duraleigh said:
Poosean wrote:
Andy, you may want to consider putting the Pool Pilot in a boost mode, or using Potassium Monopersulfate to oxidize your pool, and leave the cover off to offgas the cc's.
Hey, guys,
It's not completely clear to me just how a pool rids itself of excess CC's. I know you must have additional FC and sunlight and air play a part but how the Chloramines are actually eliminated (reduced) is a puzzle to me. I would appreciate an explanation aimed at us laymen......please type s-l-o-w-l-y!! :lol:
I'd first like Andy to try just airing out the pool before trying more shocking with either chlorine or MPS.
Combined Chlorine (CC) is a generic term representing any compounds where disinfecting chlorine (hypochlorous acid) has combined with them strongly enough to not measure in the Free Chlorine (FC) test, but weakly enough to be measured in the Combined Chlorine (CC) test. Essentially, there is a range of chemical bond strength between chlorine and various other substances.
There are three broad categories based on the type of substances and I'll talk about each.
CHLORAMINES (Chlorine combined with ammonia) -- measures as CC
Monochloramine, NH
2Cl
Dichloramine, NHCl
2
Nitrogen trichloride (aka Trichloramine), NCl
3
CHLORINATED CYANURATES (Chlorine combined with CYA) -- measures as FC
6 different related substances -- CYA combined with 1, 2 or 3 chlorine plus dissociated acid relatives
Other organic compounds that mimic CYA such as Glycoluril and maybe Dimethylhydantoin (DMH)
OTHER CHLORO-ORGANICS (Chlorine combined with organic compounds more strongly than CYA) -- measures as CC
Many compounds including byproducts that are possible carcinogens including Trihalomethane (Trichloromethane aka Chloroform) and many others (many are relatively innocuous while others are not). The two most common reactions with chlorine are substitution reactions where chlorine replaces a hydrogen attached to a carbon or nitrogen and addition reactions with chlorine (and/or oxygen) attaching to carbons that had a double bond (i.e. unsaturated hydrocarbons as found in oils like suntan lotion).
Urea is a special case since it is an organic that can behave like ammonia since it can breakdown (oxidize) into carbon dioxide and ammonia, though generally does so slowly.
Free Chlorine (FC) will measure the truly free chlorine that hasn't combined with anything, namely hypochlorous acid and hypochlorite ion, but will also measure chlorine loosely bound to organics such as Cyanuric Acid (CYA). Combined Chlorine (CC) will measure the chloramines and chlorine combined with most other organics.
We've talked about chlorine and CYA ad nauseam in many threads so let's start with the chloramines. Chlorine combines with ammonia to form monochloramine very quickly, with 90% conversion in about 30 seconds at our normal FC/CYA levels and even within 5 minutes at low chlorine levels (i.e. 1 ppm FC with 100 ppm CYA). However, further combination of monochloramine with ammonia to get broken down completely to nitrogen gas and water takes longer -- at our usual FC around 10% of CYA level, it takes about 2 hours for 90% completion. During this process, several things can happen, but generally the simplified reactions are as follows.
NH
3 + HOCl --> NH
2Cl + H
2O
Ammonia + Chlorine --> Monochloramine + Water FAST
NH
2Cl + HOCl --> NHCl
2 + H
2O
Monochloramine + Chlorine --> Dichloramine + Water MODERATE
NHCl
2 + NH
2Cl --> N
2(g) + 3H
+ + 3Cl
-
Dichloramine + Monochloramine --> Nitrogen gas + Hydrochloric Acid SLOW
NHCl
2 + HOCl --> NCl
3 + H
2O
Dichloramine + Chlorine --> Nitrogen trichloride + Water SLOW
The last two reactions compete and which one occurs determines whether you end up breaking down the ammonia completely into nitrogen gas vs. whether you end up with Nitrogen trichloride which doesn't break down readily and is very smelly and irritating (and volatile). It turns out that higher levels of chlorine and lower pH favor the production of Nitrogen trichloride so this is yet another reason to use at least some CYA in an indoor pool and keep the pH at 7.5 or above. This is also why I want Andy to air out the pool since IF the Combined Chlorine is Nitrogen trichloride, then airing it out will help since this is a volatile chemical.
You can also see that the Dichloramine is a temporary intermediate chemical in the process and it is also smelly and volatile, more than monochloramine but not as bad as Nitrogen trichloride. So having good air flow during the above process helps ensure that the breakpoint will be complete as it helps reduce the smellier byproducts and sweeps away whatever Nitrogen trichloride ends up getting produced.
Technically speaking, the above reactions are always occurring when ammonia and chlorine are both present. It is not necessary to shock the pool to have these reactions occur. Shocking just has them go faster which is not always better as I indicated above since it can end up actually producing more Nitrogen trichloride. Also, the 10x rule isn't correct, but I won't get into that here.
As for other organics that have combined with chlorine, things are not so neat and simple. Some of these chloro-organics as well as Nitrogen trichloride can be persistent Combined Chlorine. At least with the Nitrogen trichloride you can remove it by airing out the water since it is so volatile, but with non-volatile chloro-organics there isn't much you can do. You can try using a non-chlorine shock (MPS) that might help, but usually such non-chlorine shock is better to use BEFORE chloriine combines with organics. That is, MPS oxidizes organics and ammonia before chlorine gets a chance to combine with them. So MPS is more like a Combined Chlorine prevention method. It is not without it's own side effects, however, as it is still a strong oxidizer so some people have skin sensitivities to it and it might accelerate corrosion of metal so I'm particularly concerned in Andy's environment so I don't want him using it regularly (but one time is fine and we may do that if the airing out doesn't help).
Though I talked about airing out the volatile Combined Chlorine, such as Nitrogen trichloride, I didn't talk about sunlight. The UV rays of sunlight help break down various chlorine products, but its unclear how quickly this occurs. We know it breaks down chlorine rather quickly, less so when CYA is present, and Nitrogen trichloride is also relatively unstable in sunlight. Monochloramine may persist for a while though does slowly break down in sunlight. I don't know about the chloro-organics in terms of how they react in sunlight.
So the bottom line is that uncovering a pool and letting it air out (circulating the air above it with fresh air) helps to remove any volatile Combined Chlorine such as Nitrogen trichloride. Exposing the pool to sunlight (or even better, to UV lamps) can break down at least some Combined Chlorine. As for shocking with chlorine to reduce CC -- that really doesn't make sense if you've already got chlorine in the water and the CYA isn't too high (with low FC) unless you aren't patient and want to speed up the reaction. A non-chlorine shock, such as potassium monopersulfate (MPS), is best used as a preventative to stop the formation of Combined Chlorine in the first place.
Richard