Conductivity electrodes measure the specific conductance of a fluid. (Specific conductance is an outmoded term which has been replaced with "conductivity.") While alone the term "conductance" describes a material's ability to conduct electrical current, conductivity refers to the conductance within a given volume. More specifically, conductivity is defined as "conductance as measured between the opposite faces of a 1 cm cube of material."

Conductance is the reciprocal of resistance, so that conductance can be expressed as G = 1/R, where G is conductance and R is resistance. It is also true that conductivity is the reciprocal of electrical resistivity, or the ability of a material to resist an electric current within a given volume. The basic unit of conductance is the siemens (S), which is the reciprocal of the ohm (so that S = Ω^-1). For example, if a material has a resistance value of 5 Ω, its conductance value is one-fifth of a siemens, or 200 mS.

Conductivity as an expression of conductance per volume is expressed in siemens per centimeter (S/cm). It is occasionally described in terms of mhos/cm (as "mho" is the backward spelling of "ohm").

Calculating Conductivity

When measured using an electrode, conductivity can be calculated (typically using a calibrated meter) according to the formula below.

where:

G = conductivity

C = absolute conductivity at temperature t

L = length of liquid column between electrode plates

A = area of electrode plates

α = temperature coefficient

t = electrolyte temperature

t_r = reference temperature

The figure L/A is particularly important because it represents the electrode's cell constant, which is sometimes referred to as K. This parameter can be adjusted to increase accuracy when measuring solutions with varying conductivities. For example, when measuring a low-conductivity solution the electrode's poles should be made smaller or placed closer together to result in a cell constant of less than 1. This raises the conductance in order to make it more easily sensed by a meter. The opposite can be performed for high-conductivity solutions to render conductivity values more manageable.

Recommended cell constants for the measurement of certain fluids.

Image credit: Barben Analyzer Technology, LLC

Typical Conductivity Values

The table below lists typical conductivity values for electrolytic solutions. These values may be used as reference points for conductivity measurement and have been taken at the typical reference temperature of 25° C. Resistivity values have been added to illustrate the reciprocal relationship with conductivity; note that the resistivity values for the last three types are too small to be easily expressed, and so are rarely stated.

Applications

In industrial settings, conductivity electrodes are used to measure the conductivity of electrolytic solutions; these solutions may include liquids as nonconductive as ultra-pure water and as conductive as chemical process streams. Within these liquids, charge carriers which result in higher conductivity typically take the form of ions or dissolved inorganic particles. For this reason, conductivity electrodes are frequently used to measure ionization or total dissolved solids (TDS) in water quality applications. For example, dissolved solids in parts-per-million (ppm) can easily be found by dividing the conductivity in mhos/cm by 2.

While conductivity measurement is relatively inexpensive, nondestructive, and easy to perform, its major drawback is that it is not ion-specific, meaning that it cannot distinguish between the different types of ions being measured. In order to measure specific ions, conductivity electrodes and meters must be paired with other water quality determination methods.

Some specific applications for conductivity electrodes include:

• Ion-exchange chromatography
• Determination of water quality in public water systems, hospitals, and brewing applications
• Monitoring ionic impurities in boiler water

Electrode Construction and Use

Electrodes are sometimes referred to as "conductivity cells." All electrodes sense ions within a solution, then return either a measured voltage or a potential difference to a corresponding meter.

Conductivity cells usually contain either two or four poles. Three-pole devices were once common but are now generally superseded by four-pole cells. Two-pole cells consist of two platinum plates which form rings around the electrode; these rings are covered with a spongy platinum coating to reduce polarization between the poles. These cells are operated by applying a current to both poles and measuring the resulting voltage. Two-pole electrodes are typically best-used for low- to medium-conductivity measurement requiring high accuracy.

A two-pole cell diagram.

Image credit: Barben Analyzer Technology, LLC

Four-pole cells are designed to eliminate the polarization effects sometimes encountered with two-pole designs. These electrodes use two pairs of poles which effectively isolate the measurement circuit. Current is only applied to the outer poles, which allow the inner poles to measure the resulting voltage without polarization. Four-pole electrodes are used in higher-conductivity applications which allow for frequent recalibration and diagnostics.

A four-pole design, showing voltage (V) and current (I) loops.

Image credit: New Mexico State University

Toroidal Electrodes

Toroidal electrodes are specialized devices designed to avoid contact between the electrolytic solution and the poles. These use two toroidal coils—a drive coil and pick-up coil—to create a current field proportional to the solution's conductivity. Toroidal electrodes are used in applications where it is desirable to avoid contacting the sample solution, but they are typically less sensitive than the contact types listed above.

An inductive toroidal probe.

Image credit: Karlsruhe Institute of Technology

Cleaning and Storage

Maintaining and properly storing electrodes is of paramount importance. Contamination of electrode surfaces can result in severe polarization errors. To avoid this, users are typically advised to rinse electrodes with deionized water after each use; electrodes which have been used with water-immiscible solvents should be cleaned with a miscible solvent to avoid contamination. Storage of electrodes requires constant immersion in deionized water and conditioning before re-use.

Standards

Because they are common as water quality test instruments, conductivity electrodes are frequently used in conjunction with published standards. These documents describe best practices for use and methods for cleaning and calibrating electrodes. Some major standards are:

BS EN 27888 - Method for the determination of electrical conductivity

ASTM D1125 - Standard test methods for electrical conductivity and resistivity of water

ASTM D5391 - Standard test methods for electrical conductivity and resistivity of water of a flowing high-purity water sample

References

University of Crete/Radiometer Analytical - Conductivity Theory and Practice

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Endress+Hauser