Some Chlorines "designed To Burn Off Quickly"?

[CurtisT]

After reading a lot of information on this board, I think I know the answer to this, but just want to confirm. I've had numerous pool store guys as well as my pool builder (who actually seems to really know his stuff in general) tell me that using alternative forms of chlorine (like Cal-Hypo) for sanitization won't work because it is designed to burn off quickly and should only be used for shock. When I asked them about the stabilizer in the pool, they told me that it doesn't work for that chlorine. From what I read here, I'm guessing that that is mostly urban legend?

I'm thinking that the reason the chlorine burns off quickly after shocking are 1) because it's getting used up breaking down organics and chloramines and 2) because we're putting more chlorine in the pool than there is CYA to stabilize, so there's a lot of unstabilized chlorine floating around in the pool. Is that correct?

[chem geek]

After reading a lot of information on this board, I think I know the answer to this, but just want to confirm. I've had numerous pool store guys as well as my pool builder (who actually seems to really know his stuff in general) tell me that using alternative forms of chlorine (like Cal-Hypo) for sanitization won't work because it is designed to burn off quickly and should only be used for shock. When I asked them about the stabilizer in the pool, they told me that it doesn't work for that chlorine. From what I read here, I'm guessing that that is mostly urban legend?

I'm thinking that the reason the chlorine burns off quickly after shocking are 1) because it's getting used up breaking down organics and chloramines and 2) because we're putting more chlorine in the pool than there is CYA to stabilize, so there's a lot of unstabilized chlorine floating around in the pool. Is that correct?

That's the biggest bunch of B.S. I've every heard. ALL sources of chlorine form IDENTICAL chlorine in the water when dissolved/mixed. They ALL form hypochlorous acid, hypochlorite ion, and chlorinated cyanurates (chlorine bound to Cyanuric Acid, assuming CYA is present in the water). The only differences are in what they ADDITIONALLY add to the water, specifically Calcium for Cal-Hypo and Cyanuric Acid for Trichlor and Dichlor.

The chemical rules of fact that these sources of chlorine add to the water are as follows:

For every 10 ppm FC added by Trichlor, it also increases 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 Hardnes (CH) by 7 ppm.

Now your questions are different than the source of chlorine being the issue. Your questions imply that a higher shock level of chlorine is the reason it goes down more quickly and in fact that is the case. The rate that chlorine breaks down in sunlight is proportional to the chlorine level itself, so it's more like a percentage of chlorine. So if you shock to a high FC level, then a larger ABSOLUTE amount of chlorine (FC drop) will get broken down by sunlight. Also, if there are organics to oxidize and this is occurring somewhat slowly, then a higher FC level will accelerate this proportionately so that again a percentage of FC is lost per time.

As for the amount of stabilized (bound to CYA) vs. unstablized chlorine, you are correct that roughly speaking the amount of unstabilized chlorine is proportional to the FC/CYA ratio so with a constant CYA level, doubling the FC doubles the amount of unstabilized chlorine (hypochlorous acid and hypochlorite ion) so there is a larger quantity to break down by sunlight and that this is in proportion to their concentration.

Richard