FC:CYA ratios in dichlor and trichlor - wait am I a moron?

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Melt In The Sun

Melt In The Sun

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Oct 29, 2009
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Just a question that's been bugging me... :scratch:

Why is it that adding trichlor adds 0.6 ppm CYA/ppm FC, and dichlor adds 0.9 ppm CYA/ppm FC? Even though I (somehow) minored in chemistry, my chem background is more one of enthusiasm than competence :oops: ! It seems like 1 trichlor molecule (Cl3C3N3O3) would add 1 ppm CYA and 3 ppm chlorine. Likewise for dichlor, except a 2:1 ratio. Is it not a molar ratio?

:hammer: :hammer: :hammer: Maybe just answered my own question as I was typing it :idea: . Well I guess I'll finish this just in case anyone else was wondering.

3*35.5 (atomic weight Cl) + 3*12 (aw. C) + 3*14 (aw. N) + 3*16 (aw. O) = 232.5 (trichlor molecular weight)
(3*35.5)/232.5 = 0.458 (Cl% by weight in trichlor)...... :blah: :blah:

Dang, this is more complicated than I thought! I'm stuck at this point. I've looked on this site and can't find calculations for the 0.6/0.9 ratios. Does someone (Richard? :-D ) have them?
 
Probably has something to do with how much hypochlorous acid (ultimately, FC) is yielded from the chlorine in each, but that's only my guess. I'm afraid that my chem background is more one of running away screaming than competence.
 
You need to use the proper molecular weights for measurements. For chlorine, it is measured in ppm Cl2 that has a molecular weight of 70.906 g/mole while for CYA it's in ppm CYA itself which has a molecular weight of 129.075 g/mole. You do not use the molecular weights of Trichlor or Dichlor themselves since the rule is comparing ppm FC vs. ppm CYA.

Trichlor has 3 chlorine attached to a CYA core (s-triazine ring) and they all can produce hypochlorous acid in water. So, 1 mole of trichlor produces 3 moles of chlorine and 1 mole of CYA. So the ratio of ppm CYA to ppm FC is 129.075/(3*70.906) = 0.60679. Dichlor has 2 chlorine attached to a CYA core so 1 mole of dichlor produces 2 moles of chlorine and 1 mole of CYA. So the ratio of ppm CYA to ppm FC is 129.075/(2*70.906) = 0.91018.

What's really confusing is when one starts talking about "% Available Chlorine" which is a weight %, but 100% is defined relative to chlorine gas and only one of the two chlorine in chlorine gas produces hypochlorous acid while the other produces chloride (from hydrochloric acid when chlorine gas combines with water). So every chlorine atom that produces hypochlorous acid is counted twice in terms of weight %. Trichlor has a molecular weight of 232.4103 g/mole so the % Available Chlorine is 3*70.906/232.4103 = 91.527% even though the weight % of atomic chlorine in Trichlor is half that percentage. Trichlor is around 99% pure product which is why the % Available Chlorine is usually quoted as 99%*91.527% = 90.6% or thereabouts.

Similarly, Dichlor dihydrate has a molecular weight of 255.97766 g/mole so the % Available Chlorine is 2*70.906/255.97766 = 55.400%. For Cal-Hypo with a molecular weight of 142.98366 g/mole and producing 2 chlorine for every mole of calcium hypochlorite (remember that it is Ca(OCl)2), the % Available Chlorine is 2*70.906/142.98366 = 99.181% so is essentially the same as it's purity so that 65% Cal-Hypo is around 65% Available Chlorine (64.468%, actually), etc. Sodium Hypochlorite has a molecular weight of 74.4422 g/mole and each mole produces one mole of chlorine so the % Available Chlorine is 70.906/74.4422 = 95.250% so is close to the weight % (purity) of product. This is why 6% bleach (which means 6% by weight of sodium hypochlorite) is 6%*95.25% = 5.715% Available Chlorine.

Yet another confusing number that is used is Trade % which is the Volume % of Available Chlorine (as opposed to Weight %). This factors in the density of the product where the Trade % is the (weight) % Available Chlorine times the density (specific gravity). Since 6% bleach has a density of around 1.08, it's Trade % is 5.715%*1.08 = 6.172%. When one wants to easily calculate the FC rise of adding 1 gallon of chlorine to 10,000 gallons of water, it's the Trade % that is most easily used since the rise is 6.172 ppm FC.

Richard
 
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By the way, don't feel bad about not getting this figured out the first time. I had this stuff wrong for almost a year when I first started calculating these things because I didn't know the units of measurement normally used for chlorine. I used ppm HOCl until I found out it should be ppm Cl2 and had to change not only my spreadsheet, but a bunch of graphs I had made.
 
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