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Thread: Question on combined chlorine

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    eemueller's Avatar
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    Question on combined chlorine

    For the last couple of weeks, I've been getting an indication of combined chlorine when I do my drop test. The free chlorine was at 4.5 today. It only takes 1 drop of the FAS/DPD reagent to return the sample to clear, so that's telling me that there is .5 combined chlorine. Is this a problem? I've never had it before. I'm wondering if I should shock to pool to eliminate it?
    32K gal rectangular pebble sheen IG salt pool, 1200 gal pebble sheen IG salt spa
    Jandy AquaPure 1400 SWG, Jandy AquaPure 700 SWG
    Sta-Rite 2HP pump, Sta-Rite 1HP pump
    Triton II sand filters on both, Pentair MasterTemp 400K BTU nat gas heater
    CAT 1000 pH controllers w/ Stenner acid pumps on both, Levolor auto-fillers on both

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    Azgirl71's Avatar
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    Re: Question on combined chlorine

    .5 is acceptable.
    Lisa
    Intex AG vinyl 15' x 48" 4,300 gallons due to skimmer install, Krystal Clear SF pump combo 2800 GPH, Krystal Clear SWG, Pool Blaster Max vacuum, Hayward thru the wall skimmer, TF 100 test kit with speed stir, K 1766 Taylor salt test, My SLAM Process

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    Mod Squad JVTrain's Avatar
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    Re: Question on combined chlorine

    0.5 is acceptable and can be a false reading depending on if you added one drop extra of R-0871 that produced no color change for the FC test or if you waited too long before adding the R-0003 reagent.
    Joel - TFP Moderator - Minnesota - **Become a TFP Supporter!** Helpful Links: ABCs of Pool Water Chemistry - SLAM Procedure - Chlorine/CYA Chart
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    Mod Squad jblizzle's Avatar
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    Re: Question on combined chlorine

    Any more than 1 drop for the CC test is cause to start to be concerned.
    Jason, TFP Moderator
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    Re: Question on combined chlorine

    Also, the resolution of the test is only one drop (or 10%, whichever is greater) so going clear after one drop means <= 0.5 ppm. It does not mean you have 0.5 ppm CC. If you really wanted to know with more precision what you have, then use a 25 ml water sample instead of a 10 ml water sample. You add a heaping spoon or two level spoons of DPD powder, still use 5 drops of the R-0003, but each FAS R-0871 drop represents 0.2 ppm FC or CC instead of 0.5 ppm. I'll bet that your sample turns colorless after just one drop meaning you have <= 0.2 ppm CC.

    In an outdoor residential pool exposed to sunlight, CC is rarely a concern. It's really used more as an indicator that something may be wrong and as a hint that there may be ammonia or organics in the water that are not getting oxidized fast enough though even then only certain types of chemicals will show up with intermediates that last long enough to measure as CC. You can have zero CC and still have an unusual chlorine demand. So of all of our measurements, CC is probably the least critical and the absence of CC doesn't mean everything is AOK. Even when CC is present, it is rare to have someone notice it (i.e. smell it) unless it's very high or they have a serious water problem such as an algae swamp.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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    eemueller's Avatar
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    Re: Question on combined chlorine

    Thank you all for your responses. My concerns are allayed. Just out of curiosity, I'm going to do the test with the 25 ml sample.
    32K gal rectangular pebble sheen IG salt pool, 1200 gal pebble sheen IG salt spa
    Jandy AquaPure 1400 SWG, Jandy AquaPure 700 SWG
    Sta-Rite 2HP pump, Sta-Rite 1HP pump
    Triton II sand filters on both, Pentair MasterTemp 400K BTU nat gas heater
    CAT 1000 pH controllers w/ Stenner acid pumps on both, Levolor auto-fillers on both

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