Conductivity or electrical conductivity (EC) and total dissolved solids (TDS) are frequently used as water quality parameters. EC can be measured easily and inexpensively in situ, while TDS analysis (standard gravimetric techniques) is time-consuming and can only be conducted in a laboratory.
Therefore, under appropriate circumstances, the use of correlations with conductivity to determine the TDS is a popular choice for field measurement and continuous measurement due to its simplicity, speed and low-cost.
Definition:
Electrical Conductivity (EC)
Conductivity is a measure of water’s capability to pass electrical flow. These conductive ions come from dissolved salts and inorganic materials such as alkalis, chlorides, carbonate compounds, etc. Conductivity is usually measured in micro- or millisiemens per centimeter (uS/cm or mS/cm). When water is subject to pollution, contamination, or impurities, there will be a change in the conductivity value of the water. The more ions that are present, the higher the conductivity of water.
Total Dissolved Solids (TDS)
TDS is a combination of inorganic and organic substances that are dissolved in water, including wastewater. These solids can be salts, minerals, metals, cations, or anions. Common inorganic salts that are found as dissolved solids in water include calcium, potassium, magnesium, sodium, sulphates, and chlorides. Total dissolved solids are reported in mg/L.
Correlation between EC & TDS
These two parameters are correlated and can be approximated by a simple equation: TDS = k x EC
EC (in 25 °C) | Ratio TDS/EC (k) |
|---|---|
Natural water for irrigation | 0.55 – 0.75 |
Natural water (EC = 500 − 3,000μS⁄cm) | 0.55 – 0.75 |
Distillate water (EC = 1 − 10μS⁄cm) | 0.5 |
Freshwater (EC = 300 − 800μS⁄cm) | 0.55 |
Seawater (EC = 45,000 − 60,000μS⁄cm) | 0.7 |
Brine water (EC = 65,000 − 85,000μS⁄cm) | 0.75 |
Example: Correlation EC and TDS in various type of water
(Reference: https://iopscience.iop.org/article/10.1088/1755-1315/118/1/012019/pdf)
When correlated with laboratory TDS measurements, conductivity provides an approximate value for the TDS concentration, usually to within 10% accuracy.
