Clarification of "tied up" vs "Free"?

My simple understanding of pool chemistry is that the FAS DPD measurement for chlorine measures
-Total Chlorine (TC) is a total of all chlorine in the pool.
-Free Chlorine (FC) is the remaining chlorine left in the pool to do its job.

-Combined chlorine is the difference between the two and is the chlorine which is already done its job and is attached to the dead stuff.

So far this makes sense to me.

However, there is also chlorine "tied up" in the CYA.

This may be an issue of semantics, but is the "tied up" chlorine detected in both the Free Chlorine and Total Chlorine measurements.

FAS-DPD measures FC when the water turns from pink to clear. You then add R003 and if it turns pink again, you have CC (Combined Chlorine-Bad). Add R0871 until clear again and this is your CC in ppm. OTO measures only TC (total chlorine) but FC+CC always=TC.
The "Tied Up" chlorine you are speaking of will measure as FC and TC, but it is not as effective with higher CYA hence the increase in FC required with higher CYA. Hope I'm not confusing you more.

Dave

I thought "tied up" chlorine was the same as "combined chlorine."

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Originally Posted by Donna's Poolboy

I thought "tied up" chlorine was the same as "combined chlorine."

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You are right, the term does seem to imply "Combined" however his statement read "However, there is also chlorine "tied up" in the CYA" so, trying to keep the comparison of apples to apples per se, I followed suit. Sorry for the confusion. :o I would always refer to CC as Combined since technically it is not "tied" up but "used" up.

Dave

chlorioisocyanurate will test as FC from my understaning. They ARE a form of 'combined chlorine' (I have seen them referred to as 'combined chloramides'. They are not what we consider CC normally.

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Originally Posted by waterbear

chlorioisocyanurate will test as FC from my understaning. They ARE a form of 'combined chlorine' (I have seen them referred to as 'combined chloramides'. They are not what we consider CC normally.

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Evan is correct. The chlorine tests consume the free chlorine (HOCl) and this gets filled in immediatly from OCl- and quickly from the chlorinated cyanurates (the chlorine tied up in CYA) though the half-life for the release of chlorine from one of the cyanurates (HClCY-) is 4 seconds and is 1/4 second for another (ClCY(2-)) which means it's not instantaneous. This has implications for the OTO chlorine test since it relies on an immediate color measurement for free chlorine and waiting for a combined chlorine reading so the relatively slow release of chlorine from CYA could make that test even less reliable than it already is.:p The DPD and FAS-DPD tests do not have this problem since it probably takes about 10 seconds or so to add drops, do mixing, etc. before you start the measuring part of the test.

You can think of the Free Chlorine test as measuring chlorine that is "available" for disinfection even if most of it isn't in disinfecting form but is stored in other forms and will get converted "as needed". So when you measure Free Chlorine, you are mostly measuring the "reserve" of chlorine and not its absolute immediate disinfecting capability. The actual amount of disinfecting chlorine (HOCl) at various levels of Free Chlorine and CYA are shown in the following chart (for a pH of 7.5): HOCl.htm
Ignore the coloring -- these are topics for discussion in The China Shop and are not ready for official use yet.

The Combined Chlorine test will measure all forms of chlorine so you get the Free Chlorine amounts as noted above PLUS the chlorine that is found (has combined with) other chemicals including ammonia (to form chloramines) and organics (to form chlorinated organics). Though the classic approach to removing the chloramines and chlorinated organics is through high "shock" levels of chlorine, in practice it is very hard to get rid of such compounds and it is believed that they (especially monochloramine) are often broken down in outdoor pools through sunlight (UV).

Richard

Hi, Richard,

from the post above:

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Though the classic approach to removing the chloramines and chlorinated organics is through high "shock" levels of chlorine, in practice it is very hard to get rid of such compounds and it is believed that they (especially monochloramine) are often broken down in outdoor pools through sunlight (UV).

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I'm a little confused by that. It seems to me that breakpoint (shock) DOES remove these on a practical basis and results in CC's testing at zero or close to that usually within 12 hours of shocking...day or night. Am I misinterpreting what you have written?

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Originally Posted by duraleigh

Hi, Richard,

from the post above:I'm a little confused by that. It seems to me that breakpoint (shock) DOES remove these on a practical basis and results in CC's testing at zero or close to that usually within 12 hours of shocking...day or night. Am I misinterpreting what you have written?

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No, you interpreted what I said correctly. Achieving breakpoint through super-chlorination (shock) works great in theory and controlled experimental conditions and may work reasonably well when you mostly just have monochloramines (not trichloramines or chlorinated organics) and have good air circulation above your pool, but experience has shown (Ben can tell you more and you can also hear this from several other sources including PPOA articles such as Part1 and Part2) that it is often hard to achieve.

