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Thread: Show me the math

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    Show me the math

    Can someone show me the math to calculate how much a 3" trichlor tab gives in free chlorine. I've seen the "results" that tell me but I was curious about the calculation. I can figure out the CYA portion and how much it contributes in ppm, but I am missing something with the Cl and it is driving me nuts. It the overall pool care scheme of things, I don't need to know this level of detail, I just want to. So just for a fun question that the chemistry whiz out there loves to answer?????

    Also, I SLAMMED my pool last couple days. I slammed because I saw a few small deposits of algae on the steps. I've had no issue with water clarity or CCs to indicate algae starting and my Chlorine as never not dropped below levels needed for my CYA of 30. But obviously something was going on so I SLAMMED. My question is around the drop in chlorine from SLAM levels when done. After passing the 3 SLAM tests to say complete, I vacuumed out the little bit of algae that settled on bottom. FC was at 14 ppm this morning. Within 12 hours the Chlorine was down to 6 ppm. I was quite surprised at this rapid drop since my daily chlorine demand has been around 2-3 ppm. I'm in Memphis and it has been very hot this week, but 8 ppm in one day seemed high. Is it normal to have rapid drop in FC when your levels are high.
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    Casey's Avatar
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    Re: Show me the math

    The only math you're getting from me is PoolMath!

    If your water is getting really warm, the chlorine will dissipate faster than usual. If your CYA was a little higher, the FC would stick around longer too.
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    Re: Show me the math

    You can loose half your chlorine in a good hot sunny day depending on your stabilizer level. In other words if you start with 14ppm then you could be down to 7ppm by the end of the day. If you have 4ppm then you could be down to 2ppm. Raise your stablizer to 40ppm and if that dont suit you, raise it to 50ppm. Alot of people out west run 50ppm.

    The 3" tabs I have are 8 ounces. You put in 8 oz of trichlor in the effects of adding chemicals and read the results. I guess thats what your asking, not sure. Since you have trichlor tabs use those to raise your CYA. Your pH will likely go down if you have a TA lower than 100. According to your pool size, an 8oz trichlor tab will raise the CYA by 1.7ppm and will take 6 tabs to raise the CYA level 10ppm.
    Last edited by easttn; 06-19-2014 at 08:45 PM. Reason: Double Posted
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    Re: Show me the math

    Quote Originally Posted by curlytop09 View Post
    Can someone show me the math to calculate how much a 3" trichlor tab gives in free chlorine. I've seen the "results" that tell me but I was curious about the calculation. I can figure out the CYA portion and how much it contributes in ppm, but I am missing something with the Cl and it is driving me nuts. It the overall pool care scheme of things, I don't need to know this level of detail, I just want to. So just for a fun question that the chemistry whiz out there loves to answer?????
    There are some things you need to know to be able to understand the calculation. First is that chlorine (FC, CC, etc.) is measured in ppm Cl2 units so milligrams per liter of molecular chlorine which has a molecular weight of 2*35.453 = 70.906 grams/mole. Second is that 3" Trichlor tabs vary in their weight as 6, 7 or 8 ounces (I'll assume 8 ounces which is 226.796 grams for the calculation below) and Trichlor has a molecular weight of . Third, is that each molecule of Trichlor has 3 chlorine attached to it, each producing one molecule of hypochlorous acid when reacting with water. So for your 20,000 gallon pool which is 75,708 liters we have:

    (1 Trichlor puck) * (226.796 grams/puck) * (1000 milligrams/gram) * (3 * (70.906 g Cl2) / (232.41 g Trichlor)) / (75,708 liters) = 2.742 mg/L or ppm FC

    In the above I'm assuming 100% purity when Trichlor is usually 99% pure, but that's certainly close enough.
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    Re: Show me the math

    Thanks Chem Geek. I wasn't thinking of FC as Cl2. I was using same equation above but first I used only 3 Cl atoms. Then when that was too low, I thought that using the HOCl formed when dissolved in water would give me right answer. Still came up short. From an equation balance standpoint, I'm still not sure how Cl2 is used above when the MW of trichlor has only 3Cls in them, not 6.

    Casey, I'll probably just stick to the pool calculator from now on. I wasted too much time yesterday trying to figure this out when I could have been swimming

    And thanks for the rough idea of FC loss on hot days. I do plan to increase my CyA as suggested above but was saving it for when I go on vacation and have to use the tablets.
    20K IG gunite
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    Re: Show me the math

    Here is a chart so you will better understand the FC loss to CYA percentage.
    http://www.troublefreepool.com/threa...l=1#post403956
    Thanks to ChemGeek, that is how I knew. I wanted to post that on my first reply but I couldnt find it till now. That chart should really be in the pool school somewhere in an advanced section instead of buried in the archives.
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    Re: Show me the math

    Had you done any brushing during the SLAM? Disturbing the algae on the step could probably account for the CL loss, I would guess.
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    Re: Show me the math

    Quote Originally Posted by curlytop09 View Post
    From an equation balance standpoint, I'm still not sure how Cl2 is used above when the MW of trichlor has only 3Cls in them, not 6.
    It's because the ppm Cl2 is a reference equivalent and when chlorine gas dissolves in water it only produces ONE hypochlorous acid, not two:

    Cl2(g) + H2O ---> HOCl + HCl
    Chlorine Gas + Water ---> Hypochlorous Acid + Hydrochloric Acid

    So you use the molecular weight of Cl2 for each HOCl produced from the chlorine product of interest. Each Cl in Trichlor produces one HOCl as follows:

    C3Cl3N3O3 + 3H2O ---> C3H3N3O3 + 3HOCl
    Trichlor + Water ---> Cyanuric Acid + Hypochlorous Acid

    It is this sort of odd relationship that has the Available Chlorine percentage of pure Trichlor being 91.5%. Typical purity of Trichlor is 99% so Available Chlorine is usually closer to 90.6%. Obviously chlorine atoms do not make up that much of Trichlor, but it's that factor of 2 regarding Cl2 that causes this so dividing by 2 it is true that (3 * 35.453 g/mole chlorine) / (232.41 g/mole Trichlor) = 45.76% = 91.53%/2

    Trichlor by weight adds as much chlorine as 91.5% of that same weight of chlorine gas. In the Trichlor, every chlorine atom results in HOCl while in chlorine gas only one of the two chlorine atoms result in HOCl.
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    Re: Show me the math

    Chem Geek, I finally understand the numbers now. Thanks so much. It wasn't a necessity to know to keep pool in good shape but just one of those things that bugged me.

    Also, "EastTN" above shared a link regarding chlorine losses to CYA levels. You had provided the chart in the linked post. The discussion is around salt water pools. Does the loss part of the chart apply to non SWG pools?
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    Re: Show me the math

    Yes, the FC loss due to sunlight chart does apply to SWG pools. However, keep in mind that that chart assumes standard levels of sunlight, while every pool gets different actual amounts of sunlight varying dramatically from day to day, so the results are only suggestive, not something you can depend on.
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