Simply put, it is the concentration of the oxidized form of chlorine that can be used effectively for disinfection or other oxidation reactions. Chloramine is called the effective chlorine of combination, hypochlorite and hypochlorous acid are the effective chlorine of free. Once effective chlorine oxidizes some substances, it is reduced to chlorine (CL(l)-), no longer "effective". Effective chlorine is usually expressed by PPM or mg/L. Chlorine is 100% effective chlorine.
Chlorine addition at the folding point: when a large enough amount of chlorine gas is added to the water containing ammonia nitrogen, various reactions will occur, leading to the decomposition of ammonia nitrogen and the formation of free residual chlorine. Lower chlorine gas injection yields monochloramine and dichloramine, which are described on the left of the peak curve as a residual increase. When all the free ammonia nitrogen is used to form chloramine, the curve peaks. With the further increase of chlorine, chloramine no longer presents stable decomposition:
2 nh2cl (l) + HOCL (l) and N2 write + H2O + 3 HCL (l)
+ NHCL NH2CL (l) (l) 2 - N2 write + 3 HCL (l)
This tells us what happens when the curve drops to the right of the peak. When the chlorine injection amount reaches 8-10 times of the ammonia nitrogen concentration in water (the theoretical ratio is 7.6), it reaches the folding point, indicating that all ammonia nitrogen compounds are decomposed, and further increasing the chlorine addition amount will form free chlorine.
Typical folding point plus chlorine curve
In fact, ammonia nitrogen is rarely completely decomposed at the folding point. In the presence of free chlorine, some chloramines will definitely continue to exist, and a certain amount of free chlorine will exist before the folding point.
At the lower end of the peak, more dichloramine and nitrogen trichloride are produced, which have unpleasant tastes and odors. During chloramine disinfection, the amount of chlorine added should be controlled at the ascending curve of the first peak.