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What is chlorine?

Chlorine is the second element in group 17 (the halogens) in the periodic table.
There are two stable isotopes of chlorine: 35Cl (75.77 percent) and 37Cl (24.23 percent).
Chlorine is used, among other things, for bleaching, disinfection of drinking water, production of insecticides and production of PVC

Free Chlorine

Chlorine applied to water in its elemental or hypochlorite form first undergoes hydrolysis to form
freely available chlorine consisting of aqueous molecular chlorine (Cl2), hypochlorous acid (HClO) and the hypochlorite ion (ClO-).
Free chlorine reacts readily with ammonia and certain nitrogenous ones
compounds to form combined available chlorine. Both free and combined chlorine may be present
at the same time and is called total chlorine.
The sample is buffered with hydrogen-dihydrogen phosphate buffer at pH 6 without iodine ions - combined chlorine is not detected under these conditions. Then DPD (NN-dimethyl-1,4-
phenylenediamine) solution is added to develop the color. For total chloride iodide ions are added.
Combined chlorine is calculated as the difference between total and free chlorine.
The calibration is done by using a chlorine-equivalent iodate concentration in an iodide-iodate
The developed color complex is measured at 524 nm. Under these conditions, linearity is a given
up to 4 mg Cl2/L. For higher areas, a cubic formula is used.

Total Chlorine

The sum of combined chlorine and free chlorine is total chlorine (combined chlorine + free chlorine = total chlorine). Inexpensive chlorine tests usually only show total chlorine. Since pure chlorine water does not contain combined chlorine, total chlorine in a cleaned pool is the same as free chlorine

What is combined chlorine?

Combined chlorine develops when free chlorine binds to contaminants during disinfection. The presence of combined chlorine indicates that contaminants are present and that they are being actively neutralized. A clean, sanitized pool contains zero combined chlorine. Consistently maintaining the correct level of free chlorine in the pool water greatly reduces the likelihood of detecting measurable combined chlorine levels.


Chlorination of the drinking water is used by most waterworks. The method is relatively cheap and effective, and does not give the water any particular smell or taste in the doses used.


Chloramines are disinfectants used to treat drinking water. Chloramines are most often formed when ammonia is added to chlorine to treat drinking water. Chloramines provide longer lasting disinfection as the water moves through pipes to consumers. This type of disinfection is known as secondary disinfection

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