Wednesday, November 18, 2015

Today's Silly Question

Back when I was young, I learned that chlorine kills bacteria in drinking water not directly but by breaking up water molecules, releasing free oxygen, which is the actual bacteriacide.  But when I look at the electronegativity of chlorine (3.0) and oxygen (3.5), I don't see how chlorine could replace oxygen in a water molecule.  Can someone who has taken freshman chemistry recently than me explain this?


  1. 2Cl2 + 2H2O => 4HCl + O2

    Sorry no subscripts.


  2. Chlorine in water is in equilibrium with chloride anion and hypochlorite. This "breaks up the water molecule" but in such a way that the chlorine atom has a partial positive charge and the oxygen has a negative one - as you would expect from the relative electronegativeities. But when hypochlorite oxidizes organic material, it does so in a way that leaves the oxygen bound to the organic material, because that is what's thermodynamically favored - carbon-oxygen bonds are stronger than carbon-chlorine.

  3. If I may add to the question: If oxygen is the bacteriacide, could water purification be successfully accomplished by aerating water? If so, aerating to what degree? And, what is the minimum percentage of oxygen in the aerating gas necessary to accomplish successful destruction of bacteria? How successful would this process be, e.g., what percentage of bacteria wuld be destroyed?

  4. This might help:

  5. I've been exposed to a fair amount of chlorine chemistry at work.

    Chlorine in water forms hypochlorous acid -- HOCl -- which is just itching to dump the oxygen and become HCl. (Bleach, containing sodium hypochlorite or NaOCl, does much the same thing, producing NaCl.)

    The fraction of chlorine that is hypochlorous acid vs. the fraction that is hydrochloric acid depends on the pH of the water, so when we're monitoring the water, we're looking at chlorine level, pH and temperature to calculate contact times.

    Nowadays, a lot of water systems use monochloramine -- NH2Cl -- which is a weaker disinfectant. The chlorine is more tightly attached to the nitrogen, and less reactive, but when it detaches, it does the same thing that chlorine gas does in water, leaving the resulting ammonium ion to find a stray hydrogen ion.


  7. Nosmo King: The Russians use ozone instead of chlorine.

  8. The Russians are not the only ones who use ozone in water treatment, both potable and sewage. There are many cities here in the US that use ozone, Ann Arbor Mi. Is one of the larger water treatment that uses ozone.
    Many of those that do use ozone also add chlorine to prevent contamination from reoccurring after ozone treatment.
    Ozone requires a high level corrosion proof piping to maintain system integrity. This adds a cost that precludes many systems from employing using ozone.
    If you ever tour a water plant that is using ozone you will see much of the piping is stainless steel vs. galvanized or plastic where chlorine is the primary disinfectant.