Breakpoint Chlorination

[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