What influences the electrical conductivity of ionic solutions
Definition and description
The conductivity of a substance is defined as "the ability to transmit heat, electricity or sound". The unit of electrical conductivity is Siemens per meter [S / m] in SI units and microohm per centimeter [mmho / cm] in U.S. units. The symbol used is k or s.
Electrical conductivity (EC)
When the forces of an electric field act on electrically charged particles, an electric current results. Within most solids, an electrical current flow occurs due to the movement of electrons, this is known as electron conduction. In all conductors, semiconductors and in many insulators only electron conduction exists, while the electrical conduction strongly depends on how many electrons are available to participate in the conduction process. Most metals are very good electrical conductors, as they have a large number of free electrons available that can be excited into available energy levels (Readers interested in further information can learn about the valence and conduction bands, as well as p- and n-semiconductors to inform).
There can be net movement of charged ions in water and ionic materials or fluids. This phenomenon produces an electrical current and is called ionic conductivity.
The electrical conductivity is defined as the quotient of the current strength (I) and the strength of the electrical field (e) and is the reciprocal of the resistance (r, [W * m]):
s = I / e = 1 / r
Silver has the highest conductivity of all metals with 63 x 106 S / m.
Conductivity of water
Pure water is not a good electrical conductor. Ordinary distilled water, which is in equilibrium with the carbon dioxide in the air, has an electrical conductivity of 10 x 10-6 W.-1* m-1 (20 dS / m). Because the electrical current is carried by dissolved ions, the conductivity increases with increasing ion concentration.
Typical conductivity of water:
Ultrapure 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 indicates the concentration of all dissolved ions in the water. The EC can also be understood as a measurement of the ion activity of a solution in relation to its capacity to conduct electrical current. In dilute solutions, TDS and EC are somewhat comparable to one another. The TDS of a water sample is based on the measurement of the EC value and can be calculated using the following equation:
TDS (mg / l) = 0.5 x EC (dS / m or mmho / com) or TDS (mg / l) = 0.5 * 1000 x EC (mS / cm)
The above relationship can also be used to check the suitability of chemical water analyzes. However, this does not apply to wastewater.
If the concentration of the solution has a TDS> 1000 mg / l and an EC> 2000 ms / cm, the proximity of the ions to one another reduces their activity and, as a result, their ability to transport electricity. This happens although the physical value of the dissolved solids is not compromised. With high TDS values, the quotient of TDS / EC increases and the ratio tends towards TDS = 0.9 x EC.
In these cases, the above relationship cannot be used, but each value should be determined individually.
For water used for agriculture and irrigation, TDS and EC can be converted to each other with an accuracy of 10% using the following relationship:
TDS (mg / l) = 640 x EC (ds / m or mmho / cm).
In the process of reverse osmosis, the water is forced through semi-permeable membranes, leaving the impurities behind. 95-99% of the TDS is removed in order to produce pure or ultrapure water.
To calculate the TDS content of a water sample based on a water analysis and to convert the TDS to EC or vice versa, please use our calculators on the website.
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