Breakpoint chlorination

[mikefamig]

I understand breakpoint chlorination to be the free chlorine level at which you break up the combined chlorine molecules in the water and release them into the atmosphere. I have seen breakpoint defined as 10x combined chlorine and I have seen it defined as 100x CC and I have seen it defined here as 40% or greater of your cya level.

My CYA is at ~55 and I am calling it 60. I would like it a little lower but I am using liquid chlorine and it should come down eventually via rain dilution. It may take until next spring but I'm ok with that.

Now to my question. I have observed the CC level in my water go from ~0.2ppm to 0.0ppm to ~0.2 to 0.0 and back and forth while maintaining a FC between 7ppm and 9ppm. This would seem to say that 7-9 ppm is achieving breakpoint chlorination. This would also suggest that 100x CC is not necessary and 40% cya is also not necessary to achieve breakpoint.

So how am I getting CC to go from 0.2 to 0.0 with FC level as low as 7-9ppm? What exactly is breakoint?

Mike.

 

[jblizzle]

There is really no such thing as breakpoint chlorination. Search the forum and you will find many threads discussing it.

You can completely ignore any CC up to 1 drop in the test ... nothing to worry about and totally normal.

I would also recommend you use 10ml for the FAS-DPD chlorine test so that each drop is 0.5ppm.

 

[Richard320]

In my pool, CC comes and goes. I've had .5 CC every time I test for weeks and then suddenly it goes away and doesn't come back for a month or more. Don't obsess on it. Follow Jason's advice -- 10 ml sample and if CC goes away with one drop, you're good.

 

[pookiesunshine]

were you reading this?
Poolcenter

from the article, at 10x .2 cc, bp would be 2ppm FC; plus 60% for 60 cya would be 3 to 3.5 ppm FC.

the 40% of cya here is the recommended SLAM level and is not based on the amount of CC. remember that algae doesnt become CC until it is oxidized.

i dont really see the need to worry as long as you have less than 0.5 CC and a clear pool. your target of 7 to 9 FC seems to be working, yes?

am i missing your point?

 

[jblizzle]

Public Service Announcement: Do not even go read that article ... there is no point and it may cause confusion.

 

[BasicTek]

[FONT=HelveticaNeue-Light, Helvetica Neue Light, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif] From the article "[/FONT]The generally accepted formula[FONT=HelveticaNeue-Light, Helvetica Neue Light, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif] for breakpoint chlorination" is this like the generally accepted rule that you have to shock your pool every week, and the generally accepted belief that phosphates cause algae not improperly maintained CYA/FC. I know it's generally accepted that pool store testing is correct....... [/FONT]

 

[pookiesunshine]

uh, as roseanne rosannadanna would say, nevermind.

 

[JoyfulNoise]

From the following thread by chem geek - Certified Pool Operator (CPO) training -- What is not taught

Breakpoint Chlorination
The 10x rule for breakpoint chlorination is wrong and only now some industry people are starting to address this as described here and here, though they are still only accounting for the mistake of not considering that combined chlorine already used up a chlorine atom in combining with ammonia and they are not yet considering that there is still a factor of 5 error in their approach since the units used to derive the 10x rule come from chlorine (measured in ppm Cl2) oxidation of ammonia (measured in ppm N) whereas combined chlorine is measured in the same units as chlorine (i.e. ppm Cl2) where the Cl2 units are 5 times higher than the N units (that's the molecular weight ratio between the two). The oxidation of a combined chlorine that is monochlorourea (i.e. chlorine combined with urea) may require 3x, but that still far from 10x. There also isn't any getting "stuck" -- one can just add more chlorine. When one has persistent CC, it is not due to getting "stuck" but from compounds that do not oxidize as quickly. More technical details about this are described in this post.

The handbook has a newer breakpoint chlorination rule, but it is still a "10x" rule, though uses a target FC as 10x of the CC rather than using an incrementally added 10x amount. It incorrectly states that adding less than the breakpoint calculated amount may not achieve breakpoint, not recognizing that the 10x rule is wrong when starting with CC (it's correct when starting with ammonia measured in ppm N units).

