Electrical conductivity is a fundamental measurement used across various scientific and industrial fields to assess the capacity of a substance to pass an electric current. This property is especially relevant when analyzing liquids, where the electrical flow is facilitated by dissolved particles. The measurement is a simple and fast indicator of the overall purity or concentration of a solution. The common unit used to quantify this ability, particularly in water analysis, is the millisiemens per centimeter, abbreviated as mS/cm.
Decoding the Unit
Electrical conductivity is the measure of a solution’s ability to carry an electric current. The unit S, or Siemens, is the standard unit for electrical conductance, representing the inverse of electrical resistance, which is measured in Ohms.
The millisiemens (mS) is a fractional unit, representing one-thousandth of a Siemens. The measurement is expressed per centimeter (mS/cm) because it standardizes the reading to a specific distance between the measuring electrodes. This distance, usually one centimeter, allows the value to represent the specific conductance of the solution, making measurements comparable regardless of the equipment size. For many natural water samples, the smaller unit microSiemens per centimeter (\(\mu\)S/cm), which is one-thousandth of mS/cm, is more frequently used.
Conductivity and Dissolved Solids in Water
The mechanism behind a liquid’s electrical conductivity is the presence of electrolytes, which are dissolved substances that ionize in water. These substances, such as salts, minerals, and metals, break apart into positively and negatively charged ions. These mobile, charged particles act as the charge carriers that allow the electric current to flow through the water.
The electrical conductivity of water is directly proportional to the concentration of these ionic substances. As the amount of dissolved ions increases, the number of charge carriers rises, resulting in a higher conductivity reading. Monitoring conductivity is a fast and reliable proxy for assessing the overall concentration of Total Dissolved Solids (TDS) in water.
TDS measures the total weight of all dissolved solids, including both conductive ions and non-conductive materials like some organic matter. The close relationship between ionic concentration and total solids allows conductivity measurements to estimate the TDS level using a conversion factor. This factor can vary based on the specific chemical composition of the dissolved solids.
Conductivity measurements are often corrected to a standard temperature of 25°C because the mobility of ions, and thus the water’s conductivity, increases as the water temperature rises. This temperature compensation ensures that readings taken at different times or locations can be accurately compared.
Interpreting Typical Values
The mS/cm reading provides practical context for water quality, as different environments and uses have distinct conductivity ranges. Extremely pure water, such as deionized or ultrapure water used in laboratories, exhibits very low conductivity, typically around 0.05 to 0.2 \(\mu\)S/cm, because nearly all ions have been removed.
Standard drinking water generally falls within a moderate range, commonly between 50 and 800 \(\mu\)S/cm (0.05 to 0.8 mS/cm). Lower values often indicate purer water, while higher values suggest a greater presence of dissolved minerals, which can affect taste. Freshwater bodies like rivers and lakes show significant variability, ranging from about 100 to 1,500 \(\mu\)S/cm depending on the geology of the area.
A high conductivity reading, particularly one exceeding 5 mS/cm, often suggests high salinity or pollution. Seawater, due to its high concentration of dissolved salts, is highly conductive, with values typically ranging from 35 to 55 mS/cm. This high concentration of ions makes seawater unsuitable for direct human consumption or agricultural irrigation.
Excessively high conductivity in a freshwater source can indicate contamination from industrial wastewater, agricultural runoff, or road salt, which introduces a large number of conductive ions. The interpretation of the mS/cm value is dependent on the water’s intended use, providing a simple metric for assessing its overall ionic content.