Electric current will readily flow through certain solids and liquids when a voltage difference exists between two points. In solid materials it consists of electron movement through the atomic structure, it’s referred to as electronic current flow. Current carriers are of a fixed charge an almost negligible mass, any limitations on flow rate are determined by the characteristics of the conducting medium more than the carriers themselves.

Current will pass through some liquids, but the mechanism is quite different. The less ordered arrangements of the liquid molecules is not conducive to free electron movement and another sort of charged particle must serve this purpose if any current is to flow at all. In solvents where this phenomenon occurs, notably in water, ionization will provide the needed carriers. Ionization refers to the tendency of most soluble inorganic compounds to partially or completely separate into two or more elemental compounds, called ions, having opposite electrical charges. Example: NaCl will form Na+ and Cl-, AgNO3 breaks into Ag+ and NO3-. These charged particles, or ions, act as a current carrier producing electrolytic current flow, distinguished from electronic by the accompanying mass transfer. It is the physical characteristics of the carriers as much as that of the medium that determines the electrical resistance of a solution. This is the opposite of solid conductors. If the actions of these ions are sufficiently well understood, a measurement of the electrical resistance of a solution can provide valuable information concerning its chemical properties.