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How Long Before Chlorine Turns Into Free Chlorine?


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#1 Canar

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Posted 02 June 2011 - 11:21 AM

So I started this year out like many in the past. My in ground pool is just under 20k gallons. I bought shock based on the size and shocked the pool. Tested a couple days later and zero free chlorine. I read online and it said you need to get to whats called "superchlorination" with the shock and then you will start having free chlorine which is much more efficient. Also I need to add my PH is a little high. I am using the dip sticks to test everything.

Everything I read said if you put too much chlorine the negative was the time it would take to swim before the levels came back down to normal. I figured better go with more than not solve the problem. Two nights ago I put 8 gallons of liquid chlorine in the pool and one of those half gallon containers of granuals. I did think right after that I started to see the free chlorine tab start to turn color a little but I was not sure.

I just checked this AM and I STILL have zero free chlorine. I am at a loss. The water is perfectly clear. How do I get free chlorine in this thing??

#2 chem geek

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Posted 02 June 2011 - 09:28 PM

What kind of chlorine test kit are you using? If it's a DPD chlorine test where you compare shades of pink/red against a comparator, then you might have too much chlorine since very high chlorine levels will bleach out the test. If you have an OTO chlorine test where you compare shades of yellow against a comparator, then that won't bleach out so would mean you truly have no chlorine.

If you truly have no chlorine after adding it, then you might have had bacteria convert the Cyanuric Acid (CYA) in your pool into ammonia. That can take a lot of chlorine to get rid of. You would just keep adding chlorine until it holds. You can do a test using a bucket of pool water to approximate how much chlorine you'll need to add before it holds. 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC. You can read about this CYA-to-ammonia conversion technically in this post and you can read about my personal experience with this in this thread.

#3 sharkypoolpro

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Posted 05 June 2011 - 06:27 AM

i'm afraid you way over-chlorinated your pool. you are very likely 'bleaching out' your test sample. 15 gallons of chlorine usually last a few weeks with heavy swimming at my pools. what test kit are you using? if it's a taylor kit, there are instructions in the lid on how to dilute your sample to get higher readings. i have used 7.5 gallons to fire up a city pool before that had about 100k gallons. did your pool store recommend that much?...

a couple other things: liquid chlorine has a ph of about 11. that is about 10000 times more alkaline than you want your water. you will wisely test your ph and get it back in line in addition to getting your chlorine brought back into range before you let anyone in there and before it starts ruining things.

and to directly answer your question, 'free chlorine' is the chlorine that is available for sanitation. chlorine is free immediately when it is added; there is no waiting. used up or 'unavailable' chlorine becomes that way once it has attacked an impurity. it then is called a combined chlorine molecule, or a chloraMINE.

#4 waterbear

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Posted 05 June 2011 - 07:40 AM

a couple other things: liquid chlorine has a ph of about 11. that is about 10000 times more alkaline than you want your water. you will wisely test your ph and get it back in line in addition to getting your chlorine brought back into range before you let anyone in there and before it starts ruining things.



Liquid chlorine (and in fact, the other unstabilized chlorines--lithium hypochlorite and calcium hypochlorite) while alkaline when added to the water become acidic when they are 'used up' as an oxidizer and sanitizer so their net effect on pool water is basically pH neutral. This is a common misconception that they have an alkaline effect on the water.

pH rise when using unstabilized chlorine is primarily from outgassing of CO2 from the water which can be minimized by keeping the TA low (around 80 ppm).

This is why the recommended TA for unstabilized chlorine is 80-100 ppm and for stabilized chlorine (both trichlor and dichlor), MPS, DBDMH and BCDMH (which are all net acidic on usage) is 100-120 as normallhy taught in CPO courses.
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#5 waterbear

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Posted 05 June 2011 - 07:52 AM

I just checked this AM and I STILL have zero free chlorine. I am at a loss. The water is perfectly clear. How do I get free chlorine in this thing??




First we have to determine if there is truely NO chlorine in the water or if there is too much and it is bleaching out your test.
Get yourself a cheap two way test kit that has a pH test and a chlorine test that has shades of yellow on the comparator. This is called an OTO chlorine test and you can find it at Walmart, Home Depot, Lowes, many hardware stores if they sell pool supplies, and most pool supply stores. This particular chlorine test only tests total (free + combined) chlorine BUT does not bleach out (like strips or DPD testing) so it is bulletproof, so to speak.
Test the water and if it stays colorless you have NO chlorine in the water and your chlorine is either being burned off by sunlight or you have some sort of organic load that is consuming the chlorine.
If it turns yellow, orange, reddish or brown, then you have chlorine, possibly way too much! It can also all be in the form of combined chlorine which means there is free chlorine.
Post what color the OTO test turns, tell us how you are testing FC (DPD test-comparator has shades of red), FAS-DPD (drop counting test with color change from red to colroless) test, or strips (not to be trusted) and we can take it from there!
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#6 chem geek

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Posted 05 June 2011 - 10:06 AM

a couple other things: liquid chlorine has a ph of about 11. that is about 10000 times more alkaline than you want your water. you will wisely test your ph and get it back in line in addition to getting your chlorine brought back into range before you let anyone in there and before it starts ruining things.

