What is free/total free chlorine? ①. Residual chlorine (also known as free chlorine): refers to chlorine in the form of hypochlorous acid (HClO), hypochlorite ion (ClO-) and dissolved elemental chlorine (Cl 2 ). ②. Combined chlorine: refers to the chlorine existing in the form of chloramine and organic chloramine. ③. Total
residual chlorine (also known as total chlorine): refers to the chlorine that exists in the form of free chlorine or combined chlorine or the coexistence of both. Where does residual/total chlorine come from? In the disinfection process, the water quality is disinfected by adding sodium hypochlorite, calcium hypochlorite, chlorine gas, sodium dichloroisocyanurate (also known as excellent chlorine), trichloroisocyanuric acid, etc. (1) Take sodium hypochlorite disinfection as an example: the main chemical formula of sodium hypochlorite in water treatment: NaCLO+H20=HCLO+NaOHHCLO=HCL+[O] The main mode of action of sodium hypochlorite disinfection is to form hypochlorous acid through its hydrolysis, hypochlorous acid It is further decomposed to form nascent oxygen [O]. The super-oxidizing property of nascent oxygen denatures substances such as proteins on bacteria and viruses, thereby killing pathogenic microorganisms. Secondly, in the process of sterilizing and killing viruses, hypochlorous acid can not only act on the cell wall and virus shell, but also penetrate into the body of bacteria (viruses) and interact with bacteria (viruses) because the molecules of hypochlorous acid are small and uncharged. Organic macromolecules such as proteins, nucleic acids, and enzymes undergo oxidation reactions to kill pathogenic microorganisms. Furthermore, the chloride ions produced by hypochlorous acid can also significantly change the osmotic pressure of bacteria and virions, causing the cells to lose their activity and die. (2) When chlorine gas is used for disinfection on site: chlorine gas is added to water to produce a series of chemical changes. Although the chemical reaction process is different for different water qualities, the final disinfection products are hypochlorous acid and hypochlorite ions. ①. When there is no ammonia nitrogen in the water, CL2+H2O→HOCL+H++CL- HCLO is a weak dielectric, which can be decomposed into hypochlorite ion HOCL→H++OCL- (reversible) HCLO and OCL- both have Oxidizing power can disinfect where hypochlorous acid exists as a neutral molecule that can diffuse to negatively charged bacterial surfaces and penetrate to destroy them. Hypochlorite exists in the form of ions and has a negative charge, so it is difficult to get close to negatively charged bacteria, so its disinfection effect is poor, so chlorine disinfection mainly depends on hypochlorous acid. After research, it is found that the key factor affecting the percentage of hypochlorous acid in chlorine is PH, and only in the range of PH6.8~8.5, hypochlorous acid has a detoxifying effect. ②. When there is ammonia nitrogen in the water, HOCL will combine with ammonia to produce a compound called amine, (that is, what we often call chloramine). The composition depends on the pH value of the water and the ratio of CL2 and NH3 content. If the content of ammonia nitrogen in the water in a special area is high, chloramine is the main antivirus in the water. Specifically divided into the following three forms: NH3+HOCL→NH2CL+H2O (monochloramine) NH3+2HOCL→NHCL2+2H2O (dichloramine) NH3+3HOCL→NCL3+3H2O (trichloramine) The generation of chloramine lies in HOCL , but pH also plays a key role in chloramines. When the pH is 5~8, monochloramine and dichloramine exist at the same time, the lower the pH value in this range, the more dichloramine content, and the antivirus effect of dichloramine itself is also strong, so it is also conducive to disinfection. Trichloramine is only produced when the pH is lower than 4.4, which is unlikely to be produced in general drinking water. (3) When sodium dichloroisocyanurate or trichloroisocyanuric acid is used for disinfection on site: it dissolves in water to generate hypochlorous acid and cyanuric acid. As a stabilizer, cyanuric acid cannot be decomposed, and hypochlorous acid plays the main role in disinfection. effect.
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