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Thread: Questions about OTO chlorine testing and total / active / free chlorine ...

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    Questions about OTO chlorine testing and total / active / free chlorine ...

    I have a few unrelated questions that all deal with chlorine:

    1- To get a more accurate total chlorine reading via an OTO test, can I dilute my test water with distilled water? For example, if I dilute it at a 1:5 ratio and the diluted sample shows 2.0 ppm in my test, my total chlorine level is 10.0, correct?

    2- Since CYA binds 97% of the chlorine in the pool, does it release some of its bound chlorine when the active chlorine (or free chlorine) level drops? In other words, is there a chemical equilibrium that comes into play here? Does CYA-bound chlorine effectively act as a "chlorine reservoir" to replenish the active chlorine as it is used up by the sun or while sanitizing / oxidizing organics?

    3- What's the difference between Free Chlorine and Active Chlorine? Are they the same thing? Are they both measured in ppm?
    Intex 10' vinyl above-ground pool, only 1200 gallons, with 55-gallon gravity-flow sand filter, shaded by a tree in the morning then exposed to full sun all afternoon

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    JoyfulNoise's Avatar
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    Answers -

    1. Yes you can dilute but the accuracy of the test decreases with increased dilution.

    2. See this Thread, it has a very wide-ranging discussion of the question you asked.

    3. FC and Active Chlorine, for all intents and purposes are the same but different definitions have been used in the past. Free chlorine, nowadays, not only includes hypochlorous acid, but also the chlorine bound to CYA which is technically available for disinfection as the hypochlorous acid gets used up. The FC portion of the DPD-FAS test measures both. Active chlorine typically refers to hypochlorous acid alone but a lot of people use the term interchangeably with free chlorine. For the pool user, you can just treat them the same. If you were writing a peer-reviewed, scientific paper in a water chemistry journal then I would suggest being careful with usage. If you are just posting test results from your test kit, please use the term "Free Chlorine (FC)" as that is what everyone understands.
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    The OTO test measures the chlorine bound to CYA as well as the unbound chlorine. It also measures combined chlorine which is why it is said that it measures Total Chlorine.

    I usually use the term "active chlorine" to refer to hypochlorous acid since that is the chemical that is responsible for killing pathogens and algae. As noted above, FC these days means the sum of unbound and bound chlorine since it is all released from CYA in the time of the test (half is released from CYA every 1/4th of a second if the hypochlorous acid where used up at least that quickly).
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    Free chlorine includes hypochlorous acid and the chlorine bound to CYA. FC is all released from CYA in the time of the test (half is released from CYA every 1/4th of a second if the hypochlorous acid were used up at least that quickly).
    So for all practical purposes the time required for CYA to release its bound chlorine is negligible. This explains a lot, thanks.

    The FC portion of the DPD-FAS test measures hypochlorous acid and the chlorine bound to CYA. The OTO test measures both of these and also Combined Chlorine -- which is why it is said that OTO measures Total Chlorine.
    This suggests that when Combined Chlorine is zero both results should be the same. And normally, in a properly maintained pool, Combined Chlorine should be zero, correct? This brings up the questions: How long does Combined Chlorine exist? And how does it leave the pool? Does it disassociate into other molecules? Does it precipitate and then get filtered out? Or is there another mechanism involved in its removal from the pool?

    Active chlorine typically refers to hypochlorous acid alone but a lot of people use the term interchangeably with free chlorine. For the pool user, you can just treat them the same. If you were writing a peer-reviewed, scientific paper in a water chemistry journal then I would suggest being careful with usage. If you are just posting test results from your test kit, please use the term "Free Chlorine (FC)" as that is what everyone understands.
    Got it. Thanks JoyfulNoise. I won't be writing a scientific paper any time soon ...

    I usually use the term "active chlorine" to refer to hypochlorous acid since that is the chemical that is responsible for killing pathogens and algae.
    Thanks for clarifying chem geek.
    Intex 10' vinyl above-ground pool, only 1200 gallons, with 55-gallon gravity-flow sand filter, shaded by a tree in the morning then exposed to full sun all afternoon

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    JoyfulNoise's Avatar
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    Questions about OTO chlorine testing and total / active / free chlorine ...

