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Water has the ability to conduct electricity due to the presence of charged ions in solution. Ions are atoms of molecules that have a net electrical charge, and they include cations (positively charged ions) and anions (negatively charged ions). The most abundant charged ions in natural water typically include the cations sodium (Na+), potassium (K+), calcium (Ca+2) and magnesium (Mg+2) and the anions chloride (Cl-), sulfate (SO4-2), nitrate (NO3-) and bicarbonate (HCO3-). Many other ions can be also found in water, including organic ions and other inorganic ions.

These ions carry electrical charge and can move through water, which allows water to conduct an electrical current. The measure of the ability of water to carry electrical current is called its electrical conductivity. Higher concentrations of ions in water increase its ability to conduct electricity and thus its conductivity. Distilled water, on the other hand, has a very low concentration of ions and a low conductivity.

The opposite of conductivity is resistivity. Resistivity is the ability of a material (such as water) to resist the flow of electricity. Resistivity is the reciprocal of conductivity, such that

Resistivity = 1/Conductivity

From this relationship, we can see that water with a high conductivity has a low resistivity, and vice versa. For example, distilled water will have a high resistivity and a low conductivity.

The typical unit for reporting conductivity is microsiemens per cm (µS/cm). This unit is also sometimes written as micromhos per cm (µmho/cm), where 1 µS/cm equals 1 µmho/cm. Potable water typically has conductivity values ranging from 50 to 1500 µmho/cm[1]. At higher conductivities, the water starts to become too salty to drink.

Because conductivity varies slightly with temperature, conductivity values are usually reported as temperature-compensated values that represent what the conductivity would be at 25°C. This makes it easier to compare conductivity values for samples with different temperatures.

How is conductivity related to total dissolved solids (TDS)?

Total dissolved solids (TDS) refers to the total amount of dissolved material present in water. TDS is usually reported in milligrams per liter (mg/L) or ppm (parts per million). This means that, if one liter of water with a TDS of 500 mg/L was completely evaporated, 500 mg of solid residue would be left behind. Usually, the dissolved solids include mostly dissolved mineral ions such as sodium, chloride, and the other ions mentioned above. TDS can also include other inorganic ions, dissolved organic material, and non-ionic matter such as dissolved silica. Although a relatively small amount of the TDS includes non-ionic matter that does not carry electrical charge, waters with higher values of TDS generally have higher values of conductivity.

Because of this, a measurement of conductivity (which is quick and easy) can be used to estimate TDS (which is more expensive and time-consuming to measure directly). However, the relationship between conductivity and TDS varies with the chemistry of the water because ions differ in their ability to transmit electrical charge through water. Some ions carry electrical charges faster than others because of factors such as the size and mass of the ions and how they interact with water molecules.

The general equation for estimating TDS from conductivity is as follows:

TDS (mg/L) = k · EC (µS/cm)

where EC is electrical conductivity, and k is the conversion factor, which is related to the chemical composition of the water.

For typical natural waters such as stream and lake water, the value of the conversion factor is usually between 0.6 and 0.7, and a value of 0.64 is considered to be typical. For a solution containing mostly sodium and chloride ions, values of 0.49 to 0.56 are typical, depending on the concentration of salt. 

For a precise estimate of TDS from conductivity, the chemistry of the solution should be considered in the selection of the conversion factor. If the composition of the solution is known, then the true TDS of a representative sample of water can be calculated by taking the sum of the measured concentrations. Alternatively, the true TDS value of a representative sample can be directly measured. The correct value of the conversion factor can then be calculated based on the true TDS and the measured conductivity. 

If the correct value of the conversion factor cannot be calculated, then a typical or default value of the conversion factor (such as 0.64) will result in a TDS estimate that is at least in the right ballpark. 

How is conductivity related to salinity?

Salinity refers to the salt content of water. Because most dissolved solids typically consist of inorganic ions, which are the components of salts, the concepts of salinity and TDS are very similar. In fact, the two concepts are sometimes considered to be synonymous. However, salinity is often expressed in terms of mass of salt per mass of water. For example, ocean water typically has about 35 grams of salt in one kilogram of water, so its salinity can be expressed as 35/1000 or 0.035. This can also be expressed as 3.5% or 35 parts per thousand (ppt).

Salinity is often used to describe seawater and brackish water, but it can also be used to describe fresh water and brines. Because the proportions of the most important ions in seawater are nearly constant, oceanographers can use very precise formulas to estimate salinity from electrical conductivity and temperature[1]. 

In cases where salinity is measured in mg/L (for example, for lake water, swimming pools, or irrigation water), salinity can be estimated from electrical conductivity using the same formula presented for TDS in the previous section.

References

[1] American Public Health Association (APHA) (2005) Standard methods for examination of water and wastewater, 21st edn. APHA, AWWA, WPCF, Washington.