Breakpoint chlorination is very real, but as I wrote in my post the 10x rule-of-thumb that comes from it was misapplied to swimming pools with regard to getting rid of Combined Chlorine (CC). The link you gave to a presentation doesn't even go into the same level of detail as I do (in other posts) and doesn't even talk about the 10x part of breakpoint chlorination that is the part that is misapplied to CC in the pool industry. The presentation also has graphs showing chlorine concentration/effectiveness that do not apply when Cyanuric Acid (CYA) is in the water (i.e. the "Free Chlorine Distribution with pH" graph doesn't apply when CYA is present and I show the proper graphs in
this post).
So let's take this step-by-step and walk you through this. First of all, let's go through where breakpoint chlorination and specifically the 10x rule comes from since much of that information is not in the presentation you linked to. More detail is in the thread
Chloramines and FC/CYA. It comes from adding chlorine to ammonia where initially there is more ammonia then there is chlorine. That is not a normal situation in swimming pools where chlorine levels are maintained and there is usually much more chlorine than ammonia. Nevertheless, the first step is the following:
(1) HOCl + NH
3 ---> NH
2Cl
Hypochlorous Acid + Ammonia ---> Monochloramine
The above reaction occurs very quickly with 95% completion in a few seconds when no CYA is present or under a minute when CYA is present. By the way, in that presentation you linked to on the page entitled "How fast is chloramine formation?" it uses as an example for rates an HOCl concentration of 0.2x10
-3 mol/l, but did you actually calculate how much this is? It's 14.2 ppm (mg/L) with no CYA which is quite high. That's why he gets 0.2 seconds at a pH of 7. In practice the levels are lower which is why I quote a few seconds for 95% completion, but the point is still the same that the above reaction is rather fast. If you were to open a pool with ammonia in it, you would find that the chlorine demand is very rapid and forms CC with no resulting FC, but that is NOT the situation in pools maintaining chlorine and having a bather load and it is not the situation described by the original poster in this thread where they already are measuring FC so clearly there is not ammonia remaining in the pool. So that brings us to the next series of equations that are much slower than the above.
(2) HOCl + NH
2Cl ---> NHCl
2 + H
2O
Hypochlorous Acid + Monochloramine ---> Dichloramine + Water
(3) HOCl + NHCl
2 ---> NCl
3 + H
2O
Hypochlorous Acid + Dichloramine ---> Nitrogen Trichloride + Water
(4) NHCl
2 + NCl
3 + 2H
2O ---> 2HOCl + N
2(g) + 3H
+ + 3Cl
-
Dichloramine + Nitrogen Trichloride + Water ---> Hypochlorous Acid + Nitrogen Gas + Hydrogen Ion + Chloride Ion
The full set of Jafvert & Valentine equations including the above are in
this Breakpoint spreadsheet I made. Initially, equation (1) dominates and is the fastest so monochloramine is produced. Then equation (2) grows in speed and dominates (because the monochloramine builds up and the ammonia declines) and equation (3) then occurs as well and then (4) so that all three equations (2,3,4) run together so that there is not a lot of dichloramine or nitrogen trichloride lasting very long as intermediates but this process takes around 10 minutes when no CYA is present (more than 90% complete with 2 ppm FC) or around 4 hours when CYA is present (with FC/CYA ratio 7.5%).
The net reaction, ignoring some side reactions that go to nitrate, is the following
3HOCl + 2NH
3 ---> N
2(g) + 3H
2O + 3H
+ + 3Cl
+
Hypochlorous Acid + Ammonia ---> Nitrogen Gas + Water + Hydrogen Ion + Chloride Ion
You can see that there is a molar relationship of 3:2 for chlorine to ammonia. Chlorine is measured in ppm Cl
2 units where molecular chlorine has a molecular weight of 70.906 g/mole whereas ammonia is measured in ppm N units where atomic nitrogen has a molecular weight of 14.0067 so in terms of a chlorine to ammonia ppm (weight) ratio it is (3*70.906)/(2*14.0067) = 7.593. In practice due to side reactions producing nitrate, the actual weight ratio needed for chlorine oxidation of ammonia is 8 to 10 and this is where the 10x rule-of-thumb comes from.
Now let's look at how this very valid rule was misapplied in the pool industry. The pool industry took this rule and applied it against Combined Chlorine (CC). The first major flaw is that
CC is measured in molecular chlorine units (i.e. ppm Cl2), NOT ammonia nitrogen units (i.e. ppm N). So there is no factor of 70.906/14.0067 = 5.062 weight difference. The second major flaw is that
CC already has chlorine combined with ammonia presuming it is mostly monochloramine which should be the case if one starts with ammonia. So two of the 3 initial chlorine would have already been used up combining with the two ammonia. In other words, when starting with CC that is monochloramine, the net reaction is the following:
HOCl + 2NH
2Cl ---> N
2(g) + H
2O + 3H
+ + 3Cl
+
Hypochlorous Acid + Monochloramine ---> Nitrogen Gas + Water + Hydrogen Ion + Chloride Ion
So the molar ratio of what is left is only 1:2, not the original 3:2. In practice it would take a little more than this 0.5 amount, but the point is that it is nowhere near the presumed 10x rule.
Even if one goes through this same analysis using chlorination of urea one doesn't get to more than 3x at the most. The 10x rule is completely wrong in its application to CC because 1) the unit of measurement of CC is 5 times larger than that of ammonia so takes 1/5th as much chlorine compared to ammonia and 2) chlorine is already part of CC so it takes less chlorine to further oxidize it.
Finally, in pools breakpoint chlorination is continuous because chlorine levels are maintained. So there is no magic number whether it be 10x or 3x or 1x because one is not asking how much chlorine is needed to complete oxidation but rather is simply asking whether increasing chlorine concentration will make things go faster and make things better or worse. That's what I responded to in my post. Increasing chlorine concentration can have the CC decrease if the original chemical to be oxidized was ammonia, but the CC may increase if the original chemical to oxidize is urea because urea is much slower to combine with chlorine. There are other techniques for removal of organics precursors such as urea including flocculation/coagulation that may be more effective and of course supplemental oxidation (e.g. ozone) may be helpful.
Where the breakpoint chlorination of chlorine added to ammonia does apply is when opening a pool in the spring to a huge chlorine demand when bacteria have converted CYA into ammonia (see
Degradation of Cyanuric Acid (CYA)) where one can use an ammonia test kit to see how much ammonia is left to oxidize and here one can legitimately use the 10x rule-of-thumb to figure out how much more chlorine (FC) one needs to add to oxidize such ammonia. Note that one only uses this 10x rule multiplying the ppm N (or ppm NH
3 which is close enough) from an ammonia test kit and NOT the CC amount if one already started adding chlorine since CC is in ppm Cl
2 units and already has one chlorine attached to ammonia as monochloramine. However, in such degradation of CYA this will be an underestimate of the amount of chlorine needed since there is often partially degraded CYA still left (e.g. biuret, allophanate) and that doesn't show up in ammonia tests nor as CC.
Have a nice weekend.
