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Water Conductivity

Definition and description

Conductivity of a substance is defined as 'the ability or power to conduct or transmit heat, electricity, or sound'. Its units are Siemens per meter [S/m] in SI and millimhos per centimeter [mmho/cm] in U.S. customary units. Its symbol is k or s.

Electrical conductivity (EC)
An electrical current results from the motion of electrically charged particles in response to forces that act on them from an applied electric field. Within most solid materials a current arise from the flow of electrons, which is called electronic conduction. In all conductors, semiconductors, and many insulated materials only electronic conduction exists, and the electrical conductivity is strongly dependant on the number of electrons available to participate to the conduction process. Most metals are extremely good conductors of electricity, because of the large number of free electrons that can be excited in an empty and available energy state.
In water and ionic materials or fluids a net motion of charged ions can occur. This phenomenon produce an electric current and is called ionic conduction.
Electrical conductivity is defined as the ratio between the current density (J) and the electric field intensity (e) and it is the opposite of the resistivity (r, [W*m]):

s = J/e = 1/r

Silver has the highest conductivity of any metals: 63 x 106 S/m.

Water conductivity

Pure water is not a good conductor of electricity. Ordinary distilled water in equilibrium with carbon dioxide of the air has a conductivity of about 10 x 10-6 W-1*m-1 (20 dS/m). Because the electrical current is transported by the ions in solution, the conductivity increases as the concentration of ions increases.
Thus conductivity increases as water dissolved ionic species.

Typical conductivity of waters:
Ultra pure water 5.5 · 10-6 S/m
Drinking water 0.005 – 0.05 S/m
Sea water 5 S/m

Electrical Conductivity and TDS

TDS or Total Dissolved Solids is a measure of the total ions in solution. EC is actually a measure of the ionic activity of a solution in term of its capacity to transmit current. In dilute solution, TDS and EC are reasonably comparable. The TDS of a water sample based on the measured EC value can be calculated using the following equation:

TDS (mg/l) = 0.5 x EC (dS/m or mmho/cm) or = 0.5 * 1000 x EC (mS/cm)

The above relationship can also be used to check the acceptability of water chemical analyses. It does not apply to wastewater.
As the solution becomes more concentrated (TDS > 1000 mg/l, EC > 2000 ms/cm), the proximity of the solution ions to each other depresses their activity and consequently their ability to transmit current, although the physical amount of dissolved solids is not affected. At high TDS values, the ratio TDS/EC increases and the relationship tends toward TDS = 0.9 x EC.
In these cases the above-mentioned relationship should not be used and each sample should be characterized separately.
For water for agricultural and irrigation purpose the values for EC and TDS are related to each other and can be converted with an accuracy of about 10% using the following equation:

TDS (mg/l) = 640 x EC (ds/m or mmho/cm).

With the reverse osmosis process, water is forced in a semi-impermeable membrane leaving the impurities behind. This process is capable of removing 95-99 % of TDS, providing pure or ultra-pure water.

Use Lenntech calculators to calculate the TDS content from water analysis and to convert TDS in EC ou visa versa.

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