 

[JoyfulNoise]

As well, the 40% of CYA for shock levels has nothing to do with CCs. It's based on the 3-log CT kill times for algae. Algae can vary a lot by species and there's no way to know exactly what type of algae a pool will develop without knowing a lot of details about the nutrient levels in the water but the shock level was chosen such that there is enough active chlorine in the water to kill algae at a much faster rate than its colony doubling time (~4-6 hours). This ensures that an algae bloom can be effectively stopped and reversed within a short period of time (24-48 hours) but not so high as to be dangerous to pool equipment, surfaces or people.

 

[mikefamig]

I am not saying that I have a problem with my water. All is good.

I just want to learn what does it take to break up a CC combined molecule. I understood that it took shock/breakpoint level FC to release chloramines from the water but I'm seeing them disappear at much lower than shock levels.

What I'm seeing appears to prove the 10x CC rule as 10 x 0.2 = 2ppm. I am certainly not raising FC to 40% of my 60ppm cya level.

Mike.

 

[jblizzle]

What you are failing to realize is the the SUN breaks down the CC too.

 

[mikefamig]

What you are failing to realize is the the SUN breaks down the CC too.

That's what I was looking for. I thought that it had to be under shock or breakpoint FC level. Thanks!

 

[JoyfulNoise]

The thing to understand is that in chemistry, reactions are not binary with respect to concentration (reaction does happen / reaction does not happen). All of these chemical reactions are subject 1st order reaction kinetics which simply means that the rate at which chlorine oxidizes chloramines is proportional to the concentration of FC and the concentration of CCs. Therefore, the oxidation of CCs is always happening, the rate at which happens depends on concentration. This is why the idea that a "breakpoint" occurs is wrong - CCs are always oxidized and how fast that reaction happens depends on how much FC you're willing to put into the water. The presence of CYA also greatly modifies the mix of CC's that will be created. When CYA is low or zero, the chlorine oxidation reactions are very fast and will tend to favor the formation of very irritating nitrogen trichloride. This why public pools often smell so harsh and bad - the CCs created are some of the most irritating chemicals known. When CYA is present to buffer active chlorine levels, then the formation of monochloramine is favored (monochloramine is much less irritating and harmful). UV from the sun helps to increase the clearing of CCs through oxidation and those reactions are also first-order.

The amount of FC in the water simply dictates how fast most of these reactions will occur.

 

[mikefamig]

The thing to understand is that in chemistry, reactions are not binary with respect to concentration (reaction does happen / reaction does not happen). All of these chemical reactions are subject 1st order reaction kinetics which simply means that the rate at which chlorine oxidizes chloramines is proportional to the concentration of FC and the concentration of CCs. Therefore, the oxidation of CCs is always happening, the rate at which happens depends on concentration. This is why the idea that a "breakpoint" occurs is wrong - CCs are always oxidized and how fast that reaction happens depends on how much FC you're willing to put into the water. The presence of CYA also greatly modifies the mix of CC's that will be created. When CYA is low or zero, the chlorine oxidation reactions are very fast and will tend to favor the formation of very irritating nitrogen trichloride. This why public pools often smell so harsh and bad - the CCs created are some of the most irritating chemicals known. When CYA is present to buffer active chlorine levels, then the formation of monochloramine is favored (monochloramine is much less irritating and harmful). UV from the sun helps to increase the clearing of CCs through oxidation and those reactions are also first-order.

The amount of FC in the water simply dictates how fast most of these reactions will occur.

Thanks that was very helpful and very well written. I did have the idea that it was a "binary" reaction which is what caused my confusion.

Mike.

 

[JeanZ1]

I think you're confusing breakpoint chlorination of chorAMINATED water vs "breakpoint" chlorination of the chlorine/cyanuric acid complex.

 

[mikefamig]

I think you're confusing breakpoint chlorination of chorAMINATED water vs "breakpoint" chlorination of the chlorine/cyanuric acid complex.

No, I understand the difference.

