Electrical Conductivity – Water Quality Measurements

Electrical Conductivity, electrolytes, ions, and salinity

Electrical conductivity measured in Siemens (S) and represents a fluid’s ability to conduct electricity or pass an electrical flow. Temperature and concentration of free ions in the fluid determine electrical conductivity. Measuring the variables concurrently provides accurate results.

Electrical conductivity increases with salinity. More dissolved salts and solids means there is a higher concentration of free ions to carry charge.
Free ions to carry a charge.
In deionised water the LED would not be illuminated, by adding salt (NaCl) we would see the LED turn on. Its brightness will increase as salinity increases.

Hence, water’s electrical conductivity will tell us the concentration of salts or solids dissolved in the fluid; higher concentration means higher conductivity.

The electrical conductivity of Deionized water (pure H2O) is around 5.5 uS/cm (at 25C) whereas seawater is approximately 50,000 uS/cm.

Therefore we can deduct it is the salt which affects electrical conductivity, not the water itself.

Conductivity increases when compounds dissociate into ions when they meet water. For Salt (Sodium Chloride) splitting into Na and Cl. The ions are then free to ‘move about’ and conduct electricity. More free ions equal greater electrical conductivity. Note we are using salt as a basic example, various other dissolved solids alter a fluid’s electrical conductivity and salinity.

Green Span Electrical Conductivity 1500/1550 Toroidal Sensor
Green Span Electrical Conductivity 1500/1550 Toroidal Sensor

Increases in the liquid’s temperature increase ion mobility and with it, conductivity. It is necessary to use temperature as a key variable to yield accurate results. To account for this, modern sensors measure temperature and EC concurrently and give a temperature-compensated output equivalent to 25°C.

Electrical conductivity sensors

Sensors such as the EC-1500/1550, EC-2500A/L, and The Greenspan Multi-Parameter Water Quality Sensor are ideal for this application. Instead of electrical contact probes, it uses an inductive (or magnetic) method to determine conductivity. By
using this approach, there is no direct contact with the liquid. Although more difficult to implement, this toroidal method is inherently more reliable and has a very low drift compared with electrode type sensors, and will operate for many years, even in difficult environments.

Salinity

Salinity is a broad definition of the concentration of all dissolved salts. As mentioned, when dissolved, salts dissociate into ions such as sodium, chloride, sulphate, calcium, potassium, and bromine. All of these free ions can carry a charge. Organic compounds such as oil, sugar, and alcohol do not alter salinity. Such compounds do not dissociate to ions thus no charge can be carried.

It is important to note that electrical conductivity sensors cannot differentiate between ions, they simply report salinity. As the salinity of most bodies of water remains relatively constant, an unusual spike or drop can be the first warning of contamination. These would prompt further more accurate but time-consuming chemical analyses.

Why is knowing a fluid’s electrical conductivity important?

Conservation efforts

The water in these Bolivian lakes is too caustic for humans to drink, but Flamingos thrive in these waters by eating the algae and shrimp which grow there.

Organisms evolve to thrive in their environment, often forming complex ecosystems.  Flamingos, for example, eat algae and brine shrimp which thrive in lakes that are too caustic for most other organisms. However, change the salinity of the brine shrimps’ environment dramatically and they will likely die very quickly.

Conservationists must consider salinity when looking to rehabilitate aquatic organisms into the wild. They could not simply pick a lake at random if they wanted it to thrive. The environment must match what they are used to, such parameters will range from salinity to the size of the body of water.

Ocean currents

Thermohaline circulation is the process that drives deep-ocean currents. The two components of the thermohaline current are temperature (thermo) and salinity (haline) [1]. Currents form through various actions. One such action is the increase in the salinity of pockets of water where water has turned to ice, leaving the salt behind. These denser pockets sink, causing less dense water to replace it. As the ocean’s currents play a huge role in the Earth’s climate and weather patterns it is important to understand their salinity to predict the currents’ movements. Data on salinity allows climatologists and meteorologists to make more accurate models and predictions.

Ground water extraction

According to the National Groundwater Strategic Framework “increased groundwater salinity in response to groundwater extraction is a potential risk in many groundwater systems due to inter-aquifer leakage or accessions of saline irrigation drainage to groundwater or due to vulnerability to seawater intrusion” [2]

Salinity is a limiting factor for groundwater extraction regimes, the quality of extracted water can vary dramatically. As groundwater is the only supply in parts of Australia monitoring and maintaining the quality is of strategic importance for such communities. The water must be fit for purpose, whether it be for drinking water, industrial use, or agricultural irrigation.

If you would like to know more about the range of Greenspan water quality sensors then contact ESS Earth Sciences.