A glass ion-selective electrode develops a voltage due to the ion exchange that occurs between the solution and the sensing membrane. The selective glass membrane blown on the tip of the sensor exchanges ions with the solution being measured. The charge imbalance between the membrane surface and internal cell of the sensor produces a voltage that changes in response to the sample’s ion activity. When the ionic strength of the solution is fixed by the addition of ISA, the voltage is proportional to the concentration of free ions in solution. When the ionic strength of the sample is fixed by the addition of ISA, the voltage is proportional to the concentration of sodium ions in solution and the electrode follows the Nernst equation.
This category also includes the probes used when inferring sodium concretion from a chloride titration.