 

[mikefamig]

The thing to understand is that in chemistry, reactions are not binary with respect to concentration (reaction does happen / reaction does not happen). All of these chemical reactions are subject 1st order reaction kinetics which simply means that the rate at which chlorine oxidizes chloramines is proportional to the concentration of FC and the concentration of CCs. Therefore, the oxidation of CCs is always happening, the rate at which happens depends on concentration. This is why the idea that a "breakpoint" occurs is wrong - CCs are always oxidized and how fast that reaction happens depends on how much FC you're willing to put into the water. The presence of CYA also greatly modifies the mix of CC's that will be created. When CYA is low or zero, the chlorine oxidation reactions are very fast and will tend to favor the formation of very irritating nitrogen trichloride. This why public pools often smell so harsh and bad - the CCs created are some of the most irritating chemicals known. When CYA is present to buffer active chlorine levels, then the formation of monochloramine is favored (monochloramine is much less irritating and harmful). UV from the sun helps to increase the clearing of CCs through oxidation and those reactions are also first-order.

The amount of FC in the water simply dictates how fast most of these reactions will occur.

In light of the fact that chemical reactions are not binary and that CC is always being created and released into the air as long as you maintain enough FC to combine with organic materials then what is the significance of the "shock" level of FC that is stated in the chlorine/FC chart? What happens at that point that is unique or significant?

Pool School - Chlorine / CYA Chart

Mike.

 

[pabeader]

That is for killing off an algae bloom. As long as you maintain the FC/CYA relationship at the proper level, you don't need that column.

We use a process called SLAM where you maintain a higher FC level for an extended period of time to kill off and clean a green, swampy pool.

 

[JoyfulNoise]

In light of the fact that chemical reactions are not binary and that CC is always being created and released into the air as long as you maintain enough FC to combine with organic materials then what is the significance of the "shock" level of FC that is stated in the chlorine/FC chart? What happens at that point that is unique or significant?

Pool School - Chlorine / CYA Chart

Mike.

That is for killing off an algae bloom. As long as you maintain the FC/CYA relationship at the proper level, you don't need that column.

We use a process called SLAM where you maintain a higher FC level for an extended period of time to kill off and clean a green, swampy pool.

Bob is correct.

The 40% FC/CYA ratio for "shock" level chlorination is designed solely to achieve an algae kill time that greatly outpaces the colony doubling times (about 4-6 hours for many types of green and blue-green algae). This ensures that a green pool can be brought under control in a reasonable period of time (24-48 hours) without doing any damage to liners (typically, medium blue dyes in liners are the most susceptible to bleaching effects of chlorine).

CC's will form and be eliminated at any level of chlorine. The pool industry limits on CCs are not rational as treatment levels are set such that anything above 200ppb (0.2ppm) requires a chemical addition. This is simply far too low and causes the pool owner to be pushed into purchasing additional chemicals (often non-chlorine shock) that are unnecessary and costly. When one considers that the EPA allows for up to 4ppm of monochloramine in drinking water as OK, trying to drive down CCs in a swimming pool at 0.2ppm is just silly. Perhaps if one is trying to control for the production of nitrogen trichloride, then a 0.2ppm level makes sense, but that only happens in swimming pools with no CYA and high levels of chlorine and bather waste....not conditions one finds in a residential, outdoor pool.

 

[mikefamig]

That is for killing off an algae bloom. As long as you maintain the FC/CYA relationship at the proper level, you don't need that column.

We use a process called SLAM where you maintain a higher FC level for an extended period of time to kill off and clean a green, swampy pool.

I maintain FC=7-9 with a cya of 50 - 60 and still get the beginning of visible algae in low circulation areas of the pool even with 0.0 CC. I just brush, raise the chlorine to a little over 10ppm and run the filter for 24 hours and the algae doesn't come back for a few days. I don't believe that I need to raise the FC to 24 each time I see algae.

The 24ppm shock level at 60cya seems to be an ambiguous number to me. What is it derived from or how do we come to that particular number considering that our FC is always killing algae at any level. I seem to be able to control the algae at much lower FC level and I don't have to wait for very long before the FC drops to a swimmable level.

Mike.