    Quote Originally Posted by owkaye View Post
    This suggests that when Combined Chlorine is zero both results should be the same. And normally, in a properly maintained pool, Combined Chlorine should be zero, correct? This brings up the questions: How long does Combined Chlorine exist? And how does it leave the pool? Does it disassociate into other molecules? Does it precipitate and then get filtered out? Or is there another mechanism involved in its removal from the pool?
    Technical discussion is HERE.



    Matt
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    Quote Originally Posted by owkaye View Post
    This suggests that when Combined Chlorine is zero both results should be the same. And normally, in a properly maintained pool, Combined Chlorine should be zero, correct? This brings up the questions: How long does Combined Chlorine exist? And how does it leave the pool? Does it disassociate into other molecules? Does it precipitate and then get filtered out? Or is there another mechanism involved in its removal from the pool?
    Normally CC is close to zero in an outdoor residential pool exposed to at least some sunlight. The link JoyfulNoise gave shows technical details of how chlorine oxidizes CC, but in addition sunlight breaks down chlorine into hydroxyl radicals that are very short-lived but powerful oxidizers and these may help oxidize urea from sweat and urine that would otherwise be slow to oxidize by chlorine alone.

    Generally speaking, ammonia and nitrogenous organics get oxidized into nitrogen gas and carbon dioxide or get partially oxidized. Some of the smaller end-product molecules are volatile and outgas. There is also some nitrate that is formed and that stays in the pool similar to salt (it's an algae nutrient but with sufficient FC/CYA levels algae growth is prevented regardless of algae nutrient level).
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    [I know this thread is a couple weeks old, but I didn't see this mentioned anywhere above, and the math matters {this coming from a total math-o-phobe } ]
    So....

    When you are working with dilutions, you have to multiply your result by the total number of "parts" used to create the diluted sample.

    So in your example in the first post: a 1:5 dilution means you would multiply your result (2) by the total number of parts (1+5=6) so your result would be (2 x 6=12).

    Obviously, this is an important distinction. I made myself remember this "rule" by reminding myself that I have to account for every part of every thing that I am using in my test. So if I mix up a dilution of 1 part pool H2O and 2 parts distilled H2O, then I have to account for all 3 parts of H2O in my calculations.

    Hopefully this will still be helpful to someone...😀


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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    1 to 5 dilution means one part of pool water and four parts of tap or distilled water, so you multiply by five, hope this help
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    Quote Originally Posted by Piolin View Post
    1 to 5 dilution means one part of pool water and four parts of tap or distilled water, so you multiply by five, hope this help
    This is technically true. Where 1:5 means more like 1 in 5 so you have a total of 5 parts.
    Which seems a little odd if you are thinking 1 part to 5 parts which would be 6 parts.

    Regardless of the terminology, what April says is correct. You need to multiple by the total number of parts.
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    Giving examples, as April did, always helps. Some people use 1:5 to mean 1 part A and 5 parts B; others use 1:5 to mean 1 part A in 5 parts total (i.e. only 4 parts B). Even if one usage is technically correct and the other isn't, there is still confusion among the largely non-technical population on the forum, which is why examples are great.
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    And now I understand that. It's been driving me crazy and I didn't even know why!!
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    Well, a 1:1 dilution is a 50/50 mixture so diluting by one-half.
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    oh no! now I'm confused again? No wait I'm okay still. 1:1 is 1 part in 2 so multiply by 2. 1:5 is 1 part in 6 so multiply by 6.
    Right?
    Bob - Palm Beach by San Juan Pools. approx 5000 gals., Pentair 320 cartridge filter (all new guts installed by me), Goldline SWG, 'New to me' Kreepy Krauly Sand Shark, Intermec 104 Timer Test kit: TF-100 w/Speed Stir

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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    Yes, that is correct. The x:y is x parts of one thing and y parts of another with the total number of parts being x+y. The multiplier is technically (x+y)/x if "x" is what is getting diluted by "y" (so a 2:3 dilution would have a multiplier of 5/2 = 2.5) but since we are always using x=1 then this becomes y+1. So 1:5 is 1+5=6 total parts so multiply by 6.

    See this link and this link for more info. The latter link gives a bunch of worded examples that mean the same thing, but illustrate the confusion of using different terms. Namely, 1:9 and 1 part to 9 parts is one way to describe it (where the total number of parts is 1+9=10), but a 1 to 10 dilution or a 1/10th dilution is another way to describe the very same thing.
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    Re: Questions about OTO chlorine testing and total / active / free chlorine ...

    Now we need a new member named "math geek" lol
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