This thread is to discuss breakpoint chlorination. Does it work? If so, under what conditions? If not, then how do you get rid of Combined Chlorines?
Let's start by answering a topic Evan (waterbear) had from this thread.
The first thing to do is to realize the confusion that arises from the 5:1 or 10:1 quantities quoted for the amount of chlorine (FC) needed to breakpoint combined chlorine (CC). It really takes one chlorine to produce monochloramine (NH2Cl) and another to breakpoint this to nitrogen gas (2NH2Cl + HOCl --> N2(g) + 3H+ + 3Cl- + H2O). You can instead produce dichloramine (NHCl2) which would then take two chlorine to go to trichloramine (NCl3) and then nitrogen gas. So for safety, it takes 3 chlorine (moles or molecules) to ensure breakpoint, starting from ammonia with no chlorine.Originally Posted by waterbear
The molecular weight of hypochlorous acid is 52.46 while that of ammonia is 17.03 so there is a weight difference of 52.46/17.03 = 3.08 so this is where we get 3*3.08 = 9 which is about 10 for the 10:1 relationship in ppm. If you already have monochloramine, then you only need 2*3.08 = 6 so I'm not sure where the 5:1 comes from except it probably assumes that not all of the monochloramine will go to NHCl2 and NCl3 and that some will breakpoint using less chlorine with the other pathway I showed.
[EDIT] CORRECTION: Chlorine is measured in ppm Cl2 (chlorine gas equivalent) with a molecular weight of 70.906 g/mole while ammonia is measured in ppm N (elemental nitrogen) with a molecular weight of 14.0067 g/mole so there is a weight difference of 70.906/14.0067 = 5.06. The normal breakpoint process has 3 chlorine for 2 ammonia (2 of the chlorine convert 2 ammonia to monochloramine and then these two combine with an additional chlorine to produce nitrogen gas) so 5.06 * (3/2) = 7.6 and this is the actual stochiometric ratio. However, because other reactions can occur, the actual ideal ratio is between 8 and 10 so to be safe 10 is what is quoted. [END-EDIT]
One gallon of liquid ammonia (anhydrous, with no water in it) weighs about 5 pounds which in 20000 gallons is (5 lb.) * (453.59237 g/lb.) * (1000 mg/g) / ( (20000 gal) * (3.7854118 l/gal) = 30.0 ppm.
Eight gallons of 12.5% liquid chlorine is (8 gal) * (3785.4118 ml/gal) * (0.125 g NaOCl / g solution) * (1.16 g/ml 12% NaOCl solution) * (52.46 g/mole HOCl) * (1000 mg/g) / (74.4424 g/mole NaOCl) / (20000 gal) * (3.7854118 l/gal) = 40.9 ppm
I believe you got to 50 ppm by neglecting to account for the difference in molecular weights between sodium hypochlorite (NaOCl) and hypochloric acid (HOCl) assuming that the 12.5% refers to the percent weight of sodium hypochlorite and you also neglected the density of the 12% NaOCl solution. You probably just did the simple calculation of (8/20000)*0.125 which is a fraction so ppm multiplies this by 1 million to get 50.
The mixture of 30 ppm ammonia with 40.9 ppm of chlorine will likely result in only some of the ammonia getting converted to monochloramine and nothing will get to breakpoint! Maybe this old-time pool guy was using a diluted ammonia that really was not anhydrous. To achieve breakpoint, you should have 3*3 = 9, so round and say 10 times as much ppm chlorine as ammonia.
Richard
Bookmarks