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Ions in water - what does ppM actually mean?

  1. Jun 1, 2008 #1
    "A Copper test is performed much as you would test for chlorine residue. A level of copper in the range of 0.15 through 0.20 ppM"

    The above is an extract from a page on swimming pool ionisers. The ionisers produce copper ions for water cleaning purposes.

    0.2ppM (parts per Million) - I don't understand how you can quantify copper ions in this way. Is this a ratio by weight? or does it man 1 atom of ionised copper per 5 million water atoms.

    Any thoughts on the actual meaning of this appreciated.

    What I'm really looking for is an idea of how much copper I need to electrolyse to prevent algae growth.

    I am attempting to construct a simple ioniser for a small decorative fountain we have. Currently we use chlorine tablets but that kills the surrounding plants which get splashed - and more importantly displeases 'she who must be obeyed'.

  2. jcsd
  3. Jun 1, 2008 #2


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    IIRC from high school chemistry, using Avogadro's number you can convert particle numbers into moles. Then you can turn the ppM quantities into molarities.
  4. Jun 3, 2008 #3
    I would imagine that this is based on weight (mass). Rarely have I seen it based on number of atoms.

    Then, 0.2 g Copper per (10^6) g H2O = (0.2/1000000)= 2*(10^-7) Wt %

    MW Cu = 63.5 g/mol
    MW H2O = 18

    Then, .0000002 * (18/63.5) = 5.67*(10^-8) Mol %
    Last edited: Jun 3, 2008
  5. Jun 4, 2008 #4
    The question is not as trivial as it might at first seem. It could be by volume (tricky for gases with changing pressures), by mass, by atoms as I suggested earlier. As any of these will yield different results it seems a pretty arbitrary thing.

  6. Jun 4, 2008 #5
    Indeed it can be arbitrary, but usually for liquids it is on a mass basis, and for gases it is on a volumetric basis. Personally, I would go with this assumption first because it seems the most reasonable.
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