Breakpoint Chlorination

[chem geek]

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.

I came across an 'old time pool guy' recipe for reaching breakpoint. Not recommending you try this it and Richard (chem geek) or Ben might want to comment on this.
(Ben, feel free to move this to china shop if it really should be there!)
For each 20000 gallons add 1 gallon of anhydrous ammonia and 8 gallons of 12.5% liquid chloine (this will raise the FC 50 ppm!) with pump off for 24 hours. Turn pump back on and let chorine levels drop to normal.

Seems like overkill to me!

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.

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

[waterbear]

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 the density of the 12% NaOCl solution.
Actually, I used BleachCalc to figure it out...I'm lazy!
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.
This acually came from the book "The Ultimate Pool Maintenance Manual, second edition by Terry Tamminen. 7th printing McGraw-Hill 2001. The author suggested that monochoramines would form and help kill any algae that might be present. However, he did give it as a recipe for breakpoint chlorination.

Richard

Just goes to show how much misinformation there is about pools. This book is considered to be one of the "best" referrences for pool maintenance professionals generally available! I have found that much of the info is quite useful but much of it is just a rehash of 'old wives tales'. The problem lies with sorting out the truth from the tales!

[KurtV]

Richard,
Are you saying that BleachCalc is wrong? It does indeed return 50ppm when you enter with 8 gallons of 12.5% sodium hypochlorite in 20000 gallons of water.
k

[chem geek]

First regarding the book that talks about forming monochloramines to help kill the algae, well this is true that you'd be forming a LOT of monochloramines with that method. But you WON'T achieve breakpoint chlorination without adding a lot more chlorine (unless I screwed up in my calculations).

As for BleachCalc, yes it is "technically" wrong, but as you can tell it's not wrong by much and the error in measuring cups plus the degradation of chlorine over time means that you probably do need to use a little more to end up with what you expect. WHOOPS! This means the error in BleachCalc is in the wrong direction! BleachCalc probably has you add even less than you need rather than more.

Also, the density of the 12.5% liquid chlorine being 1.16 g/ml is not true for bleach which is probably closer to water density, probably around 1.07 g/ml, so BleachCalc would be even a little more "too high" in this case (that is, reporting that the change in ppm is more than it really is). I have verified my calculations with my own pool and the use of 12.5% chlorine to the extent possible, but this isn't easy since I do suspect that the chlorine does degrade somewhat at the pool store.

Mmmmm. I think we need to verify this with some more real-world experience before we get Michael to change BleachCalc. What is your experience?

Richard

[waterbear]

My experience with BleachCalc and chlorine is zero. I have never needed to add bleach to my pool with the SWG. Never had any any CC to speak of!

[chem geek]

My experience with BleachCalc and chlorine is zero. I have never needed to add bleach to my pool with the SWG. Never had any any CC to speak of!

Oh yeah, I forgot. Rub it in, why don't you! One of these days I'll join the SWG club...one of these days...

[waterbear]

Just stating the facts....I had my nephew, his wife and their three kids staying with me this week (they left yesterday) and I checked the pool once during the week (Wednesday) after a day of heavy use and there was still no CC. (I didn't check the water until I got home from work at 1 am and the pump does run for 3 hours in the evening but it seems that was enough to take care of any that might have formed. I also suspect that my 3.5 year old grand nephew might have 'gone number one' in the pool that afternoon from an overheard converstaion between him and his mother in the pool while I was geting ready to go to work....that's why I checked it that evening!

[chem geek]

The consensus so far on this thread seems to be that people manually dosing chlorine shock from zero to 3 times per season -- most only once or twice. So assuming some of them also have kids with, well, #1 releases, my guess that a regular manual dosing of chlorine is pretty similar to what an SWG does may be correct. It's obviously not as good since even once or twice a day chlorine addition isn't the same as several hours each day, but it does show that, as CarlD says, "Stay ahead of your water!" really works.

Richard

[KurtV]

That small error in the BleachCalc alogorithm may explain why the Best Guess Table seems to sometimes understate the chlorine needs for reaching "shock levels". Some people may not actually ever be getting to the breakpoint if they're dosing based on BleachCalc. Or would that be overstating the effect?

I certainly don't want to sound ungrateful for Michaels fine software. It has made the pool maintenance part of my life much simpler on many occasions.

[chem geek]

I don't think it is BleachCalc that is the root cause of any underestimation of how much chlorine is needed to shock, though you are correct that if I am right about the error, then it will underestimate how much to add. I believe the much bigger potential problem (that we don't know for certain yet) is that Ben's table tracks reasonably well with the ppm HOCl theory for his "min" and "max" columns, but not well at all for the "shock" column. Look at my FC.htm chart and you can see that (ignoring very low CYA since the required FC is so low as to be impractical to maintain) the 0.02 ppm column roughly correlates to Ben's "min" while the 0.05 ppm roughly corrleates to Ben's "max", but there is no good correlation for his "shock" column. We don't know which way the error is, if any, but if I were to hazard a guess, I would say that the proper "shock" level is at around 0.5 ppm though possibly 0.2 ppm *might* do the trick in some cases. If 0.5 ppm is correct to kill algae, then the high CYA levels in Ben's chart underestimate the amount of chlorine truly needed.

Only real-world experience will tell what we should do to go forward -- if the ppm HOCl is valid for the shock to kill algae and if so, what level is appropriate.

Richard