For technical details about waterbear's explanation for chlorine usage/consumption being acidic, see this post. There are tens of thousands of pool owners on multiple pool forums using chorinating liquid or bleach without having significant pH rise in their pools because we understand that TA is a SOURCE of rising pH due to carbon dioxide outgassing -- I am one of them.

#7 sharkypoolpro

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Posted 06 June 2011 - 05:10 AM

interesting. i haven't studied this stuff as much as i should have, just experienced it! why are we constantly adding acid to liquid chlorinated pools to keep the Ph in check?

#8 sharkypoolpro

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Posted 06 June 2011 - 05:12 AM

ok i will check the link. i didn't see that last post before i put mine...

#9 sharkypoolpro

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Posted 06 June 2011 - 05:50 AM

k just read your article. nice work. i tried to email you this direct but couldn't find a way to contact individually so my response will have to go here. and yes, this is still related to the original post and question.

your article was about the effects of cya in the water under different conditions and levels, etc. i only read it once, so obviously i missed some things, but basically that's what it was about. the only part that talked about Ph was the short paragraph that discussed the outgassing of CO2, but it didn't discuss the actual ph of liquid chlorine. so i still am of the opinion that when this member of the forum added 8 gallons of an above-11 ph chemical to what was probably a 7.5 ph body of water, they will definitely have a portion of the pool (the spot where the liquid was poured in) that is instantly way too alkaline for a human to get into(10000 times too alkaline). yes there are other factors too, like the granular chlorine they also put in, so at that spot they will have a too-low-ph area. and once all this is circulated together, the overall ph of that pool is going to be well above the 8.0 that our regular test kits will read accurately.

i don't think there's any way in the work a residential pool has so many contaminants in it that all of the liquid he put in is gone already (after only a day or two). his pool probably has a fairly high CYA level because he has been buying at the local pool store (and likely putting in tabs). so last year's residual CYA would have been plenty of protection for the free chlorine that was inserted this spring. i still think you are bleaching out. in fact i just re-read his initial post. he put in shock (a couple of 1lb bags, i assume) then put in 8 gal liquid, then prob about 5-8 pounds granular (dichlor?). all into a 20k gal pool. it's just been way way way over chlorinated. better go pull $100 out from under your mattress because you are buying your wife a new suit if she got in...

(i would love to know more about the outgassing and the way to keep the ph in check without having to dose a lot of acid, if that's what you were saying.

#10 waterbear

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Posted 06 June 2011 - 08:57 AM

i would love to know more about the outgassing and the way to keep the ph in check without having to dose a lot of acid, if that's what you were saying.


interesting. i haven't studied this stuff as much as i should have, just experienced it! why are we constantly adding acid to liquid chlorinated pools to keep the Ph in check?


Simple, the TA is too high. Drop it to about 70-80 ppm (and adjsut calcium up to maintain calcium saturation balance) and your acid demand will go down. Also, keep the pH at 7.6 and lower it when it rises to 7.8. The lower you put the pH the FASTER it will rise (because you convert more bicarbonate ions, which is what TA really is, into carbonic acid (carbon dioxide in the water) so it outgasses faster. The result of this is less carbonic acid in the water. Operative word here is acid. Less acid in the water= pH rise.

Also, adding borates to 50 ppm adds a secondary pH buffer that ,unlike the bicarbonate buffer in the pool that is better at keeping pH from dropping, is effective at slowing pH rise. The borates are also an effective algaestat. (commercial borate products include Proteam Supreme and Supreme Plus, Bioguard Optimizer Plus, Guardex Maximizer, and Pool Life Endure. However, generic borax (sodium tetreaborare) or boric acid and also be used if you know how.)


We are assuming that there is no acid demand from new plaster, which is a different source of pH rise and not really a normal water balance issue since it is not a permanent problem.

Next step is learning how to properly lower TAPosted Image
(particularly if you are of the camp that still believe that you lower TA by slugging acid and lower pH by walking acid even though this particular myth has been debunked for at least 15 years now but refuses to die!)
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#11 chem geek

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Posted 06 June 2011 - 09:46 AM

Also, just to note that the pH DOES rise when you add a hypochlorite source of chlorine, but then it falls back down when that chlorine gets used up. So it essentially rises and falls with the FC level when using that type of chlorine. Any net pH rise when comparing the same FC levels (i.e. pH rise over time) is usually due to carbon dioxide outgassing from the TA being too high. There is also some "excess lye" in hypochlorite sources of chlorine but that is usually fairly small so would only show up in high demand pools (such as commercial/public pools) where the chlorine volumes are large. And as waterbear pointed out, there can be other sources of pH rise such as from the curing of plaster. Also, saltwater chlorine generator pools may have undissolved chlorine gas outgassing that is another source of pH rise.

Also note that increased aeration increases the rate of carbon dioxide outgassing, but if the TA is low enough then that won't matter so much. Every pool has a "sweet spot" where the TA is low enough for fairly decent pH stability. You want to get the TA low enough to minimize the pH rise from carbon dioxide outgassing. Then, any acid you need to add would only be from the other sources of rising pH as noted above.




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