Use of chlorine in water
Chlorine began to be used in water disinfection from sodium hypochlorite (NaOCl) in 1908. Today it is the most widely used disinfectant agent in water and wastewater treatment because of its high oxidizing power. It can be found as inorganic Chlorine (Chlorine gas, calcium hypochlorite, and sodium hypochlorite) or organic Chlorine (sodium dichloroisocyanurate or trichloroisocyanuric acid). Both types of Chlorine react with water to dissociate into hypochlorous acid (HClO) and hypochlorite ion (ClO-).
Types of Chlorine
In chlorination processes we can find Chlorine in three forms: Chloramines, Free Residual Chlorine, and Total Residual Chlorine. For each species that one wants to quantify there is an adequate methodology.
Chloramines
Chloramines, also called combined Chlorine, are formed from the reaction of Chlorine with ammonium compounds present in the water and are responsible for the strong chlorine odor observed in swimming pools, for example.
Chlorine in the form of hypochlorous acid reacts with ammonia to form monochloramines (NH2Cl), dichloramines (NHCl2), and trichloramines (NCl3). Dichloramines have a greater disinfectant potential than monochloramines. Trichloramines, on the other hand, have no disinfectant effect. However, both mono- and dichloramines have a lower bactericidal effect than free residual Chlorine. One advantage of Chloramines is that they are more stable compounds than free Chlorine. Thus, when it is desirable to maintain the Chlorine residual for a longer period, the formation of Chloramines is forced in a secondary chlorination step.
After all the oxidation of the Chloramines, organic and inorganic materials, the residual Chlorine is called Free Chlorine and is presented in the form of hypochlorous acid and hypochlorite ion.
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| Balance between Hypochlorous Acid and Hypochlorite Ion |
Residual Chlorine
The break point shown in the figure below is the point where all the chlorine added to the water has already reacted and oxidized the chloramines, and from this point on it becomes available as Free Residual Chlorine. The potability of water is determined by health standards, each country has its own guidelines that indicate the maximum and minimum amounts of Chlorine that should be dosed into the water. Even if another type of oxidant is used in the treatment process, such as Ozone or Hydrogen Peroxide, for example, it must be dosed respecting the limits established by the ordinance.
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| Relationship between added Chlorine and residual chlorine in the break point chlorination process |
The total residual Chlorine is nothing more than the sum of the combined Chlorine (chloramines) and the free residual Chlorine. The total Chlorine analysis is important to determine the Chloramine content by difference (between free and total). It is common to perform the Chlorine dosage in a process and, when performing the analysis, obtain a result equal to zero. Many times the problem is not with the equipment used in the analysis, but with Chloramines present in the process that consume the Chlorine and leave no residual. One way to understand this phenomenon is to measure free and total Chlorine and verify the difference between them.
Free Chlorine
When analyzing free Chlorine or total Chlorine, some points must be taken into account. In the case of colorimetric equipment, the reagents change depending on the type of Chlorine that will be analyzed. The buffer used must have in its composition the addition of Potassium Iodide (KI) so that there is a reduction of chloramines to free Chlorine. For amperometric equipment, it is not possible to use the same instrument to analyze both species, since the construction of the amperometric cell is different for each type of Chlorine.
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