Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simple aid to Brita filter for declorinating tap water.

  1. Mar 12, 2008 #1

    rbj

    User Avatar

    hi guys,

    i know something about math and physics and a lot about electronics and signal processing, but chemistry is the natural science that i have always been weakest about.

    first, how do most cities chlorinate their water? i presume they mix in some solid or liquid chemical that has Cl as part of the molecule. what are the other elements? do they precipitate out (before exiting the water treatment plant) or is there a lot more than the Cl in the tap water?

    i was told that, if you put chlorinated tap water into an uncovered pitcher and let it sit several hours, that much of the clorine will come out of solution and dissapate in the air, much like CO2 will in soda that has been opened and left sitting in air of about 1 atm. is that true?

    i presume, if the above is true, that after allowing Cl to escape, there will be less of it left in the water and if that is passed through the Brita filter, the filter will possibly last longer and maybe the filtered water will have even less chlorine in it than if the tap water was applied to the filter immediately after coming out of the tap. is this the case?

    thanks for replying with accurate info.
     
    Last edited: Mar 12, 2008
  2. jcsd
  3. Mar 12, 2008 #2

    DaveC426913

    User Avatar
    Gold Member

    It is effectively true, but I don't know if that's because it's rendered chemically inert or if it's because it actually off-gases.
     
  4. Mar 12, 2008 #3

    rbj

    User Avatar

    well, "chemically inert" means it's still in there. right? i wanna know if the Cl actually goes away, so there is less of it in the water. and, if that is true, what about the other elements in whatever solid or liquid chlorine chemical that they dump in the water? maybe the Cl separates from the rest of the molecule when in H2O solution. even if we make the Cl go away, what about the rest of that chemical? does that somehow precipitate out before we even see it at the tap or is it in our water?
     
  5. Mar 12, 2008 #4

    DaveC426913

    User Avatar
    Gold Member

    A question you may want to ask is whether there is a correlation between impurities and filter-life.

    My tap filter has a fixed life that is based on throughput. It will filter the same number of gallons of water whether that water is muddy with junk or pure as the dew on a virgin's thigh.
     
  6. Mar 12, 2008 #5

    DaveC426913

    User Avatar
    Gold Member

    "When we add Cl2 (chlorine) + H2O (water), we get a reaction which leaves us with HOCl (hypochlorous acid) + HCl (hydrochloric acid). Hypochlorous acid is the active, killing form of chlorine. This is what does the real sanitizing work. The chlorine molecule or ion kills microorganisms by slashing through the cell walls and destroying the inner enzymes, structures and processes. When this occurs, the cell has been deactivated, or oxidized. The hypochlorous molecule continues this slash & burn until it combines with a nitrogen or ammonia compound, becoming a chloramine, or it is broken down into its component atoms, becoming de-activated itself."
    from http://www.poolcenter.com/chlor.htm

    So, chlorine goes in relatively pure. What is left in the water is dead organics and chloramine ([TEX]NH_{2}Cl[/TEX]).
     
  7. Mar 12, 2008 #6

    rbj

    User Avatar

    actually, Wikipedia has something:
    http://en.wikipedia.org/wiki/Water_purification
    http://en.wikipedia.org/wiki/Chlorination

    but i still can't find a good reference that tells me what is left after letting tap water "stand" for a day. the Cl might go away, but isn't there something else, like Na left? do those charcoal filters take it out?

    i would think that there are some kinda chemical components in the Brita filter for neutralizing Cl and other impurities with a pH far from 7. that these chemical components in the filter will stop functioning if they get used up to the last molecule by reacting with Cl or whatever it is they encounter.

    so, with your tap filter, what changes inside of it when you pass distilled water (taken off the virgin's thigh) through it? if something does change in the filter, does the water (that was distilled going it) come out as pure?
     
