Over thinking the chemistry

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EasyE94

New member
May 27, 2020
1
Daphne, AL
Hey Everyone.

I've been lurking the forum for awhile now, trying to learn as much as I can. I just refilled my pool (cya too high, previous owner) and using what I've learned from y'all has been working like a charm. Even though my levels are great, a few things just aren't adding up for me and I want a better understanding of the chemistry, specifically the chlorine and CYA relationship.

1. So, if CYA binds to some of the chlorine and renders it inactive wouldn't that just leave all your FC just as vulnerable to the sun as if there were no CYA at all? At that point, how does it actually work at prolonging FC?

2. And if say, my targets were 5ppm FC and my target CC is 0 but I have 40ppm CYA. How is that possible? If I had any CYA, shouldn't that create some CC? Also, wondering how a reading of CC ppm isn't a direct indication of CYA ppm? The dot test sucks..

3. Lastly, with the 2005K and 2006K (I bought 2006) the 2005 tests for FC and TC, but not CC. And the 2006 tests for FC and CC but not TC. Wouldn't FC+CC=TC Can't you figure out the missing variable with both kits? Which brings to to wonder why they are made differently, and/or don't include a note like "add findings from step 1 and 2 to determine TC" and visa versa.

I been thinking that "if it works, it works, don't ask any questions" but its been bothering me because there most likely a simple explanation to answer all 3 questions... or my understanding of it is just totally wrong. Thanks for reading, and getting me this far!
 
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EasyE94 said:
1. So, if CYA binds to some of the chlorine and renders it inactive wouldn't that just leave all your FC just as vulnerable to the sun as if there were no CYA at all? At that point, how does it actually work at prolonging FC?
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Lots of misunderstood concepts here. Chlorine is not "rendered inactive" by CYA. Like all chemical reactions, these are equilibrium reactions and you have to think of it as a chlorine atom constantly bonding to and unbonding from the CYA molecule. When the chlorine atom is bonded to the CYA, it forms a chlorinated cyanurate that is immune (mostly) to UV photolysis. When CYA is present in pool water, around 95% of the chlorine is "bound" to it at any given point in time and it leaves the remaining fraction of chlorine to form hypochlorous acid and hypochlorite anion. The amount of HOCl/OCl- that forms is pH dependent with roughly a 1:1 ratio at a pH of 7.5. Hypochlorous acid (HOCl) is the primary oxidizer and sanitizer in the water and does most of the work keeping your water clean. Hypochlorite (OCl-) is a much slower sanitizer and poor oxidizer BUT it is the chemical component that is the most affected by UV light. UV light will decompose OCl- into chloride (Cl-) and oxygen gas. So, in an outdoor pool, it is the small fraction of OCl- that represents the largest fraction of UV loss. As the OCl- and HOCl get used up, the chlorine bound to the CYA takes its place and, because of the forces of chemistry, the solution tries to maintain a constant level of HOCl/OCl- for a given pH and concentration of FC and CYA. Over time, the chlorine slowly degrades, the HOCl levels fall below what is needed to keep the water sanitary and algae will begin to form.


EasyE94 said:
2. And if say, my targets were 5ppm FC and my target CC is 0 but I have 40ppm CYA. How is that possible? If I had any CYA, shouldn't that create some CC? Also, wondering how a reading of CC ppm isn't a direct indication of CYA ppm? The dot test sucks..
Click to expand...

Just because chlorine is in the water doesn't necessarily mean it will form a combined chlorine (CC) compound. Even in pools with algae and lots of biological contamination, CC's don't necessarily form. High chlorine levels can slowly oxidize CYA, but it will almost always form carbon dioxide and nitrates or nitrogen gas. CC's don't form from the CYA in the water.

In reality, the CC test is only really measuring monochloramine and dichloramine for the most part which are by-products of chlorine reacting with bather waste. There are plenty of other disinfection by-products (DBPs) and trihalomethane (THM) compounds that can form that will never register on a CC test. This is why it is so important to always know what you are adding to your pool and to not add anything that the water does not need. A classic example of this was a person who setup a small above ground pool with an SWG that was left to run at 100% all the time and the pool was covered. The water had in excess of 30ppm FC and no CYA in it at all. When the person got in and soaked for about 45mins, they became horribly sick and dizzy but the water registered no CCs. What likely happened was the chlorine was forming by-products like chloroform and nitrogen trichloride from the sweat on their skin and the sunscreen they were using. They were inhaling that chemical mix and it made them very sick but there was no indication from the test results other than the astronomical FC and low CYA.

EasyE94 said:
3. Lastly, with the 2005K and 2006K (I bought 2006) the 2005 tests for FC and TC, but not CC. And the 2006 tests for FC and CC but not TC. Wouldn't FC+CC=TC Can't you figure out the missing variable with both kits? Which brings to to wonder why they are made differently, and/or don't include a note like "add findings from step 1 and 2 to determine TC" and visa versa.
Click to expand...

The K-2005 should come with R-0001/R-0002 and R-0003. The R-0001/R-0002 measures FC on the color comparator and then you add the R-0003 to see if the color changes (gets darker). After adding R-0003, your result is now TC. CC can be determined by subtracting FC from TC. The K-2006 simple splits up the test differently and does a titration. The first titration result is FC where the FAS titrant (R-0871) has reduced all of the FC. When you add the R-0003, the mixture reacts with any CCs in the water and causes the dye to turn pink again. The next titration is then the CC results. So in the 2006, you get TC = FC + CC.

The test kits are doing the same test, it's just a little different. Also, TC is really not a useful number. It's always best to know FC and CC independently.
 
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EasyE94 said:
If I had any CYA, shouldn't that create some CC?
Click to expand...
Chlorine does "combine" with cyanuric acid.

So, technically, it is "combined chlorine".

However, we don't consider it as combined chlorine.

It's mostly a technical distinction.

During the fc part of the test, the chlorine dissociates from the cyanuric acid and measures as free chlorine.

Chlorine bound to ammonia does not dissociate from the ammonia during the fc part of the test.

Reagent R-0003 is iodide, which gets oxidized by the chlorine from the CC.

So, it's iodine (hypoiodous acid and hypoiodite) that you're measuring in the CC part of the test.

Note that iodide is so easily oxidized that it basically reacts to any oxidizer that didn't react in the fc part of the test.

This can be ozone, oxygen, MPS, bromate, trihalomethane, chloramine, chlorine combined with sulfamic acid etc.
 
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Yes, it's more like there are little elves in CYA that hold on to some of the chlorine. They put some SPF 1 million on the chlorine so it doesn't get sunburned. They let it go when you pee in your pool, so it can burn the pee and create CC.

It's not hard.😀
 
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