Publisher Summary Under the influence of a potential gradient, the ion in solution can carry an electric charge, and thus if a voltage is applied across two electrodes situated in the solution, a current will flow between the electrodes and the solution is said to be conducting. Such a system could be used as a method of detecting ionic species in liquid chromatography (LC), and can be called as the “electrical conductivity detector (ECD).” In ECDs, it is the resistance of the solution that is actually monitored, and it is the change in electrical resistance of the mobile phase in the presence of a solute that provides the output from the detector. For this reason, the functioning of the detector is considered in terms of resistance measurement. An ECD cell consists of a small chamber containing two electrodes across which an AC potential is applied. The electrical capacity of the cell should also be kept to a minimum by ensuring that the surface area of the electrodes is as small as possible. If the cell has significant electrical capacity and a bridge circuit is used, then as an AC potential has to be employed to eliminate polarization; electrical balance is difficult to achieve because of the out-of-phase capacity current across the bridge. The chapter also discusses the properties of ions in solution, a commercial example of an ECD, and the areas of application of ECD.