The electrical conductivity of liquids is an important analytical measurement in many industrial processes. This measurement is one of the more non-specific types of analytical technologies, because it does not discriminate between different conductive substances dissolved in the solution. For this reason, conductivity measurement is found in process applications where the type of conductive substance is irrelevant (e.g. ultra-pure water treatment for semiconductor “chip” manufacturing, where any conductive substance dissolved in the water is undesirable), or where the substance of interest is known to be the only conductive substance present in significant quantity (e.g. controlling the salinity of a brine solution, where large quantities of salt are added to water).
Electrical conductivity in metals is the result of free electrons drifting within a “lattice” of atomic nuclei comprising the metal object. When a voltage is applied across two points of a metal object, these free electrons immediately drift toward the positive pole (anode) and away from the negative pole (cathode).
Electrical conductivity in liquids is another matter entirely. Here, the charge carriers are ions: electrically imbalanced atoms or molecules that are free to drift because they are not “locked” into a lattice structure as is the case with solid substances. The degree of electrical conductivity of any liquid is therefore dependent on the ion density of the solution (how many ions freely exist per unit volume of liquid). When a voltage is applied across two points of a liquid solution, negative ions will drift toward the positive pole (anode) and positive ions will drift toward the negative pole (cathode). In honor of this directional drifting, negative ions are sometimes called anions (attracted to the anode), while positive ions are sometimes called cations (attracted to the cathode).
Electrical conductivity in gases is much the same: ions are the charge carriers. However, with gases at room temperature, ionic activity is virtually nonexistent. A gas must be superheated into a plasma state before substantial ions exist which can support an electric current.
Types of Conductivity Probes