# Waterguide - Conductivity

## What is conductivity?

The conductivity value tells you how well electrical current will be conducted. It is a physical variable and has the symbol δ (sigma) or κ (kappa) in electrochemistry and electrotechnology. Its SI unit is S/m (Siemens per metre). If you use it to form the reciprocal value, you get the specific resistance. Conductivity is a good guideline to use to determine the purity of a body of water.

## Principle of conductivity in water

You immerse two electrodes into a water-based solution and then apply a voltage (volt V). There is also a simultaneous flow of current (ampere I).

The formula for this is:

Conductivity is described as follows:

Some of the well-known values include the area (A) and the distance (I) between the electrodes.

The conductivity level depends on the availability and density of movable charge carriers.

In water, the principle of electrical conductivity is based on dissolved ions being able to transport the electrical current. Each ion is important for the conductivity, as the value is proportional to the quantity of ions in the water.

The conductivity therefore does not tell you anything about the type of ions dissolved in the water, but just about their concentration. The higher the conductivity, the saltier, or more polluted the water is.

## Conductivity measurement in water-related application areas

• To check for plausibility when creating an ion balance
• As a replacement variable for ionic strength when calculating the calcite saturation of a body of water (conductivity is much easier to measure)
• To monitor the product quality and determine the demineralisation rate for ion exchangers and reverse osmosis (used instead of the more time-consuming TDS measurement approach)
• To differentiate between different waters in the pipeline network (only possible for large differences in conductivity)
• To differentiate between freshwater and saltwater, e.g. for wells that are close to the coast
• To monitor waste water discharge into natural bodies of water (if measured continuously)

## Typical measured values

• Ultrapure water: 0.042 μS/cm

• Distilled water: 0.5 – 5 μS/cm

• Waters that are low in minerals: 100 – 300 μS/cm

• Seawater: 45,000 – 55,000 μS/cm

Threshold for drinking water: 2,790 μS/cm (measured at 25 °C)

Ultrapure water can be produced using a reverse osmosis unit and electrodeionisation or membrane degasification, if applicable.