mycotheologist
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Swimming Pool Chemistry - Why does UV cause chlorine loss?
I read that calcium hypochlorite sold by swimming pool suppliers contains trace amounts of cyanuric acid which acts as a chlorine stabiliser. I read
that UV radiation in the sunlight causes the chlorine to dissipate and that cyanuric acid prevents this. How does UV radiation cause the chlorine in
the pool to dissipate? I know that UV causes homolysis of the Cl2 molecule and thus catalyses free radical reactions but I can't see how this would
cause the chlorine to be lost to the atmosphere? What would be the mechanism of the reaction?
Also, how does cyanuric acid prevent the reaction? I notice that cyanuric chlorides are superseding hypochlorites as chlorinating agents for swimming
pools. Its a pretty interesting compound cyanuric acid, next time I drop into a swimming pool supplier, I'm going to pick up some TCCA.
EDIT: I just read that when the concentration of cyanuric acid in a swimming pool is too high, it stabilises the chlorine to the extent that it is no
longer a disinfectant. I see now that cyanuric somehow renders Cl2 relatively inert. I'm sure its been tried but I have the urge to add some cyanuric
acid to a brine electrolysis cell to see if it inhibits hypochlorite formation.
[Edited on 22-4-2012 by mycotheologist]
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weiming1998
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Quote: Originally posted by mycotheologist  | I read that calcium hypochlorite sold by swimming pool suppliers contains trace amounts of cyanuric acid which acts as a chlorine stabiliser. I read
that UV radiation in the sunlight causes the chlorine to dissipate and that cyanuric acid prevents this. How does UV radiation cause the chlorine in
the pool to dissipate? I know that UV causes homolysis of the Cl2 molecule and thus catalyses free radical reactions but I can't see how this would
cause the chlorine to be lost to the atmosphere? What would be the mechanism of the reaction?
Also, how does cyanuric acid prevent the reaction? I notice that cyanuric chlorides are superseding hypochlorites as chlorinating agents for swimming
pools. Its a pretty interesting compound cyanuric acid, next time I drop into a swimming pool supplier, I'm going to pick up some TCCA.
EDIT: I just read that when the concentration of cyanuric acid in a swimming pool is too high, it stabilises the chlorine to the extent that it is no
longer a disinfectant. I see now that cyanuric somehow renders Cl2 relatively inert. I'm sure its been tried but I have the urge to add some cyanuric
acid to a brine electrolysis cell to see if it inhibits hypochlorite formation.
[Edited on 22-4-2012 by mycotheologist] |
Calcium hypochlorite is at an equilibrium with HOCl in aqueous solution:
Ca(ClO)2+2H2O<===>Ca(OH)2+2HClO
HOCl is the active ingredient in the chlorination of pools, as it acts as a disinfectant. But UV light catalyses its decomposition into HCl and O2,
which causes the equilibrium to shift to the right, generating more HOCl, which is again decomposed by UV light until all the calcium hypochlorite are
reacted away into Ca(OH)2/CaCl2. The same thing happens with NaClO. When all of the hypochlorites has been reacted away, the chlorine has "dissipated"
Now, this link says that cyanuric acid forms a weak bond with the free chlorine in water http://www.lincoln.ne.gov/city/health/environ/pollu/pdf/Cyan...
But a pool's pH is about 7.5, so any chlorine dissolved within would be in the form of HOCl/OCl- ions. Why would there be any free chlorine in
solution?
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AJKOER
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There could be free Chlorine as a result of the following reactions:
HOCl --uv--> HCl + O
HCl + HOCl <---> Cl2 + H2O
In general, the lower the pool's pH, the more free Cl2.
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