Conductivity measurement is an extremely widespread and useful method, especially for quality control purposes. Surveillance of feedwater purity, control of drinking water and process water quality, estimation of the total number of ions in a solution, or direct measurement of components in process solutions can all be performed using conductivity measurements.
The high reliability and sensitivity, and relatively low cost, of conductivity instrumentation make it a potential primary parameter of any good monitoring program. Some applications are measured in units of resistivity, the inverse of conductivity.
Other applications require the measurement of total dissolved solids, which is related to conductivity by a factor dependent upon the level and type of ions present. Conductivity measurements cover a wide range of solution conductivity from pure water at less than 1x10-7 Siemens (S)/cubic meter (cm) to values of greater than 1 S/cm for concentrated solutions.
In general, the measurement of conductivity is a rapid and inexpensive way to determine the ionic strength of a solution. However, it is a nonspecific technique, unable to distinguish between different types of ions, giving instead a reading that is proportional to the combined effect of all the ions present.
Conductivity is the ability of a solution, a metal, or a gas to pass an electric current. In solutions, the current is carried by cations and anions; in metals, it is carried by electrons.
How well a solution conducts electricity depends on a number of factors:
Mobility of ions
Valence of ions
All substances possess some degree of conductivity. In aqueous solutions the level of ionic strength varies from the low conductivity of ultra pure water to the high conductivity of concentrated chemical samples. Conductivity may be measured by applying an alternating electrical current (I) to two electrodes immersed in a solution and measuring the resulting voltage (V).
During this process, the cations migrate to the negative electrode, the anions to the positive electrode and the solution acts as an electrical conductor. Conductivity is typically measured in aqueous solutions of electrolytes. Electrolytes are substances containing ions, i.e., solutions of ionic salts or of compounds that ionize in solution. The ions formed in solution are responsible for carrying the electric current.
Electrolytes include acids, bases, and salts and can be either strong or weak. Most conductive solutions measured are aqueous solutions, as water has the capability of stabilizing the ions formed by a process called solvation.
Strong electrolytes are substances that are fully ionized in solution. As a result, the concentration of ions in solution is proportional to the concentration of the electrolyte added. They include ionic solids and strong acids, for example hydrochloric acid (HCl).
Solutions of strong electrolytes conduct electricity because the positive and negative ions can migrate largely independently under the influence of an electric field.
Weak electrolytes are substances that are not fully ionized in solution. For example, acetic acid partially dissociates into acetate ions and hydrogen ions, so that an acetic acid solution contains both molecules and ions.
A solution of a weak electrolyte can conduct electricity, but usually not as well as a strong electrolyte solution because there are fewer ions to carry the charge from one electrode to the other.