect the measured potential.
The measurment error can be reduced because adding the ISA enables to maintain the ion strength constant  and to adjust the activity coefficients of the sample solution and the standard solution almost at the same value.

(Example) Activity coefficient for monovalent ion concentration