Theory of conductivity measurement

Electricity is a flow of electrons. This indicates that charged particles (ions) will conduct electricity.

Conductivity is the ability of a solution to pass current. It follows that the amount of current flowing is proportional to the number of ions present in the conducting solution. Therefore, a measure of the conductivity will give a direct reading of the solution concentration.

All substances conduct to some extent. In solution the level of ionic strength varies from the low conductivity of ultra pure water to the high conductivity of concentrated chemical samples.

Conductivity is measured in a wide range of industries and gives a readout of total ionic concentration within the sample solution.

In some industries the nature of the ions is known and may be present as one molecular species. In food manufacturing processes, the conductivity meter becomes a salt meter and is used for quality control measurement.

By contrast, industrial effluent and the resultant polluted waterways may contain many ionic species. Conductivity is used to give total ionic strength readings. It gives a means of monitoring the build-up of contaminants and also the success of water treatment programmes. Industrial cleaning and sterilising processes use conductivity to monitor their effectiveness and also the strength of residual, spent solutions. Depending on their ionic strength readings, they may be recycled for further use, or replenished.

The plating industry will also use conductivity in a similar way, to monitor the strength of etching, cleaning and waste solutions.

In general the measurement of conductivity is rapid and an inexpensive way of determining the ionic strength of a solution. The main drawback is that it is a non-specific technique.

Various ions in solution contribute to a total conductivity reading. Some ions contribute more than others. The presence of organic alcohols and sugars will give unusable conductivity results. Some materials also reduce the accuracy of the technique by coating the sensor. Temperature effects also contribute to experimental inaccuracies.

Total Dissolved Solids (TDS) is a measurement of the total concentration of ionic species in a sample. The measurement of solution conductivity gives a method by which a TDS value for the sample solution can be determined. The microprocessor controlled meter carries conversion factors, which are user selectable, in order to give a TDS value referenced to a calibration standard (usually KCl or CaCO3).

Applications

Industrial heating and cooling systems use recirculating water. Evaporation can lead to a high level of dissolved solids resulting in precipitation and scaling within the plant. Conductivity measurements can monitor the effectiveness of these systems and in particular the status of boilers and cooling towers. Swimming pools and other closed water systems are also monitored.
In the chemical industry, the conductivity of effluent streams can quickly indicate spillage or leakage problems. Similarly, the environmental sector can use conductivity measurements to highlight pollution problems caused by industrial outflows.
Chemical processes using high purity, low conductivity water can be monitored for contaminants with conductivity measuring equipment.
In solutions, H+ and OH– ions give high conductivity readings and this forms the basis for using conductivity as an endpoint detector in acid-based neutralisation systems.
The preference for a conductivity system over the use of pH electrodes is the robustness of the conductivity probes and the fact that they can operate at higher pressures and temperatures than conventional pH electrodes.