Note: For measurements using ion-selective electrodes, the measurement is made under equilibrium conditions what means that the macroscopic electric current is zero and the concentrations of all species are uniform throughout the solution. The indicator electrode is in direct contact with the analyte solution, whereas the reference electrode is usually separated from the analyte solution by a salt bridge. The potential difference between the indicator and reference electrodes is normally directly proportional to the logarithm of the activity (concentration) of the analyte in the solution (Nernst equation).