    Last edited: Mar 13, 2008
  8. Mar 14, 2008 #7


    Brita filters contain activated carbon and a special type of ionic exchange resin. The activated carbon sucks up a lot of impurities while the resin is designed to filter out ionic impurities.
     
  9. Mar 14, 2008 #8

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The brita filter contains both activated charcoal and a mixture of ionic exchange resins. The ion exhcange resins exchange cations to produce H+ and anions to produce OH-. Together they pretty effectively produce nearly distilled water. The chlorine component of drinking water starts out as some baseline level of chloride, organo chlorine compounds and HOCl from the addition of Cl2 to water. As the hypochlorous acid encounters acids, low valent transition metals and bugs/virus/organic materials, the hypochlorous acid is converted into either Cl- or an organochlorine species. These species are removed by the filter. Whatever hypochlorous acid remains is ion exchanged by the resin in the filter. Hypochlorous acid is in equilibrium with Cl2 and allowing water to set exposed to the air can, over time, remove the remaining Cl2. Of course, then you get impurities in your water from dust in the air...

    The upshot is that allowing your water to stand for a time could result in slightly longer life of the filter provided you don't have a lot of contamination from the air. Back in my homebrewing days, I found it to be much be much more effective to boil the water to both remove chlorine and a significant amount of the water hardness. I suppose you could do that and significantly increase your filter life.
     
  10. Mar 15, 2008 #9

    DaveC426913

    User Avatar
    Gold Member

    Oh. Sorry. It has a spinny widget in it that closes the filter (stops flowing) when a certain number of gallons have run through it.

    Apart from designing consumability into the product, this "filter lifetime" prevents the filter from being overused - which can result in the dumping its contaminants back into the water.
     
  11. Mar 17, 2008 #10

    rbj

    User Avatar

    thanks for the response.

    it's a covered pitcher, but not sealed. dust falling straight down might not get in, but a mosquito could fly in. i would hope that the Cl2 molecules that find themselves in the air above the water surface can escape. it "stands" for maybe a day or overnight before getting poured into the Brita pitcher, rinsed, and another liter and a half put in from the tap.

    boiling is more effort and energy. my tea water (from the Brita pitcher) does get boiled, and used right away. how does boiling remove hardness? aren't those chemicals less volatile than the water?
     
  12. Mar 17, 2008 #11

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Calcium carbonate is less soluble in hot water than in cold. It precipitates with boiling and settles upon standing. Decanting removes a substantial amount of the hardness. Have you noticed that the tea you make with water purified with a Brita is very clear and tea made otherwise is somewhat cloudy?
     
  13. Mar 18, 2008 #12

    rbj

    User Avatar

    of course! that's the whole point!

    what i didn't know was that boiling water would make some impurities precipitate and i could decant a purer water from off the top. i knew, of course, that boiling water sufficiently kills pathogens in it.

    so, if i boil tap water, let it stand, decant it, and then pass it through Brita, is that about as good as i can do without buying bottled water? better, perhaps?
     
  14. Mar 18, 2008 #13

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I would say yes. I used to treat 7 gallons at a time that way. You can review the boiling water trick on any of a number of homebrew forums. Of course the first thing I would do after treating the water was add calcium sulfate! Go figure...
     
  15. Mar 18, 2008 #14

    rbj

    User Avatar

    why would you do that?
     
  16. Mar 18, 2008 #15

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It's a taste thing. Trying to make it taste like some brook in the UK, I believe. It is pretty common. It also helped with the cold break by agglomerating some of the soluble starches.
     
  17. Mar 19, 2008 #16

    rbj

    User Avatar

    well, i've heard of Brits developing a taste for C20H24N2O2 as a "flavour" component of tonic water, but adding CaSO4 or some other chemical to make it taste like some local brook seems very odd. we could add H2SO4 to make it taste like some stream in Gary Indiana, i s'pose.

    mmmm. that's good water.

    :smile:
     
    Last edited: Mar 19, 2008
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Simple aid to Brita filter for declorinating tap water.
Loading...