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Swimming pool chlorine questions

  1. Apr 9, 2005 #1
    Something has always mystified me about swimming pool care. Why is it that owners need to keep adding chlorine products to their pools? My understanding of chemistry is that ionic salts don't evaporate with water. So, in my mind I picture water constantly evaporating and leaving the chlorine behind. This would make the pool more concentrated wouldn't it? Isn't this what made oceans salty over the eons? Whats happening to cause the loss of chlorine? Clearly I'm missing something here. Does it have something to do with the constant UV radiation of the sun (like altering the chemicals or something)? IF that is the case, would keeping the chlorinated water our of sunlight make it stay chlorinated indefinitely?

    Another similar one is this: I work in the foodservice industry. Many times I have been instructed that when making a dilute chlorine solution (using ordinary bleach as a sanitizer) that it is important to not use very hot water. The explanation was that hot water would "ruin" the bleach or "cause it to evaporate". Again, this doesn't fit at all with my knowledge of things.

    OK, thanks for any help understanding all of this.
  2. jcsd
  3. Apr 9, 2005 #2
    im guessing that the chlorine is combining with the 'bad stuff' in the pool water. So you need more chlorine to combine with new bad stuff. that was a really bad answer, sorry.
  4. Apr 9, 2005 #3
    Chlorine is a greenish-yellow gas that combines readily with nearly all other elements, like sodium.
    In swimming pools the chlorine combines with things like,ammonia, nitrogen-containing contaminants and other organics such as perspiration, urine and other swimmer waste. This combination is called chloramine.
    So you need to add fresh chlorine {called Free Avalible Chlorine or FAC}.
  5. Apr 9, 2005 #4
    The chloramine in your swimming pool is then photosynthesized by ultraviolet light and forms various salts.

    Hot water can accelerate chemical reactions. Perhaps the gases released by the hot chlorinated water are something you might not want to breath. Also, adding an acid to bleach will release chlorine gas.

  6. Apr 10, 2005 #5


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    It's because chloric acid likes to break down. This equation might be a little off, but you get the general idea:

    [tex]6H^+ + ClO_3^- + 6e^- \rightarrow Cl^- + 3H_2O[/tex]

    This is spontaneous and it happens very quickly if there is something in the water that can be oxidized.

    pH in the pool is monitored to maintain a balance between safety and effectiveness. At a high pH, there is a lack of H+ in the water, so the chlorate is not able to reduce as quickly. At a low pH, extra H+ ions floating around will join to OCl- ions in the water (present after chlorate reduces) to form HOCl, a poisonous gas.


    Just remember to keep a high-ish pH. If you dump sulphuric acid in a swimming pool, it makes a poisonous gas that will kill everybody in the pool. Don't get any ideas. :tongue:
    Last edited: Apr 10, 2005
  7. Apr 10, 2005 #6
    Thank-you Shawn, that was a really valuable addition to this thread.
  8. Feb 6, 2008 #7
    What happens to chlorine

    Chlorine in a pool or spa can get consumed in many different ways, but most have in common converting chlorine in an oxidation state of +1 into chloride ion in an oxidation state of -1. In an outdoor pool exposed to sunlight and with low bather load such as a private (homeowner) swimming pool, the greatest loss is due to the breakdown of chlorine from UV in sunlight:

    2OCl- + --[tex]\nu[/tex]--> O2(g) + 2Cl-
    Hypochlorite Ion --> Oxygen Gas + Chloride Ion

    2HOCl --[tex]\nu[/tex]--> O2(g) + 2H+ + 2Cl-
    Hypochlorous Acid --> Oxygen Gas + Hydrogen Ion + Chloride Ion

    The next greatest loss which is also the largest loss in high bather load situations and in spas and pools not exposed to sunlight is from chlorine oxidizing ammonia and urea from sweat (or urine) as follows:

    2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
    Ammonia + Hypochlorous Acid --> Nitrogen Gas + Hydrogen Ion + Chloride Ion + Water

    (NH2)2CO + 2HOCl + [tex]\frac{1}{2}[/tex]O2 --> N2(g) + CO2(g) + 2H+ + 2Cl- + 2H2O
    Urea + Hypochlorous Acid + Dissolved Oxygen --> Nitrogen Gas + Carbon Dioxide Gas + Hydrogen Ion + Chloride Ion + Water

    Chlorine can also oxidize organics similar to what is shown with urea above, but generally the oxidation is incomplete and instead there is a formation of chloramine or chlorinated organic compounds. The two most common reactions are shown generically as follows (ignoring other atoms carbon is connected to) and these are not redox reactions of chlorine (i.e. the chlorine is still in the +1 oxidation state).:

    C=C + HOCl --> Cl-C-C-OH
    C-N-H + HOCl --> C-N-Cl + H2O

    For a more complete analysis of pool water chemistry, see http://www.troublefreepools.com/viewtopic.php?t=628 [Broken].

    Last edited by a moderator: May 3, 2017
  9. Nov 26, 2009 #8
    I need to correct a couple of items in my previous post above. The reaction for the oxidation of urea by chlorine probably does not involve dissolved oxygen, but rather additional chlorine and may proceed via the following mechanism (this was proposed by Wojtowicz for the first and second part and by Jafvert & Valentine for the third part which is part of breakpoint chlorination):

    (NH2)2CO + 4HOCl --> (NCl2)2CO + 4H2O
    Urea + Hypochlorous Acid --> Quadchlorourea + Water

    (NCl2)2CO + HOCl --> NCl3 + NHCl2 + CO2
    Quadchlorourea + Hypochlorous Acid --> Nitrogen Trichloride + Dichloramine + Carbon Dioxide

    NCl3 + NHCl2 + 2H2O --> 2HOCl + N2(g) + 3H+ + 3Cl-
    Nitrogen Trichloride + Dichloramine + Water --> Hypochlorous Acid + Nitrogen Gas + Hydrogen Ion + Chloride Ion

    (NH2)2CO + 3HOCl --> N2(g) + CO2 + 3H+ + 3Cl- + 2H2O
    Urea + Hypochlorous Acid --> Nitrogen Gas + Carbon Dioxide + Hydrogen Ion + Chloride Ion + Water

    The hypochlorous acid reaction with unsaturated bonds (e.g. double bonds of carbon) is generally slow or neglible. The electrophilic substitution reaction at nucleophilic sites (especially with nitrogen) is far more common.
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