Chlorinating, maintaining the right chlorine levels,
chlorine problems. Dichlor, trichlor, cal hypo, bleach,
granules, chlorine pucks and chlorine sticks.
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Postby poolideas » Wed 21 Oct, 2009 00:54

Just a quick question.
When a salt chlorinator makes chlorine and then the spent chlorine reverts back to salt, the only chlorine in the pool should be free chlorine?
So is the need to test for total & free chlorine is inappropriate?. True or false



Postby Guest » Wed 21 Oct, 2009 07:11

the spent chlorine reverts back to salt

The spent Cl from the active free chlorine dissipates and the sodium (Na) from the salt remains in solution in the pool.
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Postby Me... » Wed 21 Oct, 2009 18:15


The SWG creates free chlorine. The "spent" chlorine would be the combined chlorine AKA chloramines. Add the free chlorine reading to the combined chlorine reading and you get total chlorine.

You DO NOT WANT any combined chlorine in the water. This in itself probably generates the need for more free chlorine than anything else since it the first thing the free chlorine will go after. If it can't get rid of the combined chlorine it just keeps trying.

If you use chlorine, any form, you need to test for both forms and superchlorinate it out, shock it out or oxidize it out, whatever you want to call it (although they are in reality all a bit different).
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Postby chem geek » Wed 21 Oct, 2009 20:56

Me... is correct. I will go into more details for those who are interested.

In the saltwater chlorine generator (SWG), the following occurs (netting out what happens when the chlorine gas that is produced dissolves in the water):

2H2O + Cl- ---> HOCl + OH- + H2(g)
Water + Chloride Ion (i.e. salt) ---> Hypochlorous Acid (i.e. Free Chlorine) + Hydroxyl Ion (aka lye) + Hydrogen Gas

Note that sodium ion (from salt) does not participate in any of these reactions.

If the chlorine combines with some ammonia (or urea, though I don't show that) from your sweat or urine, then the following occurs quickly:

HOCl + NH3 ---> NH2Cl + H2O
Hypochlorous Acid (i.e. Free Chlorine) + Ammonia ---> Monochloramine (i.e. Combined Chlorine) + Water

If you have sufficient chlorine in the water or if you shock to accelerate getting rid of Combined Chlorine, then the following happens (more slowly than the reaction above):

2NH2Cl + HOCl ---> N2(g) + 3H+ + 3Cl- + H2O
Monochloramine (i.e. Combined Chlorine) + Hypochlorous Acid (i.e. Free Chlorine) ---> Nitrogen Gas + Hydrogen Ion (i.e. acid) + Chloride Ion (i.e. salt) + Water

However, what mostly happens to the Free Chlorine in the water is the following which is breakdown from sunlight:

2HOCl ---> O2(g) + 2H+ + 2Cl-
Hypochlorous Acid (i.e. Free Chlorine) ---> Oxygen Gas + Hydrogen Ion (i.e. acid) + Chloride Ion (i.e. salt)

So the net result of chlorine being produced in the SWG and then oxidizing ammonia is the following:

2NH3 ---> N2(g) + 3H2(g)
Ammonia ---> Nitrogen Gas + Hydrogen Gas

and the following is the net result if the chlorine gets broken down by sunlight:

2H2O ---> O2(g) + 2H2(g)
Water ---> Oxygen Gas + Hydrogen Gas

So one can consider what the SWG does is to elevate chloride salt to a higher oxidation state that is an effective sanitizer and oxidizer and when this gets used up it results back in chloride salt.

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