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Saltwater electrolysis, chlorine reacts with water?

  1. Aug 10, 2014 #1

    kox

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    Hi. I'm confused about how chlorine reacts with water. Yesterday I electrolised saltwater. There was some intense bubbling at 0V electrode. I guess that was hydrogen. However there was no bubbling at positive electrode. The water had been greenish at first but then some brown mud appeared. It's probably because I used iron electrodes or impurities because it was tap water with addition of salt. So did chlorine react with water immediately after it had been produced, hence no bubbling?
    How do they make chlorine via electrolysis if chlorine reacts with water?
     
  2. jcsd
  3. Aug 10, 2014 #2

    Borek

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    Staff: Mentor

    Brown mud most likely means you dissolved some of the iron.

    Collect and test the gas produced, that will save you guessing.

    Generally speaking chlorine doesn't react with water, or rather - as long as the solution is not alkaline - it reacts only slightly. In alkaline solutions chlorine disproportionates into chlorides and hypochlorite. Still, once the solution is saturated with these chlorine will just evolve. To make things difficult it will be always contaminated with oxygen from water electrolysis, so salt water electrolysis is not the best way of making chlorine. Electrolyzing melted NaCl is much better, although at the same time it is quite challenging because of high temperatures involved and because both sodium and chlorine are highly reactive (IOW: don't try it at home).
     
  4. Aug 10, 2014 #3

    kox

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    I'm not looking to produce chlorine. I'm just curious why there was no bubbling at positive electrode (that would be anode in electrochemistry?). I will try running it a little longer tomorrow to see if chlorine will be produced.
    Wouldn't electrolysis make it an alkali solution? (2)Cl- is oxidized to Cl2 while Na+ is still there, so chlorine could be reacting with the solution, because the solution is alkali, did I get that right?
     
  5. Aug 10, 2014 #4

    Borek

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    If chlorine is being produced, solution is definitely getting alkaline, albeit rather slowly. But it is not clear what you are producing.
     
  6. Aug 11, 2014 #5

    kox

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    I've run it twice today. First electroylsis was done using 2 iron electrodes. I don't know if
    solution was saturated, because I forgot to weigh water and took a guess. There was some salt
    left undissolved, so I think it was saturated solution. I ran it for some time and I didn't
    see chlorine. Hydrogen was there, I tested it.
    I think conductivity of the solution increased. When I started electrodes were close
    together, current measured at 9 amps with ampmeter in series. The ampmeter has some
    resistance and its rated for 10amps so I couldn't have it connected all the time. After I
    disconnected it I didn't have to move away the electrodes, without ampmeter it could be
    something like 13 amps. PSU is rated for 23 amps, it shuts down if too much current is drawn.
    But after some time I had to move electrodes away from each other. They had to be away as
    much as they could be in a jar.
    Do OH- ions conduct better than Cl- ?
    There was some NaOH, I dropped aluminum foil and it was reacting.

    In second one I used iron for cathode and copper for anode, so there was no mud. This time I
    weighted water and I'm pretty sure the solution was saturated. Water around copper was
    turning green. Is it because of copper or chlorine compoounds? Soultion became pale green.
    After 90 seconds I saw bubbling at anode and a green gas was leaving the water.

    I'm not sure, maybe I ran first electrolysis for shorter amount of time, I didn't measure time. I would run it for longer, but psu kept shutting down and the jar was pretty hot. But there was more NaOH in the first one - more reactive with aluminium.
     
  7. Aug 12, 2014 #6

    Borek

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    Yes, definitely.
     
  8. Aug 12, 2014 #7

    Lok

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    A bit o remembrance of my electrolysis of low salt water. I used a 2V and up to 12V continuous source. Bubbling was on both for the higher voltage, and the stench of chlorine was obvious. I did get a murky green precipitate in the beginning, but when the chlorine got depleted (did not test it) the bubbling turned to O2 and H2 (no stench of chlorine so other gases were excluded) I continued to get a green precipitate. After drying the precipitate (it turned brown) and heating it it turned into very fine highly magnetic Iron rust (intense rust red). At some point I did also get a white precipitate but that one seemed to have a limited lifespan.

    I suppose in the beginning FeCl2 (green solute) was obtained which got electrolyzed further into Fe(OH)2 (green precipitate) which dries in air to Fe(OH)3 (brown precipitate) which can be heated to Fe2O3. After Chlorine was depleted the Iron was attacked by the OH- directly to Fe(OH)2.
    All this is a supposition as I never tested beyond the observation. I would like some input on these if possible.
     
  9. May 12, 2015 #8

    jim hardy

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    i remember doing this as a kid with 6 volts.

    as best i recall , at the O2 electrode my metal wires reacted with the oxygen , so i made rust or verdigris that discolored the water just as you describe..

    I took apart an old zinc-carbon battery and used the big graphite center rod for an O2 electrode. Then it bubbled nicely.

    Do drafting supply stores still exist? A graphite pencil lead should work, i'd think...
     
  10. May 12, 2015 #9

    James Pelezo

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    I know the anion is chloride, but what is the cation; that is, what salt did you add to the tap water?
     
  11. May 15, 2015 #10

    Lok

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    Hi,

    While not directly expressed and in all cases where nobody mentions it or bothers with it in a few posts, I believe and assume the salt to be table salt.
     
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