In an indoor pool, it is very hard to get good air circulation while in an outdoor pool you are usually using CYA so that getting a high enough concentration of chlorine to achieve breakpoint is difficult. I do not know enough about the detailed chemistry to know if the 10:1 ratios normally quoted are only referring to the stochiometry (in which case CYA wouldn't matter) or if they are at least partly referring to reaction kinetics and the lifetime of complex intermediate chemical species (in which case CYA can make a difference by causing the actual concentration of HOCl to be quite low). [EDIT]From the PPOA article and from other articles I've seen written by water treatment districts, there are less disinfection by-products (DPBs), such as chlorinated organics, formed when the chlorination process is done at a higher pH. But again, I haven't seen the detailed chemistry for these reactions so can only go by such qualititive information. The use of a maintenance level of non-chlorine shock (such as potassium monopersulfate - MPS - trade-name Oxone from DuPoint and sold as many different brands) is supposed to oxidize organics (and ammonia) before chlorine gets a chance to do so and therefore may avoid the creation of DPBs. So it may make sense to use a maintenance level of MPS for indoor pools, though it's not as clear if this is needed in outdoor pools where sunlight may break down DPBs (I don't know if this is true).[END-EDIT]

Richard

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Originally Posted by DavidD

You are right, the term does seem to imply "Combined" however his statement read "However, there is also chlorine "tied up" in the CYA" so, trying to keep the comparison of apples to apples per se, I followed suit. Sorry for the confusion. :o I would always refer to CC as Combined since technically it is not "tied" up but "used" up.

Dave

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This thread has gotten a bit over my head and I'm having a hard time deciphering the chemistry.

I'm not sure if Dave was saying that Donna's PoolBoy was correct in saying "I thought "tied up" chlorine was the same as "combined chlorine."

or if I was correct to say there not the same.

In my KISS (Keep It Simple Stupid) level of chemistry, I'm confused about the following types of chlorine.
-Free Chlorine, stuff available to do its work
-Combined Chlorine, stuff that's attached to the dead stuff.
-I'm not sure about chlorine attached to the CYA which for lack of a different name I called "tied up" chlorine. My understanding is that this is buffer being stored for future work.

Since adding CYA, will increase the tied up chlorine, but doesn't seem affect CC, I figure that tied chlorine cannot be part of CC.

So I have 3 questions.
1. Is chlorine tied up in CYA a part of CC or not.
2. If they ar different, is there an actual name for the "tied up" chlorine?
3. If they are different, than the tied up chlorine must be measured in both TC and FC, or neither so as not to affect on CC. [TC-FC=CC]
Which is the case?

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Originally Posted by cschnurr

This thread has gotten a bit over my head as I'm not a chemist.
So I have 3 questions.
1. Is chlorine tied up in CYA a part of CC or not.
2. If they ar different, is there a better name for the "tied up"chlorine?
3. If they are different, than the tied up chlorine must be measured in both TC and FC, or neither so as not to affect on CC. [TC-FC=CC]
Which is the case?

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Sorry I got too technical for you. Some people want the technical details while others (most) do not.

1. Chlorine tied up in CYA is NOT part of calculated CC which is TC-FC.

2. How about "buffered chlorine" or "reserved chlorine"? Anyone else have a better suggestion? Whatever terminology that is used should convey the fact that the chlorine is available to do work but is not "ready" for work right now. It's kind of like the army reserves. They aren't on active duty so if there were an immediate attack they aren't available, but as active duty people get captured or killed then reserves are called up to fill in and become newly active duty people.

3. Yes, the chlorine tied up in CYA is measured in both FC and TC.

[EDIT]Another way to look at it is that chlorine tied up in CYA is measured along with untied chlorine as FC. Your test kit cannot distinguish beween chlorine tied up in CYA and chlorine that is not tied up (untied chlorine is both free and immediately available to disinfect and prevent algae).[END-EDIT]

Richard

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Originally Posted by cschnurr

I'm not sure if Dave was saying.....

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Well, I blew that now didn’t I?:o

Let’s try this, I’ll recap and take one last stab:

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-Free Chlorine, stuff available to do its work

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Correct

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-Combined Chlorine, stuff that's attached to the dead stuff.

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I think you should think of CC as used up chlorine or better yet, chlorine waste.

The chlorine that is "attached to the CYA” is measured as FC and TC. FC plus CC equals TC always. Just remember that it is less effective than it would be in it were not attached to the CYA.

Dave

Thanks for the clarification and interesting discussion.