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Conductivity is the ability of a material to pass an electric current. It is the reciprocal of resistivity. It is typically measured in aqueous solutions, and in this context, it can be defined as the ability of a solution to conduct an electric current between two electrodes. It is used as a measure of the number of ions dissolved in a solution. Measuring conductivity is a convenient and relatively inexpensive way to analyse the quality of a water sample and there are many applications in which the measurement provides valuable data on water purity, and can also be used to estimate total dissolved solids of a sample. Common applications include cooling tower monitoring, effluent monitoring and laboratory research.
Conductivity is measured by passing an alternating electrical current between two electrodes that are immersed in a solution and measuring the voltage. In practise, this involves using a conductivity electrode, which usually has two electrode plates (or in some cases 4 plates) and connecting the sensor to a conductivity meter. As a result of the electrical current being applied, the solution acts as an electrical conductor and ions move to the oppositely charged electrodes – cations move towards the negative electrode and anions towards the positive electrode. Solutions with high ionic concentration are highly conductive. Conductivity electrodes come in many materials, shapes and sizes to suit a range of samples and measuring ranges.
When measuring conductivity, the sensor must be calibrated; this is done by assigning a cell constant. The cell constant is determined by the ratio of the distance between the poles and the surface area – however the exact cell constant is always determined by experimental means rather than calculation based on the cell geometry. This involves using a conductivity standard of known value, and calibrating using the conductivity meter. Many meters have software available that will auto-recognise the standard which the cell has been placed in, and use an auto-cal function to give the cell constant. Most frequently, 1413µS/cm solution is used for calibration, which is a 0.01 molar potassium chloride solution. The cell constant of a conductivity cell will affect the measuring range; therefore, it is important to consider this when choosing a conductivity sensor. If the measuring range is low (e.g. 0-500µS), then a low cell constant is desirable (e.g. K=0.1)
The unit of measurement used for conductivity measurement is Siemens/cm, also commonly expressed in millisiemens/cm (mS/cm) or microsiemens/cm (µS/cm). Measurements can give an idea of the concentration of ions present in a sample; however, they do not provide information about the specific ions present. Despite this, the measurement still holds valuable information that can be used for many processes and industries.
Conductivity can be measured in laboratory processes or in the field. The measurement is temperature dependent and the degree to which temperature affects conductivity varies depending on the composition of the sample being measured. It is common to have an integrated temperature sensor in a conductivity electrode, which will work alongside the software in a conductivity meter to ensure that the measurement is as accurate as possible. Most meters have fixed temperature compensation of 2% per °C.
Measuring conductivity is a vital part of monitoring and quality processes – it can provide useful information about the chemical concentration of a sample. It is often necessary to know if a certain threshold of ions has been reached, or if a water source has been contaminated, particularly in industrial and environmental applications.
The applications commonly used include;
High purity water: Purified water is used in many applications, such as laboratories, cosmetics, pharmaceuticals and the food industry and is monitored by conductivity measurement.
Environmental: Significant changes (usually increases) in conductivity may indicate that a discharge or some other source of disturbance has decreased the relative condition or health of the water body and its associated plant and animal life.
Agricultural: The measurement of soil conductivity is widely used in agriculture. It correlates with various soil properties, such as soil texture and water holding capacity. It can also be used for ‘precision fertilisation management’.
Industrial: Conductivity measurement is used to determine the effectiveness of water treatment processes in preventing corrosion in plant equipment and in leak detection. When cleaning in place is used in process equipment it is important to know that the appropriate amounts of cleaning solutions are used and that those solutions have been removed completely before the next processing run. Conductivity measurement can help confirm these processes. Conductivity measurement is used in chemical processing and food and beverage manufacturing. Conductivity is used to determine the effectiveness of desalination.
Pharmaceutical: The quality of highly-purified water is crucial to the pharmaceutical industry and conductivity measurement is the most widely accepted quality control method.
Here at Sentek we make a range of conductivity measurement probes for use in the laboratory or in the field. It is important to consider the following when selecting a probe for your application;
To find out more about our range of electrodes and for more technical insight, visit us online or contact us by phone to speak to a member of the team.
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