What % of trichlor is actually chlorine?

docsobeck

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May 29, 2008
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#1
hi all,

i'm a committed BBB method user, but like to have some pucks on hand for when i'm away, and when i need to refurbish CYA levels. i'm also kind of a nut about getting the best deal, so i like to figure out the actual PPM of chlorine i'm adding per dollar for each chlorine source. (for those in the midwest, it turns out the best deal i've found is the tundra bland bleach at the Aldi chain of bargain grocery stores - $0.99 per 96 ozs., yielding 2.27 PPM chlorine per dollar in my 20k gallon pool.)

anyhow, when you look at the label on trichlor packaging, it tells you what percent is trichlor, and what percent is inert. what it doesn't tell you is what percent of the trichlor is actually chlorine, and what percent is CYA and whatever other stuff makes up trichlor. anyone know?

thanks.
 

chem geek

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Mar 28, 2007
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San Rafael, CA USA
#2
A cost comparison of chlorine sources that also has some of the other information you are looking for is here. The thing most people forget is that with a very acidic source of chlorine such as Trichlor, you also have to add some chemical to raise the pH and that also costs money. Trichlor is nearly 100% pure. It would be 91.5% Available Chlorine if it were pure and most Trichlor is quoted at 90% or so Available Chlorine so is 98% or more pure. Available Chlorine is the weight percentage of equivalent Free Chlorine.

As for the percent of Trichlor that is chlorine vs. how much is CYA, that's unfortunately not the way to look at it as the chlorine that is attached to CYA combines with water to form hypochlorous acid while chlorine levels themselves are measured in a parts-per-million (ppm) that is weight based on chlorine gas equivalent. The 90+% Available Chlorine does not mean that Trichlor is only 10% CYA (Trichlor is actually 55.5% CYA-equivalent by weight). So instead, there are simple chemical rules that you can use that apply at any purity level:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

Richard
 

JasonLion

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May 7, 2007
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Silver Spring, MD
#4
For every 10 ppm of FC from bleach you will get about 16 ppm of salt. Keep in mind that all forms of chlorine will put salt in the water, though not quite that much. Given typical rates of water replacement the salt level normally stabilizes below 1000. That is more salt than is normally in municipal water, but less salt than many people with water softeners have.

Mentioning caustic soda is misleading. Once the caustic soda is in the water it disassociates into sodium and OH ions, which are essentially salt and water. The quantities involved are very small, hardly worth calculating, aside from their effect on PH. A little more salt and water has no real impact compared to the salt and water already in bleach in their usual form.
 

chem geek

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Mar 28, 2007
12,082
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San Rafael, CA USA
#6
The amount of salt that all chlorine produces when it gets used up and converted to chloride is 8 ppm (from an original 10 ppm FC) so bleach and chlorinating liquid add an extra 8 ppm above that. The amount of excess lye (caustic soda) is negligible and has virtually no effect on pH relative to other things like outgassing. In my own pool with a pool cover that is on most of the time, I have virtually no rise in pH yet add around 1 ppm FC average each day. In theory, the pH would be rising by 0.3 per week if one just looked at the pH of chlorine due to mostly being hypochlorite, but because the process of chlorine getting used up is acidic, it exactly compensates for the initial rise in pH with the net result being virtually no rise in pH at all over time.

Richard