Physics

Conductivity of water

Pure water cannot conduct electricity. The presence of ions in the water enables it to conduct electricity. Salt dissolved in water dissociates into ions and increases the conductivity of the solution. The conductivity is also affected by the temperature and concentration of the solution. In this article, we discuss the conductivity of different types of water and the factors affecting it.

What is conductivity?

Conductivity is proportional inversely to resistance. It is the property of a material to allow an electric current to pass when voltage is passed through it. Resistance is measured in ohms and conductance is measured in mhos. (as ohms become mhos because of applied conditions). Resistivity is measured in Ωm and conductivity has an SI unit of S/m.

Conductivity formula

Formula of conductivity is the inverse of resistivity.

σ = 1/ρ

Where σ = conductivity, ρ = resistivity

Electrical Conductivity of Water

The ability of water to pass electrical flow through it is called the conductivity of water or in other words, the degree to which water conducts or transmits electricity. It is denoted as S. It directly depends on the concentration of conductive ions present inside the water. The source for conductive ions is inorganic materials like chloride, carbonate, and alkalies, dissolved salts.

There are multiple units that can be used to measure the conductivity of water.

SI units- Siemens/meter (s/m).
U.S units- millimhos per centimeter (mmho/cm).

Electrical Conductivity of different types of Water

When chemicals or salts dissolve in water as negative and positive charge ions, these free ions in water conduct electricity. Therefore electrical conductivity depends upon the concentration of ions. Conductivity is less for pure water(therefore it’s not a good conductor of electricity).

Typical conductivity of water:

Ultra-pure water: 5.5*10^-6 S/m
Tap water:0.0005-0.0008 s/m
Sea water:5 s/m

Factors affecting the conductivity of water

Type of substance that is dissolving into water.
The temperature
The concentration of ions in the solution is present. (Higher the number of ions, higher will be the conductivity) If the inflow is a freshwater source then the salinity and conductivity value decreases.

Why does distilled water not conduct electricity?

Distilled water is a pure form of water. It is used in chemical reactions as it does not contain ions. Since there are no ions to conduct electricity, distilled water does not conduct electricity. At the same time, tap water contains ions like magnesium (II), calcium (II), etc which conduct electricity. Even rainwater sometimes contains ions(due to the acidic nature of the rain due to pollutants in the air) that conduct electricity.

What are the applications of the conductivity of water?

Conductivity can be used as a measure of the quality of water. It can be used to check the salinity of the water during water treatment. Certain industries require highly pure water like in research labs. Environmental pollution like agriculture run-offs, acid rains, etc can also be detected by measuring the conductivity of the samples.