What is conductivity and why should it be measured?

Conductivity depends on the concentration of ions and temperature.

Electrical conductivity is an inherent property of most materials and ranges from extremely conductive materials, such as metals, to non-conductive materials, like plastic or glass. In between the two extremes are aqueous solutions, such as sea water and plating baths. In metals, the electrical current is carried by electrons while in water it is carried by charged ions. In both cases, the conductivity is determined by the number of charge carriers, how fast they move, and the capacity of the carrier. Thus for most water solutions, the higher the ion concentration from dissolved salts, generally the higher the conductivity. Conductivity will increase with an increase in ion concentration until the solution becomes too crowded, thus restricting the freedom of the ions to move. Thereafter, conductivity may actually decrease with increasing ion concentration. This can result in two different concentrations of a salt having the same conductivity.

Conductance is defined as the reciprocal of resistance and is measured in Siemens (S), which was formerly referred to as mho (ohm spelled backwards). Conductivity is an inherent property of any given solution.A measurement results in the conductance of the sample and it is converted to conductivity. This is done by determining the cell constant (K) for each setup using a known conductivity standard solution.

Conductivity = (Cell conductance X Cell constant)

The cell constant is related to the physical characteristics of the measuring cell.  For a cell comprised of two flat, parallel measuring electrodes, K is defined as the electrode separation distance (d) divided by the electrode area (a).

In practice, measured cell constant is entered into the meter (directly or by user calibration) whereby the conversion from conductance to conductivity is calculated and presented.

Applications

Conductivity meters measure the ion capacity in aqueous solution to carry electrical current. As the ranges in aqueous solutions are usually small, the basic units of measurements are milliSiemens/cm (mS/cm) and microSiemens/cm (μS/cm). Conductivity is used widely to determine the level of impurities in water supplies for domestic consumption, wastewater, water quality testing, as well as industrial use. Industries that employ this method include the chemical, semi-conductor, power generation, hospitals, textile, iron and steel, food and beverage, mining, electroplating, pulp and paper, petroleum and marine industries. Specific applications include chemical streams, demineralizer output, reverse osmosis, stream boilers, condensate return, waste streams, boiler blowdown, cooling towers, desalinization, laboratory analysis, fruit peeling and salinity level detection in oceanography. In the table below are examples of solutions and their known conductivities.

We use conductivity measurements to determine the amount of dissolved ions present in a sample, which in water, serves as a measure of water quality

Although conductivity measurements are generally simple, not accounting for tempature will greatly affect the validity of the data generated. Applying temperature compensation is a way to account for temperature effects, and helps ensure the reliability and accuracy of your measurements. Temperature compensation uses the measured conductivity and temperature readings of the sample and applies a coefficient or algorithm to calculate and report the conductivity value of the sample at the selected reference temperature. When reported at 25 degrees C, this is known as specific conductance.